How do you know if you have enough loft insulation?

Sandy Hickey

by Sandy Hickey

27/02/2024, in Energy focus | Main Blog | South Warwickshire Blog

For many people their loft is a mystery, a dark place that is difficult to access, so they never go there.

I lived in my last house for two years before I finally got up the nerve to pop my head through the hatch and discover the almost total lack of insulation. That was soon rectified, with an immediate improvement in comfort, so I definitely hoped to do the same thing here.

Well, this is my new loft and yes it has potential, (and some of my friends have loft envy) but my aim at the moment is to make the part of the house I actually use 24/7 comfortable and take the stress out of thinking about the energy bill.

It is not a dark space and it’s easily accessed at the moment, so there was no excuse not to investigate. But I needed a bit of help.

So I got out my largest saucepan and cooked up a jackfruit and black bean chilli and called on some friends to come and lend a hand with investigating the current situation. 



This is what we discovered when we got a crowbar out and lifted boards in various areas.

As you can see, there was insulation between the ceiling rafters in some places, but as it was wider than the space it had to fill, it had been folded and overly compressed. It is the air that is ‘captured’ in an insulation product that is doing all the work, squash the air out and you loose the insulating effect. So where I actually had some insulation in the loft, it wasn’t working effectively enough.

The house had been rewired and the electrician had removed the insulation where he had laid cables between the joists. He should of ensured the new cables were oversized to cope with being surrounded by insulation and positioned them to allow the insulation to be put back properly. 

At the eaves the insulation stopped short of the walls, allowing heat to escape at the edge of the room below and creating a much colder surface area. This is where warm moist air in the house could condense, allowing black mould spots to develop. One area must of been insulated as far back as the 1970s as there was only a 25mm depth (U-value of 1.5W/sqmk) of loft insulation. Early adopters of insulation, what a shame they didn’t top it up!

The latest Building Regulations update in 2019 requires a minimum U-value of 0.13W/sqmk, unless you are surpassing the insulation requirements for the walls or floor of your house. This equates to a 338mm depth of standard loft insulation. 

The main loft definitely needed some serious attention!


The next place we needed to investigate was the area above the sitting room that has it’s own separate roof space. There is no loft hatch into it and a brick cavity wall separates it from the main loft. How could I find out what, if any, insulation was in there?

Fuelled with chilli, some friends got busy with a hammer drill and removed a brick so we could see into the wall cavity. Excitement! We could see blown in fibrous cavity wall insulation. I knew from the deeds that it had been installed in 1992, so it was good to be reassured that they hadn’t skimped on the installation. We then drilled a hole through the second brick leaf. This allowed us to insert an endoscope through the wall. There is a tiny camera and light on the end of the cable.

We had chosen to remove the fourth brick up, to try and get a good view, but were a bit confused when we saw strange fuzzy images on the screen. As a test we explored the cavity insulation and got similar images.

I already had a hope that there was some insulation above the sitting room, as despite it having three external walls it didn’t seem to be markedly cooler than the main part of the house.

Had the cavity wall installers also insulated this loft space, or was it just overspill? We needed to check a different part of the space.

Rather than taking out another brick further up, we decided to see what was happening at the eaves. This is the advantage of investigating a single storey house – you just need a chair to stand on! 

We popped out one of the eaves vents in the soffit, and started exploring the void above. We could see a reassuring blanket of insulation along the top of the eaves coming over the inner brick leaf and meeting the insulation in the cavity wall. This couldn’t of spilled over from filling the cavity. It was clear they had endeavoured to insulate the sitting room ceiling. But how much was up there? In 1992 the required U-value would of been 0.4W/sqmk, probably equivalent to about 110mm depth. We knew there was at least 270mm depth where we did our first check. To ensure there was the minimum requirement over the whole ceiling area, was it possible they had over filled to ensure the middle section had enough? That is what I am hoping, and that I have adequate insulation.

I am not planning any further invasive exploration, but the final thing I did was wait for low (7degC) temperatures outside, and the heating to be on, then I used my infrared thermometer to measurement the surface temperature across the house ceilings. The variation in surface temperature readings did seem to corroborate my theory. Where there is 350mm of insulation in the main part of the house the surface temperature was 16.5degC (room temperature 17.5degC), where there was no insulation (by the still to be completed insulated loft hatch) it was 13.5degC. In the sitting room with the mystery depth of insulation it varied from 14.2degC to 15.5degC.

I surmise there is insulation, but not as much as I had hoped. There is definitely room for improvement. I will be considering my options for adding more insulation in the future.