One or Two layers of PIR insulation in a warm roof?

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Steven Dickinson
London Flat Roofing
07802300099
info@londonflatroofing.co.uk
One or two layers of PIR insulation is the question nobody seems to be talking about, so in this video, I express my opinions on the subject.
This particular viewer sent over some information and asked the question. He also posed the question of whether or not he could mix the two different types of insulation: foil-faced and tissue-faced insulation. So let's answer some questions.
First of all, should you be using foil-faced insulation in a warm roof construction? And my opinion is no, and the reason behind that is you have one vapour control layer in a roof construction, and it should be at the bottom of the insulation, what we call the warm side of the insulation. This is because on the warm side of the insulation the temperature changes will be absolutely minimal because of the thermal properties of the insulation above. In which case you are never going to get any kind of interstitial condensation. What is interstitial condensation? It is basically condensation forming inside the roof construction. It's just a fancy word that is used in the construction industry.
So if you had more vapour control layers in a roof construction even though you've got a good air vapour control layer at the bottom, surely there shouldn't be a problem, you may ask? And that's absolutely right, there shouldn't be a problem, but why go there if you don't have to? Theoretically, any moisture that does get through the proper air vapour controller has the chance of migrating up through the rest of the buildup of the roof and getting out through the roofing membrane. So, at this point, you must note this: the most vapour-resistant product in the buildup should be the air vapour controller. Everything above it should be more vapour open. Thus, if for example some vapour gets past the air vapour controller, it can migrate all the way up and get out to the open atmosphere, penetrating through all the upper materials including the roofing product such as felt or a single ply. This is the principle that we're trying to follow. If you use foil-faced insulation, you are putting in other possible air vapour control layers in different places. These places will be higher in the buildup and could be more prone to having interstitial condensation forming. This is why it's not a good idea to use foil-faced PIR. However, we use it all the time and it's very rare that we see a problem. Why do we use it all the time? Because it's cheaper than using tissue-faced insulation. Generally speaking, we do not get many problems with interstitial condensation in warm roof construction. Therefore, over time we at London Flat Roofing have gone from using tissue-faced insulation because of the cost to using foil-faced insulation just like the rest of the country.
So now let's get to the point about adding different layers of insulation. We think this is a good idea. Basically, it helps stagger joints in the insulation and therefore stops any potential movement of air or moisture from accumulating in one area where there may be a gap between insulation and then getting up towards the bottom of the top decking and causing that to rot. We are now seeing lots of different studies on this being a good idea. These are coming out of America, where they seem to be a lot further forward than we are and have a lot more data to work with. Inside the buildup of a warm roof, there is a microclimate going on. It's above the air vapour control layer, and the movement of minute particles with moisture in them will move around small cavities and gaps in the insulation. In America, they tend to find that when they open a roof up with rotted decking, it is above where there are joints because of bad workmanship. You're never going to stop bad workmanship; however, by adapting the construction by using two layers of insulation with staggered joints, you can dramatically stop this migration of moisture and air.
Hopefully, this is informative. If you need any more information, please don't hesitate to contact me.
Steven Dickinson
London Flat Roofing
07802300099
info@londonflatroofing.co.uk
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3 окт 2024

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Комментарии : 24

@MrKarlPrince 3 месяца назад

As well over overlapping joints reducing leakage, other advantages of multiple layers: less bowing (especially after cutting) and easier to cut, especially around corners and protrusions.

@SteveRoofer 3 месяца назад

Yes, all of the above are also good advantages

@tombeever6309 10 дней назад

Use a lot of woodfibre insulation over here in Europe, they have rigid water proof stuff (waterproof if the roof pitch is over 16º) but its all tonge and groove as well so no joint gaps.... unless your building is out of square 🥴

@whalebonefields Месяц назад

I've often wondered about this, and the cost of PIR without the foil, and have also found it odd and annoying that the the vapour resistance value of PIR is usually very hard to find. Maybe the answer you could be looking for here is that the vapour resistance value of the foil bit isn't THAT much higher than the rest of it. In other words, because the breathability of the PIR foam bit isn't good either, then having multiple VCLs in the form of the foil facings on a double layer of PIR, is merely adding to the problem of trapping moisture and not a sole cause of? i.e. 2 layers of 50mm foil faced PIR might have a similar overall non-permeability as 1 layer of 150mm foil faced PIR - so there’s not enough in it to worry about unless you’re using for 3 or 4 layers? I found an old not about Celotex products CTB4000, GA4000 and XR4000: ”The foil-facings have a water vapour resistance of 1000 MNsg-1 and the insulation core has a water vapour resistivity of 300 MN·s·g-1·m-1 […]” As a DIYer I can’t get my head around the units or the numbers to be honest, but perhaps you can? Does it really mean the foil is only about 3.3 times more effective at blocking moisture movement through it than the foamy bit? p.s. Have you ever tried just peeling the foil off?

@philipoakley5498 3 месяца назад

The "if" part of a two layer construction (rather than single layer) is whether sufficient moisture could accumulate at the layer join for it to actually condense out and then propagate the potential mould/rot. The solid PIR has nowhere for there to be that condensable moisture (compare with mineral wool etc). It's a long slow game (decade of life) to ensure that the moisture doesn't have opportunities to ruin everything. So many things to consider!

@SteveRoofer 3 месяца назад

So many things to consider is assault right

@smellypunks 3 месяца назад

Can you do a video about insulating solid walls (i.e. no cavity) and the issues around interstitial condensation, air gap or not, breathable insulation or not etc. There are millions of homes in the UK that have solid walls and would be good to hear from you on the subject if you can speak to it?

@SteveRoofer 3 месяца назад

Ill put that on my list of video to do

@BroughamBaker 3 месяца назад

I would guess that you would say the existing felt is a bad AVCL when converting an uninsulated roof to a warm roof. My bungalow had a secong floor with dormers added and flat roof extension 18 years ago. Not a scrap of insulation anywhere. Expensive to heat in the winter and an oven in the summer. At least th electrician was good out i would be much more concerned.

@SteveRoofer 3 месяца назад

no the existing roof can make a very good air vapour controller because the problem is tucking the roofing around the edge which was the subject of my last video. You've gotta make sure that you tuck it in around the edge. Bring the thermal element of the insulation that's gonna go over the top of the roof down to the next thermal element which is the wall this is so often missed and therefore realistically you're losing 7080% of the performance of the roof by not doing it

@captainwiffwaff6201 2 месяца назад

Sorry steve, I love your videos and I find them really useful. But can I tuck in the insulation in below a lower deck but above the wall. Or should I should I have the lower deck short of the wall so the tucked in insulation and warm roof insulation meet. FYI it’s having a top decked fitted as well.

@adamkingdon8063 3 месяца назад

It certainly has to be a warm roof when it’s located in Scotland, do you ever work up that way or are you just looking for work in London? I can possibly put you in touch with a roofer that also specialises in water damage etc that you could liase with he lives up that way.

@SteveRoofer 3 месяца назад

Thanks for the offer, but we're London-based in really don't get any work up there

@northeastcorals 3 месяца назад

I could be wrong but I think I can spot some black plastic sheeting just say sticking up out of the 1st insulation layer at the corner after the window & the top right corner which may have been used as the vapour control layer. I'm not saying that would be correct, just pointing out what I think I can see.

@SteveRoofer 3 месяца назад

You could be right; however, plastic is the wrong material to use on a warm roof. Additionally, plastic Air Vapour Control Layers (AVCL) are typically green. Black is normally the color used for Damp Proof Course (DPC) or Damp Proof Membranes (DPM), all of which do not use virgin plastic and therefore do not offer the correct vapor resistance.

@abescheele 2 месяца назад

Is there any issue with using two different types of insulation. For example I currently have 50mm of fibreglass wool in a cold roof. Wondering if I can stick inculcated plasterboard underneath?