What is a “Warm Roof”?
By placing insulation over the top of the roof structure, we form a warm roof.
Think of it like this:
“You sleep under your quilt, not on top!”
Because the insulation is over the top, the timbers of the roof are warm, therefore this is called a warm roof.
The makeup of a cold roof is like sleeping on top of your quilt. Everything above the quilt is cold and everything below is warm. With the insulation under the roof covering, the roof is cold, hence the term Cold Roof.
Grants for warm roofs.
At the moment there are not grants for warm roofs installation (2013) however the vat for the installation has been dropped to 5% and if all the works undertaken on the flat roof are substantial then those works are also 5% vat
Depending on the size of the flat roof and just how much of the flat roof is covering the house warm roof insulation could save you around £190 and 800kg of carbon dioxide a year.
Warm Roof Construction
Warm roofs construction is one of the easiest methods of insulating a roof, because the roof insulation is placed over the roof and not under it its accessibility and ease of laying make it a relatively unskilled job. A quick inspection by a qualified roofer to make sure all the joints between the insulation has been filled and away you go. The design process for a new warm roof should take in.
- Can the roof be raised by the height of the insulation without conflict.
- What vapour barrier should be used
- What depth of insulation should be used to reach the new building regulation
- What the chosen roofing membrane will be
Warm Roof Construction Health and Safety
Warm roof construction means taking large quantity of rigid insulation up onto your roof, therefor scaffolding is nearly alway needed for access, and to safeguard the roofers as they conduct their work. The insulation itself is very safe apart from the dust that is formed when we are cutting it there are now unknown problems with the materials. The most common rigid roof insulation used on warm roofs is a high performance rigid thermoset polyisocyanurate (PIR) insulation manufactured with a blowing agent that has zero Ozone Depletion Potential (ODP) and low Global Warming Potential (GWP).
Over the years we have installed many warm roofs, and the warm roof market is getting bigger and bigger. Unfortunately we are finding that the attention to detail on installing a warm roof is not what it should be.
Let me explain.
Firstly we are in the retrofit market of warm roof, meaning that we are upgrading old buildings from cold roofs to warm roofs.
To do this first we have to make sure we close off all the old vents and find all the areas that will thermally bridge the warm roof. If a warm roof is installed without doing this, it won’t perform, and worse still, in the colder areas (cold bridging) it will probably sweat.
So in the design stages these areas should be recognised and built into the job programme. Air tightness and vapour control is the second thing that is not being dealt with in new warm roof installations. Roofing trade organisations know that the majority of roofers are using the wrong vapour control system, and even the wrong insulation in the new warm roof. Luckily the “Warm Roof” construction technique is very robust, and usually will work even if the wrong materials have been used. Typically a roofer will use a vapour barrier that is a sheet of material (usually plastic) and just because it’s got a label on it that states it’s a vapour barrier, he will use it. Unfortunately it’s not that easy. With warm roof construction, a minimum requirement of moisture resistance needs to be achieved, and most cheap plastic vapour barriers don’t achieve this, then the roofer fixes the warm roof insulation down over the vapour control layer (VCL) and punches holes through it, thus making it even less likely to perform correctly. If this happens and thermal bridging is present, then the possibility of some sort of sweating occurring is likely at some times of the year. If the rooms under the warm roof are high in moisture, this is even more likely, and in some very severe cases we have experienced water dripping through the ceilings of the rooms below. The manufacturers of the insulation aren’t helping the poor old flat roofer, as their product information isn’t particularly clear. The small, independent roofer may be good, but without the correct information, they can get into a lot of problems. Even larger roofing companies are getting the specifications for warm roofs wrong. I talked to Visqueen, a leading supplier of vapour control layers, and they reported that one of the largest flat roofing companies in the UK is using one of their products, even after repeatedly telling them they are still using the wrong product.
Try calling an insulation supplier and telling them that you want to form a warm roof, then asking what insulation you should use. The answer you would normally get will probably be a choice of different insulations, some of which will have silver foil on both faces and some will have tissue on both faces. Ask the question, can I use the foil as the VCL and the answer will be yes you can, and if you’re really lucky, they will also add that it’s part of the thermal element of the insulation. They should guide you to the relevant technical documents, but by now the roofer is off looking for the cheapest insulation and he finds that everybody stocks insulation with foil on it and it comes in big sheets, so bingo! To him, he sees it as the perfect stuff for that new warm flat roof he’s building, and not only that, he can use the silver foil for the VCL. Anyone would go down that line. I did, myself, when I first started, but luckily for me I’ve worked with lots of different surveyors, architects and engineers over the years; I’ve been able to pick the brains of the ones who know what they are doing. Believe me, the majority of them don’t!
Going back to using insulation with foil on each side.
I once read an article about vapour control layers and “when is a vapour barrier not a vapour barrier” was the question. The answer is when there is another vapour barrier in situ that works better. Let me explain. If the roofer doing your new warm flat roof uses silver-backed insulation and he installs it as per the manufacturer’s instructions, (virtually impossible as you can’t get to the back to seal the joints together) you now have insulation with a continuous vapour barrier at the underside (the warm side, hence the name warm roof). However you also have another silver layer on the top that’s equally as good as the one below, although the roofer should not have sealed all the joints, given that he has probably not sealed all the joints on the bottom (I’ve only ever seen this method carried out correctly once). Both VCLs are the same, so which one will work? We don’t know if the roof insulation is cut in neatly or not, in which case you could have movement of air with moisture in it, and then there is the possibility of thermal bridging where sweating can occur. The way round this is to place a continuous VCL over timber decking at the lowest point and use tissue-faced insulation thus forming what’s considered the best method of building a warm roof. I document this all over the internet and all of my videos show this, yet each week I get customers, builders, architects and surveyors calling to ask me for details on how to do this correctly. The information is just not out there and we are heading for some big disaster to do with warm roof construction.
So what’s the key to all of this?
Why not go back to cold roofs? That’s always been OK, hasn’t it? Forget the new warm roof way of constructing! Unfortunately there is no way back.
Most cold roofs worked, but as we have changed the way we live and the materials we use inside our house cold roofs that worked, are now starting not to work, and are incurring problems. It’s a fact that we spend more time inside our houses than we ever did; we heat them more and we have more internal washing and cooking areas, creating a lot more water vapour. Then add the fact that we insulate everything and fill all cracks, stopping natural movement of air and, yes you can see where I’m going with this
Ok so let’s look at all of this again.
You have an old property that has lots of changes made to it over the years – new windows, doors, central heating bathrooms and kitchens, probably a loft conversion and even a rear or side extension.
The old flat roof needs replacing and you want to insulate it. You think the old flat roof is about 40m², so you invite some roofer to give you some estimate for a new roof. Along the way you find out that what you are looking for is called a “warm roof”. As this is the new standard, two roofers come back and tell you the cost is going to be around 4 or 5 thousand pounds, but the third roofer comes back with the following:
Before I come out to look at your roof, I need to send over an engineer to evaluate the complete property – all the rooms on each floor. The engineer will be taking note of all changes that have occurred to the property over the years; he will take note of what windows and doors you have, whether they have trickle vents or not, he will look to see what tiles you have on the main roof and what insulation you have in the loft. He will want to know if you have had external wall insulation and if the any of the walls have been sealed with masonry paint; he will note the amount of people living in the property and the amount of bathrooms you have. Armed with all of this information, he can then go back and work out what vapour control will be necessary for your new warm roof. Too much vapour control could cause problems to other areas of the property; too little and the roof could sweat.
As soon as the engineer passes over the information, the roofer can then put together your quotation for your new roof.
What a performance! But, interestingly it’s the correct way of doing things, and yet it’s rarely done.
If you are not going to look at the complete buildings performance, the roofer really only has one choice:
Use the best vapour barrier on the market, and make sure that the new warm roof doesn’t sweat.