Build Science Expert - Windows and Insulation Part 1
In our latest episode, we had the pleasure of sitting down with Dr. Cameron Munro from Passive Analytics to really dig into the world of insulation. If you’ve ever wondered why your house feels too hot or too cold, or if you’re curious about how the colour of your roof actually affects your comfort, this one’s for you.
What Is Insulation, Really?
We kicked things off by asking Cam the most basic question: what is insulation? As it turns out, insulation is all about reducing heat transfer. But to understand how insulation works, you first need to understand how heat moves. Cam broke it down for us into three main mechanisms: conduction, convection, and radiation.
Conduction is the transfer of heat through materials. Think of how a metal spoon gets hot in a pot of soup—heat moves atom by atom through the solid.
Convection is about the movement of air (or fluids), carrying heat from one place to another. This is why air tightness in buildings is so important.
Radiation is electromagnetic energy—heat radiating from a hot surface to a cooler one, and this is where colour can come into play.
Does Colour Matter? The Dark vs. Light Roof Debate
We’ve all heard the debate about dark roofs versus light roofs, especially with councils and local governments pushing for lighter colours to combat urban heat. Cam explained that while dark roofs do absorb more heat (which can be beneficial in colder climates), the real impact depends on the building’s design and context. In high-performance homes or passive houses, the colour of the cladding or roof becomes less critical compared to other factors like insulation quality and ventilation.
Interestingly, Cam pointed out that the urban heat island effect is influenced more by things like roads and concrete than by roof colour alone. So, while colour does play a role, it’s just one piece of a much bigger puzzle.
Ventilated Cavities and Building Performance
One of the most eye-opening parts of our chat was about ventilated cavities. We often think of air gaps as good insulators, but Cam clarified that it’s only true if the air is still. In ventilated cavities, air movement (convection) can actually carry heat away, reducing the insulating effect. This is why the design and placement of insulation matter so much—trapping small pockets of still air is key.
Windows: More Than Just a View
We couldn’t talk about insulation without diving into windows and glazing. Cam explained that windows are a form of insulation, but their performance depends on more than just the thickness of the glass. Double and triple glazing, the size of the air gap, and the use of gases like argon all play a role. Plus, coatings like low-E (low emissivity) can dramatically affect how much heat is retained or reflected, depending on the climate and the specific needs of the building.
Rules of Thumb and Real-World Context
A recurring theme in our conversation was the importance of context. Building codes and rules of thumb are based on studies from specific climates and wall types, but every project is unique. Understanding the physics behind heat transfer allows you to make better decisions for your particular situation, rather than relying solely on generic guidelines.
Moisture, Durability, and the Balancing Act
We also touched on the trade-offs between thermal performance and moisture management. Sometimes, ventilating a cavity is more about preventing moisture problems than maximising insulation. It’s all about finding the right balance for your climate and building type.
Key Takeaways
Insulation is about controlling heat transfer—conduction, convection, and radiation all matter.
The colour of your roof or cladding has an impact, but it’s less important in well-designed, high-performance buildings.
Still air is a great insulator; moving air (convection) is not.
Windows are complex—double/triple glazing, air gaps, and coatings all affect performance.
Context is everything: what works in one climate or building type may not work in another.
Moisture management is just as important as thermal performance.
A huge thanks to Dr. Cameron Munro for sharing his expertise and helping us unpack the science (and myths) around insulation. If you’re designing, building, or renovating, we hope this episode—and this article—gives you a clearer understanding of what really matters when it comes to keeping your home comfortable and efficient.
We recorded this episode in two parts, so look out for part two where we move further into wall and roof insulation.
LINKS:
Connect with us on Instagram: @themindfulbuilderpod
Connect with Hamish:
Instagram: @sanctumhomes
Website: www.yoursanctum.com.au/
Connect with Matt:
Instagram: @carlandconstructions
Website: www.carlandconstructions.com/
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[00:00:00] Hamish: we’re going to talk about insulation.
[00:00:02] Hamish: Cam, what is insulation?
[00:00:06] Cam: Well, I think we need to start, well, well what is insulation is about reducing heat transfer.
[00:00:11] Hamish: Yep.
[00:00:12] Cam: But in order to understand how insulation works, we need to understand how heat transfer works. So there are three ways in which heat can transfer, conduction, convection and radiation.
[00:00:24] So we take each of those one at a time. Conduction is a material property. So different materials have different rates at which heat conduct across them. And conduction is about the, um, the atoms within a, a ma a, a lattice of a, of a solid material like a mele or glass for example. Or even timber And heat is, is energy.
[00:00:46] It's the excitation of those atoms. So they vibrate. Yep. And when they vibrate, they hit the atom next to 'em and they transfer that energy and they do that across that solid material that's conduction. So that conduction is something that happens. Cross [00:01:00] salt convection is the movement of air. It's air, warm air from one space, moving to the cooler space, carrying heat energy with it.
[00:01:11] And that happens a lot in our buildings and is one of the reasons why air tightness is so important to Enish. And the third one is probably the trickiest one, conceptually to under step. And that's radiation, that's electromagnetic, uh, energy. So a, a material when it's heated will emit radiation in all directions.
[00:01:33] And then that will transfer to, uh, an adjacent material that be absorbed. So this is about the characteristic, and this is where color becomes important because different materials have different weight rates at which they emit electro radiation and absorb electromagnetic radiation. So a material that's a dark color, like black, we'll have a higher absorption.
[00:01:56] Which is a better ability to absorb radiant [00:02:00] energy and a light color will tend to, uh, have lower absorbed.
[00:02:04] Hamish: So you're saying that with all these cancels and local governments now wanting to remove black roofs that could potentially be in a, in a climate where cool climate, but we need a heat that could be a bad thing.
[00:02:15] Cam: Well, it depends. So, so colors like dark claddings can be beneficial too. So, uh, in a healing dominated climate like an Melbourne or a Canberra or somewhere, it is true that a darker colored roof sheet will obviously make that roof space significantly warmer on a warm day than a light colored roof sheet.
[00:02:34] Uh, but it also means that the systems is drier because you have much Grady drier and drying potential within that roof space because of the, the ability for that to absorb the heat. And then allow, uh, they allow drying.
[00:02:48] Hamish: So maybe a bit off topic for a second with this. Is it the heat urban island effect?
[00:02:52] Are the roofs that we're designing, like should we be going more lighter color or darkened color? I think heat urban, um, I think [00:03:00] that is about landscape and concrete mass. Yeah. Rather than, and and lack of trees. Trees. And this is where I'm actually, this is actually what I wanted to get at is like, is are we blaming the wrong thing for this issue?
[00:03:12] Cam: Well, I think it, it can contribute. I mean, if you looked at a satellite image of a, an urban suburban area, obviously the roof area is a significant part of the tunnel fraction of that surface area. But the roads asphalt. Yeah. A dark color. Yeah. And is, you know, 30% of our surface area of our seas is road.
[00:03:30] Yeah. So that's gonna contribute disproportionately. So, yes, to an extent, I think this is somewhat overplayed, but if we just bring it back to buildings for a second. If you design your building and the load well. The color of the cladding and the roof sheet are far less material than if you design your building poorly.
[00:03:51] Hamish: I, I actually have a note down here saying dark colors, does it matter in passive house? Or we could extend that to high performance homes. And I think we've kind of [00:04:00] touched on this before with, in other podcasts where we start talking about, um, that ventilated and drainage cavity because does a light or dark roof actually have that impact given its, I guess, separation from that insulation layer?
[00:04:14] Cam: If that cavity is really well ventilated, then clearly the cover of the cladding is far less important.
[00:04:20] Matt: Yep.
[00:04:20] Cam: But they're, they're will always be some heat transfer from the roof sheet sheet through the cladding into your insulation and then into the building itself.
[00:04:29] Matt: Yeah.
[00:04:29] Cam: But it, as you go up through the, from code to high performance to passive house, the effect of the cladding color on performance of the building declines.
[00:04:41] Yeah. So yes, it's still important, but it's secondary or tertiary versus to so many other things that you can do with the building. Whereas if you have a really poor building, pre-built building, then yes, the cladding color absolutely has an effect. And so if you put monument on the roof, then in the summer in Western Sydney, you are gonna bake.
[00:04:59] Hamish: So [00:05:00] you say poor, poor performing house. But we had a passive house in Mascot Vale where originally the designer designed it with a darker color to get the PHBP across line. But actually it was a white, and by the end of the project, which I had no idea about this, and the clients had no idea. It was actually white.
[00:05:16] I, we lost one kilowatt hour across the whole boom, which meant we were now failing.
[00:05:19] Cam: And so this is where context matters, I think. 'cause one kilowatt hour per square meter for annum, which is like part of these metrics for using passive house is really significant when you are targeting something like 50.
[00:05:31] Yeah. Matches this magic number. So one divide by 15 is obviously a large number, but we need to bear in mind your coil homes are the north of a hundred. Yeah, yeah, yeah. So one unit out of a hundred is dly squat one dead of 15 is really interior.
[00:05:45] Hamish: And just to close out the whole dark color versus light color thing, that would probably have an impact more or less of an impact if you had a cold roof scenario and a warm roof scenario in Washington because, and I'll probably let you [00:06:00] expand on that a little bit because that does then feed into insulation then where insulation sits and its impact in the building.
[00:06:06] Cam: Yeah. Yeah, it can, it can do. Uh, so if it's a cold roof, again, it depends how well isolated that roof space is. Yep. Um, but you've got a, a bigger,
[00:06:15] Hamish: a lot of these sort of like, well that depends because there is, every situation is different. And I think that's actually really important to acknowledge.
[00:06:25] Cam: And I think this is where we come back to rules of thumb.
[00:06:27] You know, we talk about like, uh, like the NCC talks about this, you know, you need so many square millimeters per linear meter of opening in order to ventilate a wall over a roof. And that we have all these rules of thumb and they are based on one or two empirical studies in Germany, Austria, New Zealand, wherever it may be with one context, one wall type, maybe a few different ones.
[00:06:52] That doesn't necessarily apply to your particular brick ER project in kind. Or wherever it may be. And so context [00:07:00] matters. Understanding the physics of what's going on matters because you can use that rule of thumb to then say, well, that's my starting point, but my particular project has these particular attributes, which makes it riskier.
[00:07:12] Therefore I err towards having more ventilation. Yeah. But on the other hand, this project has the attributes which makes it less risky, so I can be a bit more, uh, not, not have to require as much in the way to as those openings.
[00:07:26] Hamish: So history of insulation, we probably think back to like the house I live in now, something built around like the fifties, sixties, nothing.
[00:07:34] We've gone from nothing to having something but now introduced a problem.
[00:07:37] Cam: Yeah. And, and so what are we doing insulation for? We're trying to reduce the rate of heat movement and in so doing that also reduces the ability for the system to dry.
[00:07:46] Hamish: And just to go back to those three things that you talked before, conduction, convection, radiation.
[00:07:50] We're talking about convection here,
[00:07:53] Cam: so we're talking about all three. So, okay. Heat transfer a possibility. Is this by all three of those. Okay. In different [00:08:00] ratios depending on what element it is. So conduction is a material property. Yeah. If I have a block of wood and I heat one side of that wood and I measure the temperature at the other end of that wood.
[00:08:10] Yeah. Then obviously I'm expecting very little heat transfer. Intuitively we know that to be right because wood is a, you know, uh, has a low, so conductory. Yep. By comparison, if I get a bit of steel, uh, the other end will be hot to touch. And if I get aluminum, it'll be even hotter again.
[00:08:28] Hamish: Yeah. So it's no still houses, no aluminum windows with foil suing
[00:08:33] Cam: all of those things.
[00:08:34] So really, yeah. Intuitively, Dion is the thing that we, I think we all kind of get, we know that low thermally conducting materials are better.
[00:08:43] Hamish: Well, I get it now.
[00:08:44] Cam: Yeah. And, and, and, and so this comes to things like, like glass. Yeah. So everybody with their standard four mil glass, although my windows are terrible, if only I had thicker glass, so we'll know because glass as a material is highly thermally [00:09:00] conductive.
[00:09:00] So if you can have a little bit of glass, it's poor performers. If I have a lot of glass, it's still port with thermal performers. Yeah. It's a material property. Yes. Thicker glass has less heat transfer than thinner glass. But not significantly so in the way that changing the material has,
[00:09:19] Hamish: so it's the, it's with the glass, it's the, the Lowe coating, the spacing, that's really adding the value to spacer bars.
[00:09:25] Like that's where performance is coming through. And,
[00:09:27] Cam: and so if you look at a window and say, you have a single pane of glass, so how can I improve the performance of this glass? Let's set aside the frame for a moment. Then the first thing you do is look at the conduction across that. Yeah. So what do I do?
[00:09:42] I could go double blazing. And so what I'm trying to do then is to separate my two highly conductive elements, glass on either side and trapped in between a, a volume of a gas, the air, so in the usual or the easiest case. [00:10:00] And by trapping air, the conductance of air is much, much lower than that of, of the glass.
[00:10:07] And so I reduce the conductance across that buildup. Yeah, it's the, the huge benefit you'll see in going to double players. Now the next one is convection. And so this again is an air mass that's holding heat energy transferring from a warm space, so internally inside in winter to the outside, and it carries that energy away, and that's not what we want to have happen.
[00:10:33] Now, if we use that glass of buildup as a example of this for a second, it's if you we accept that it's the air that's doing all the heavy lifting in that double waste unit, then it would be intuitively reasonable to say, well, why do we just build these with say, 10 mil gaps or 16 mil gaps? Why don't we build them with 30 or 50 mil air gaps instead?
[00:10:55] Shouldn't that perform better? And in terms of conductance, yes, [00:11:00] absolutely it is better, but the problem there arises, so you get convection. So you've got your interior surface of glass. So again, we're thinking a warm heating dominated climate. So it's warm that interior face of glass. The exterior bit of glass is cold, and so the air that's trapped in that cavity, when that gla and that air gets near to the cold surface on the outside equals what does cool air do it?
[00:11:26] It drops and then cycles. It hits the glass on the inside, it goes warm, and you get exactly what my mission do I to out. It cycles around. It moves Ed currents that this convection. So when I do that, I've now got this air that's moving, and so the air is carrying the heat energy from the interior face of glass towards the exterior.
[00:11:49] It touches the exterior face of glass. What does it do now? It transfers by conduction the heat energy into the outside pane of glass, which then [00:12:00] conducts out towards the exterior. Is your brain try.
[00:12:03] Hamish: No, it's not. 'cause it actually makes sort of sense. Is it? Because then, because then it makes sense now that um, when you start, I guess talking about double verse triple glazed and cam please jump in if I do this, say this wrong.
[00:12:18] 'cause you can have a triple glazed unit with external glass, middle glass, internal glass. And that might be overall 32 mil thick. Right? So I guess then ask the question, well why don't it still double glaze? That's overall 32 mil thick. But the reason is that you've got the convection that happens within that DGU and you don't get that same impact in a triple glaze because that convection is limited to a smaller space outside.
[00:12:48] And then as you move through that wall, that bit of glass, it's lessens on the inside. My kind of spot on right up.
[00:12:55] Cam: Yeah. So, exactly. So let's, if we just borrow, let's just say you can put a 24 [00:13:00] mil egg out. Yeah, in between a double glaze, if you now whack a third pane in the glass down the middle and split that into two 12 mil gaps, you are, you've got the same total air layer conductance 24 mil of air, but you've got two 12 mil gaps of air and you don't get that convection falling as much.
[00:13:19] So if you've got a tracked air void, depending on the temperature difference inside and out, but in, certainly in the Australian climate, say, where it gets to zero outside and maybe 20 degrees inside in a wind, then at about 12 mil below a tracted air mass won't have much convection to it.
[00:13:37] Hamish: So, so am I hearing right?
[00:13:39] And, and, and please Don, now fancy this with, well that depends. Um, am I hearing right that there is like an optimal gap between balance that depends?
[00:13:50] Cam: It it does depend because it depends on the temperature difference.
[00:13:53] Hamish: And so it's really clum climatic.
[00:13:56] Cam: And this is where we get so confused in the glass [00:14:00] space.
[00:14:00] Yeah. Between different test standards. Yeah. So in Australia, you know, we probably shouldn't go down this rabbit hole here, but in Australia we go to A FRC, which is adopted for an NFRC, the North American Test standard, which uses very cold exterior air temperatures when it's more than a 16 or something.
[00:14:18] I can't re, I might have heard that wrong, but it's minus something far colder than we would experience in almost all Australian climates and, and 20 degrees say inside, when you have that very high delta tube, that temperature difference, say 35 odd degrees, then even at minus gaps, sort of 12 to 14 mil, you'll drive that conducted air movement more rapidly.
[00:14:39] Whereas if you are, if it's only five degrees outside, plus five and 20 beers inside, then the tendency for that air to start convecting for those I currents to form is far reduced. And so you could still go for say, a 14 milli gap and not had significant
[00:14:57] Hamish: Yeah.
[00:14:57] Cam: Can reference,
[00:14:58] Hamish: I wanna go back a little bit for a second though, [00:15:00] because we're sitting here talking about glazing and windows.
[00:15:03] But windows are a form of insulation. Yeah. Obviously this, we just wanted to, I wanted, was just thinking before there's potentially a whole podcast on talking about flash. So while we're, while we're talking about insulation here. What we're talking about I think is probably, uh, yeah, windows are formed, insulation, we've got this big structure and all of a sudden they're cutting a hole in it and it needs to perform as close to as we can in the walls.
[00:15:25] It's pretty much where, yeah, just wanted to keep everyone on track of.
[00:15:28] Cam: And so let's jump across insulation for a second. So our standard insulation is our bulk insulation, our bat, you know, the, the woven glass wool usually thing that we, we get from our hardware or how is that working? Well, it's glass firstly, and we've already established that glass is highly conductive.
[00:15:46] Hamish: Yeah. Can I just stop just for one second because, um, and I just, and I do just wanna take it back to glass for a second. And this is probably just sort of closing out that comment before when you said conduction, convection and radiation is all [00:16:00] relevant. And if we, we've talked about conduction, talked about convection in glass.
[00:16:05] Could, let's quickly talk about how radiation. Is also um, a part of that glass unit.
[00:16:12] Cam: This is one of my big gripes. Yeah. Is everybody talks about, oh look I've got double waste. Fantastic. But that's like saying, I've got a new car. What did you get? A Toyota Getting a car doesn't mean anything. 'cause the next question always ask 'cause what's sort of car?
[00:16:26] Matt: Yeah.
[00:16:27] Cam: There's a hell of a big difference in double glazing is exactly the same. There's double glazing and then there's double glazing. Yeah. There's a huge difference. And a lot of the di a part of the difference is the size of that cavity we've talked about. Forget about the thickness of the glass.
[00:16:42] That's tertiary. It doesn't matter. Yes. The how deep is the cavity. Has that cavity got a noble gas like Argonne in it? Because Argonne has less conductance than than hair. And you contend to go through a deeper cavity with Argonne before you get those [00:17:00] ED counts for me
[00:17:01] Hamish: and the air is to be still because this is gonna lead to my second part.
[00:17:04] And is and is that the reason why we have started to put. Gases in there rather. Okay, cool. So just to slow that convection.
[00:17:11] Cam: Uh, and the better can lower conductance of argonne versus air.
[00:17:15] Hamish: Okay. So conduct. Okay. Yeah. All right. Now,
[00:17:17] Cam: so, so adding a Argonne to AGL double glazed unit gives you a improvement, you know, thrown around numbers, but maybe T cent.
[00:17:25] No, it's, it's material and it's relatively cheap and easy to do, and therefore we do it, but it's not massively changing performance. They glas it. Where you get the real benefit is going for those things we call low ES Let's again, constraint on a he dominated climate. We've got long wave of radiation coming from the sun.
[00:17:45] So hot, hot, um, things. The, the spectrum of heat energy generated tends to be longer wave a longer a wavelength. Yep. So the sun obviously being massively hot. Long wavelength. It comes [00:18:00] in, it strikes our bus, it gets towards an interior. It warms our interior spaces in winter, which is just what we want to have happen.
[00:18:06] And it heats our sofa and the floor and, and everything else. And then those materials warm up, but nowhere in the air is hot as the sun and they emit short wave radiation.
[00:18:17] Hamish: Is that, is, is that like a bit of a thermal mass thing? Like is that what you were saying? The furniture, because I've noticed in a house we just hand over, it's beautiful and comfortable, but it seems to not, there's like an emptiness of the heat, if that makes sense.
[00:18:31] Yeah. But until that, there was no furniture in his part yet. So the house is just open, we're about to hand over. When you go back there after it, it's the same, it feels like there's more heat in it.
[00:18:41] Cam: Well, depending on what the, the house is made from, by the time you add all of those, the furniture stuff, that's gonna contribute massively to the thermal acid.
[00:18:48] Yeah. I, that's function, I think tons of stuff we put in L in, but, but we've got this short wave radiation being admitted. And then it goes back and it goes through that glass towards the exterior. Now the [00:19:00] low e coating is a spluttered surface of fancy material. Silver is a collage component in it, and it's coated in your heating dominated climate.
[00:19:11] You'll be coated on surface free. Now you count the glass surface from the exterior to the interior. I was about to ask where, where does, does, does it matter where it is? Yeah, so you've got, so one is the outside bit that gets hit by the rain surface. Two is the inside of the outer pan. So there's three, the one I'm talking about is on the outside of the inner pain.
[00:19:31] Yeah. So it's facing towards our tra air gap cap, and you put this spluttered silver stuff on there and that reduces the emissivity of that surface. So the radiation that can emit from that interior paned glass is reduced to towards the exterior. Can you please
[00:19:52] Hamish: explain what, and I don't even know how to say this.
[00:19:54] That E in low EIF int, what does [00:20:00] that
[00:20:00] Cam: mean? It's a material characteristic that is the rate at which electromagnetic radiation can be emitted and be seen away from T material. Okay, so different materials of different color? Yeah. Have different LA rates of emmi
[00:20:16] Hamish: and that low E works by it in summer. It bounces the heat back in and in winter it kind of allows to come through.
[00:20:23] Cam: So this is where I think low E gets really, really tricky and people get confused because I think we inherently think of low E first as a solar control glass, like something you will put on the house in the cans of Darwin, your, our sole objective is to stop that spray and heat from the outside getting it.
[00:20:40] And in that sort of climate you would put the low E coat on surface two. So on the inside of the outer pain because you were trying to reduce the rate at which that heat, that waxs the glass and then emit can radiate into the, the space. But it's a different chemical composition you use because [00:21:00] you tune it to try and reduce the long wave of radiation because that's what's coming in from the sun.
[00:21:06] Hamish: So this is where, so I know this houses where the accidentally put the gla in the wrong way around. So it's very important then we that outside that stick our should always be on the outside and you need to check your windows.
[00:21:15] Cam: Yeah. Yeah. It depends. It depends. It kind of does because it also depends on the type of low eco I whole bunch of other stuff.
[00:21:24] Matt: Yeah.
[00:21:24] Cam: But, but what's, what, rather than the, the, the sur the coat, the surface onto which it's applied is the chemistry of that coating itself. So where I think we really go wrong, it will, and this, to be fair, there is just debate about this, but. When you look up your glass specification or go your, your glass manufacturer, they will have a, a suite of options for lowly coatings.
[00:21:50] Mm. And some of those are designed for your very heating do or your warm, sorry, your hop climates, your darlings and your cams. And they are [00:22:00] designed to reduce that long wave radiation into the space and reduce the, the overheating. If you are a Melbourne Hobart cradle mountain, then that's probably not what you want other than two or three months of the year.
[00:22:12] In the summer you've got the other eight months, a year when you want to get that heat into the space. And so then you use a different low coating that has different characteristics to reduce the miss of the, uh, short wave radiation. We typically put it on that surface three to try and improve the rate of heat retention of that class.
[00:22:34] Wicken, the space
[00:22:35] Hamish: are, are we talking about solar, heat and coefficient right now? Correct. Yep. Okay. SHGC.
[00:22:42] Cam: Yes. So, so there's two phrases. There's S-H-H-G-C that we would use in words and what we use in Australia, A FRC. And, and then in the European context we use a thing called igo Fair. Similar but slightly different is, is solar and
[00:22:57] Hamish: coefficient represented as a number?
[00:22:59] Yes. [00:23:00] And, and lower the better. Am I right? I thought it was the opposite to you. Values
[00:23:04] Cam: solar, heat gain, coefficient, or G
[00:23:07] Hamish: Yeah. Are a coefficient. So, so heat gain coefficient is represented as a G, is that, no, no. No. Or G. Or G. G. Value for pass. Sorry. Yeah, sorry. G,
[00:23:15] Cam: G value is the phrase or the acronym where you shopping.
[00:23:18] European quota.
[00:23:19] Hamish: Yes. Okay. Yeah. Um, which is actually easy to understand too, I find
[00:23:23] Cam: Possibly, but both of them are coefficients. They have a value from zero to one.
[00:23:28] Matt: Yeah.
[00:23:29] Cam: If, if the coefficient, the SHGC, let's just use that one, is one that means all of the rodent. People that just strikes that class. It's penetrates through on the other side.
[00:23:41] Matt: Yeah.
[00:23:41] Cam: And that, and that's what you want if you are in a ate.
[00:23:44] Hamish: Yes. Okay. 'cause you wanna bring, you wanna bring the heap in. So which one have no low E You just want to have clear bias.
[00:23:52] Cam: Yes. Except
[00:23:53] Hamish: you wanna it the other way around. You want it to bounce back in. Just to remind everyone right now, we are, we're currently talking about [00:24:00] radiation.
[00:24:00] Yes. But currently it's where radiations.
[00:24:04] Cam: And so, so we've established that ans HDC value one means all the radiant heat comes through a value of zero intuitively meets, none of it comes through. And so green lanes, you want it as close to one as possible. In Cairns, you want it as close to zero as possible.
[00:24:22] Now in reality, you can't get either of those extreme values and most of your, and this again, depends on how you, whether it's singles level or triple ways. But your SHGC, you know. Heating dominant liner, say Canberra Mellon. Hobar tends to have an SHC something in the 0.5 to 0.6, 5.7. And whereas in Cairns it'll be closer to 0.3.
[00:24:48] Hamish: And does your solar heat and coefficient change? And I think under the answer to this, from single to double, to triple to whether you've got low E to whether you've got, yeah. So so that's where [00:25:00] it's it's actual, it's looking at the entire unit. That's right. What? It's a glass. He's not glass.
[00:25:06] Cam: No. Yeah, that, that, that's right.
[00:25:08] These glass is, and these low E coats are not usually visible to the naked eye, except maybe on the certain angle, or
[00:25:14] Hamish: this is definitely not too far. If you've said a comment before that air is a great insulator, uh, foot glazing. And I then wanna now quickly move that to a conversation about ventilator cavities because we have an air gap.
[00:25:29] Does that mean the air in the ventilator cavities is also brand insulator?
[00:25:32] Cam: No. Could f.
[00:25:34] Hamish: And so, which means a convection is happen at a very fast rate.
[00:25:37] Cam: Uh, uh, you are, depending on how you do it, you are des hopefully designing that ventilator cavity to be just that ventilated. You wanted ventilated because you wanna carry away moisture and help drying of the, of the assembly.
[00:25:50] And so doing that, inherently you are gonna be taking any heat energy in that cavity. And trucking next door is the outside. Yes. So we, it [00:26:00] offers almost zero, uh, heat retention of heat, uh, benefits.
[00:26:05] Hamish: So then when we have this metal sarp thing, as people would say, it's the insulation, it's a radiant barrier because that's the, it needs that air gap though.
[00:26:15] Is it actually insulating?
[00:26:16] Cam: Yeah. Okay. So, so it comes back to a couple of things here. So still air gaps. Yeah. Yeah. So, so a is a shiny surface has a, a, a much, uh. Lower sensitivity Yeah. Than a, the, a matte dark color surface. And so the idea is it reflects that heat away, which is obviously desirable in a, in a hot climate.
[00:26:41] Uh, if you want to get the optimum performance out of that foil or that shiny surface, though, you need two things. Firstly, you need to keep it shiny. And if you are putting it on a flat roof and you're not gonna see it for the next 50 years, it's unlikely to remain shy if there,
[00:26:59] Hamish: oh, I just [00:27:00] was at a house doing the inspection last week and you, it is kind of exposed when we went up to attic and it's always brought it away.
[00:27:07] Cam: And then secondly, you need a still bag gap adjacent. And usually we're putting these falls on the outside of our star frame in no egg gap. And there is still cavity's no egg gap. Well, and if we do have a cav, a cavity in the dust worlds, it's no longer a still egg gap. And so the as built. Performance of that assembly is significantly different to what you and might test in a lab where you have a steel air extra, a clean set sheet of fault.
[00:27:33] Hamish: So you could say these metal sightings are a scam. I, I, in in, in a housing environment, in, in a, in a lab. It works. Oh, in a house it wouldn't, yeah. I'm certainly
[00:27:43] Cam: not
[00:27:43] Hamish: gonna say there're scare. Can I, can I also point out too that, um, this probably also pokes a couple of holes in our star ratings as well, because a, a lot of the time the star rating includes your 1.5 r, 1.5 anticon, which [00:28:00] sits on top of your batten claddings, your roof claddings.
[00:28:04] And if you then got that ventilator cavity underneath that's moving, you can't then include that R value in your overall R value. But, but Hurs does do that.
[00:28:16] Cam: Yeah. And, and again, this is one of those, it depends on because ventilated as well, this is all brief and from an insulation point of view, you don't want to ventilate that caver at all.
[00:28:27] But from a moisture point of view, you absolutely do want to ventilate that. So where does the balance lie? Yeah. And assuming as I think we would all agree with on durable weldings that don't ride out in five years, then we urge towards, we've gotta do with moisture first. Bugger the energy. Yeah. And so you ventilate the space, in which case the thermal benefit of that for faced, uh, uh, blankets sitting up on the top of the roof truss is massively declined in a, in, in a heat transfer towards the exterior context
[00:28:58] Hamish: with that risk of going too [00:29:00] much standard like a tangent.
[00:29:01] So in that ventilator cavity, what we're seeing is convection. Is that right? It depends. Or, or, or like
[00:29:10] Cam: you are also seeing radiation, but predominantly that'll be convection. Yeah.
[00:29:13] Hamish: Okay. So then if we're thinking about that, how we explain that scenario in a double glazed unit, does that impact that convection?
[00:29:22] And I'm thinking about like the size of the cavity year. Does that then impact how the insulation performs inside, like underneath that WIB? Or is it not impacted by the convection that's sitting on top?
[00:29:35] Cam: What we didn't say at the outset is the insulation is all about still air gaps. Yes. That's all we're trying to do is achieve a still air gap because air is a really excellent insulator if we maintain small pockets of air that can allow convection.
[00:29:49] So you to our,
[00:29:50] Hamish: sorry, what's a maximum? Maybe like 20, 22 mil? It depends.
[00:29:54] Cam: It depends on temperature difference. Yeah. Yeah. And so, you know, an Australian climate where we don't tend to have minus [00:30:00] 20 temperatures outside the Dells are tea. The difference between the inside and the outside temperature on a hot day or a cold day is probably less than 20 degrees.
[00:30:07] Yeah. You know, it's 40 degrees outside, 25 inside. We're at zero out and 20. Yeah. And so the ability for that air to convect is somewhat reduced and if temperature difference was larger. But what we are fundamentally trying to do is create lots of tiny little pockets of air in our glassful, or in our fo now XPSE, P-S-P-I-R, whatever material links.
[00:30:28] Lots of tiny, little trapped
[00:30:30] Hamish: Yeah.
[00:30:31] Cam: Voids of air.
[00:30:32] Hamish: We, we had a, a situation recently on a project where we're, um, working on together. We, we had a C cavity and then the, the art, like the bats were only gonna take up to, there was about 20 mil at the top of that where, um, there was gonna be a void because the bats didn't quite fill the void.
[00:30:50] And I just thought, oh, beautiful. Um, there's trapped down there. And you, I'll, I'll let you kind of finish off where we got to with this one because I learned [00:31:00] something that day.
[00:31:01] Cam: So the issue we grew, I think is sort of a, a, a halfway house if, if I have a trapped air void towards the cold side of a construction.
[00:31:09] Yep. So imagine, say a roof truss construction where I run my insulation of the ceiling fine. And then I have a void above before I get to my, my socking. And then yeah, my outside. I had this trapped mass of coal air in the woods, period. And that's a, a big depth so far, more than 12 or 40 mil. So you, there's a chance that you end convection and that air will move and any moisture that gets up into that space can then be picked up by those e currents and carried around in ways that're not entirely predictable.
[00:31:43] Hamish: So when we build our, we build, so itlo cavity, batten plaster, is that air gap still still? If we didn't put in installation, like how still is it? So, so that's because on the inside it's not running, so,
[00:31:55] Cam: so let's assume you're putting a 35 on button. Yeah. Pretty typical. Yeah, probably. Um, [00:32:00] that, that will have some air movement, but not much because there's not a big delta t it's only once you get into where the insulation is between your in or your external grab that you, you have a temperature difference.
[00:32:15] Yeah. And, and a risk of convection for,
[00:32:17] Hamish: so when you're doing the PHBP, do you allow that, if there's no insulation in zone, do you put a value in there still?
[00:32:24] Cam: So if you've gone be conservative, you've given no value?
[00:32:27] Hamish: Yeah.
[00:32:28] Cam: Um, we would normally give it a value because it is, as you say, essentially a still a, yeah.
[00:32:33] Hamish: Yeah. So what Mel was saying before there, there's probably, there's less risk on the warm side. And in our scenario there was a higher risk because there was quite a potential of a drastic.
[00:32:45] Cam: Temperature differential from the inside to the outside because we had a significant amount of the insulation. It was like R four or something?
[00:32:51] Yes, on the warm side. So if you think, again, zero degrees outside, 20 degrees inside. If I've got R four of insulation, by the time I get to the outside [00:33:00] face that I'm getting pretty cold, if I then put, say a photo face blanket a palat, which is a 1.5 ish, then I've, I've already reduced, you know, a good two thirds of my temperature.
[00:33:10] So I've gone from 20 degrees on the top side of that R four installation. I'm now maybe around 12 or less degrees and that almost certainly below dew point. Yeah. And so I've got this trapped massive of cold air that possibly is gonna vec and there is a chance that I'm gonna have moisture issues.
[00:33:29] Hamish: So prevent, even though that we, we probably did in some areas, compress the insulation a little bit.
[00:33:37] The risk was, well, the benefit of that is that, that we weren't allowing any end move. That in summary,
[00:33:43] Cam: that's, that's right. So your glass wall is of a sufficient density. Yeah. That you are trying to restrict the ability for convection air movement through that material.
[00:33:54] Matt: Yeah.
[00:33:54] Cam: Now that doesn't mean if you've got pressure difference in and out, obviously glassful is not an air tightness.
[00:33:59] You know, you [00:34:00] can get ahead on one side and you'll feel and blowing through the other side. That's not what we're talking about here. It's the temperature driven air movement. So delta C across that glass, is it gonna drive those ed currents in that, the
[00:34:13] Hamish: con And that's conviction.
[00:34:14]