Insulating a Roof Space: Lessons Learned

Recently, I have been involved in a project where I’ve been picking up the pieces of something done in the wrong order. Essentially, a hole in a roof was made and an internal wall was taken down, in winter, before the upstairs space was insulated.

To try and make the space liable, I decided it would be best to insulate a small boxed-in section above a recently installed staircase, which joins the downstairs to an uninsulated loft space. There is insulation between the floors, but not between the rafters of the roof, hence the need for additional insulation.

Selecting the Right Insulation Material

I considered a number of options for insulation including rockwool, fibreglass, PIR (polyisocyanurate), and spray foam. Each has its pros and cons for retrofitting an existing structure.

Comparison of Insulation Options

In the end, I decided the only way to meet the U-values for building regulations was to use PIR. To hit the current building regs, 125mm of PIR is required between the rafters and 50mm below it to stop thermal bridging. You could, in theory, reduce the depth of insulation below the rafters, but at the cost of thermal efficiency.

Working With the Existing Structure

The rafters are 170mm deep, which means that the maximum depth of PIR that can be installed is 120mm. This is because a 50mm gap for airflow must exist above the insulation to ventilate the roof space. I could have taken a different approach, but I wanted to have a solution that didn’t waste materials—as insulation is very costly. So I wanted to do it right, once. I sort of managed it; there won’t be any wastage in the end, but I will likely have to shuffle a few bits of insulation around.

Having watched a number of videos on installing PIR, I realised it was going to be a bit of an arse. My mother had previously insulated her extension during its construction and lamented to me about the frustration of having to remove PIR boards over and over and trim bits off so that they would fit snugly.

Using GAPO Tape: A Game Changer with Caveats

To avoid the recutting process, I instead chose to use GAPO tape, which is a very expensive foil-covered foam tape that attaches to the side of the insulation. It ensures that there are no gaps around the insulation. This is good because it means no draughts, so no rapid heat loss via air changes. It also, in theory, means you shouldn’t have to cut the boards as much. However, there is a learning curve.

Lesson 1: You Might Need to Cut Your Boards Smaller Than You Think

My initial issue after installing the GAPO tape on my first boards was that despite cutting the PIR 1cm smaller than the width between the rafters, I couldn’t push them into place. This was very annoying because GAPO tape sticks well and is expensive. So I didn’t particularly want to peel it off and cut the board smaller. Unfortunately, I had no other choice, and the tape came off.

I ended up cutting the boards 1.5cm smaller than the gap they would fit in, though I think I could have trimmed them further still. You will have to figure out what works for your installation. I think once you have done this a lot, you’d get very good at it.

Cutting the Boards: Finding the Right Tool

For cutting the boards, I had considered using a Festool insulation saw. But I didn’t want to hire one for £60 to do a few cuts. So in the end, I used a circular saw with a laser cutting guide. The advantage of this was it could do straight cuts. The disadvantage was that you had to line up the cuts perfectly on either side. This is not as simple as it would first appear and requires very careful measuring. Again, if you did this a lot, you’d get good pretty fast.

The Dust: The Horrible Dust

Whatever you do, don’t attempt this without dust extraction. I ended up using a Henry Hoover attached to the circular saw. Without it, both I and the surrounding area were carpeted with the most awful fine foam insulation dust you can imagine. The particles are very light and stay airborne for ages, so good ventilation and a proper dust mask are essential.

“Good Enough for Government Work”

I was aware when I took on the job that the sides of the roof would have to be insulated with thinner material – 100mm PIR. However, I had forgotten about this when ordering the insulation. So I had to undertake an executive bodge. I ended up using the 120mm PIR on the edges. The advantage of the GAPO tape is that I will be able to remove these sections easily and replace them with 100mm PIR when the time comes. These pieces can then slot directly back between the 170mm rafters.

What I Learnt from the Experience

I knew that doing insulation was a faff, which is why I tried to avoid the job for as long as possible. However, the heat pump I installed couldn’t deal with the massive heat loss through the roof. So I had to get on with it.

To try and make my life easier, I took some time to understand the problem before I took it on and did a lot of thinking and research. Even then, it wasn’t fun.

Due to the structure of the roof, this job was an easy job in the world of insulation. It was essentially just insulating between rafters. The rafters are at regular spacing, so the PIR can be cut to regular sizes and slotted in and sealed air-tight with the aid of GAPO tape.

What it made me realise (or become even more aware of) is that retrofitting insulation is a faff. It has made me much less bullish on the idea that the UK can rapidly insulate lots of old properties. Almost every job will be a faff in one way or another, and with faff comes labour costs.