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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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