Good practice guide to insulation

I wrote this guide to insulation for the Passivhaus trust. It is quite long so I have made it shorter and a bit more blunt. Here’s ‘5 reasons your insulation won’t be as good as you think’, or ’20 ways to get hot and steamy on a building site’, prizes for other tempting titles that don’t relate to the text.

What is the problem?

Passivhaus includes checks on site to ensure that things happened the way we planned them, and the way that our energy calculations assume. Unbelievably this is not the norm for UK construction, or construction in Germany (hence why they need Passivhaus too). By checking on site we often see where things have gone right, but also where they’ve gone wrong. There is still room for improvement. Here I aim to give insight into good practice, hopefully its useful for your project.

How good the insulation is depends on the manufacturing of the material and how it is installed on site. U-value calculations include the first one, but typically not the second. Even small gaps between and behind insulation have a huge effect on how effective it is, this is the performance gap!

The performance gap represented by a bad partial fill cavity
A bad partial fill wall cavity

I don’t think that this is the builder’s fault. Here are some ways we can help.


1. Thicker walls

Not because we need more insulation, but because we want to give choice later on. The thickness we have to play with is fixed at planning, or even before, when we set a wall thickness on the ground plan.

Estimate the thickness of insulation that will be required with a conductivity of 0.04W/mK, add structure and finishes, and include an extra 150mm if you have a brick or stone finish! If you’re on a constrained site make sure the depth is maximised where it can be, for example front and back of a terrace house. Don’t forget the roof thickness too, especially for flat roofs or those insulated at rafter level.

2. Check the U-value calculations

Or do them yourself. Despite numerous competent person schemes there seems to be a lot of variation, to put it politely! Has all the structure been included? Would it simplify building the thing if you included a few fixings?

3. Think about the material, and tolerance

Building materials are all over the place; lengths of timber aren’t straight, concrete is rough and wonky, and blocks/bricks can vary by 10mm thickness before they’ve been built into a wall (using a bit of string and a trowel). We know this and should design the layers of construction to account for it.

If you have to use rigid insulation boards make sure they are held against a flat surface. Don’t expect the builder to do complex joinery work when they’re paid to be quick and its raining.


4. Fitting insulation is a separate skilled task

Think carefully whose package of works it fits in with, or the person you ask to do it. There needs to be time in the programme for fitting and inspecting the insulation works.

5. Specify the conductivity and thickness of insulation, not the U-value

This makes it much easier to check on site, and ensures the supplier doesn’t have the chance to change the construction at the last minute. Be wary of ‘I can get the same U-value for less thickness’, either the material must have changed, or they’ve been too optimistic in the calculation.


6. Check delivered materials

Now you know what the thermal conductivity should be, you can check it on the product label. Also check for damaged boards and damaged or open packaging.

7. Show what ‘good’ looks like

Look at photos of similar insulation done well. Some projects have built a sample section of construction and this works really well. Aim for no gaps, then fill any gaps that couldn’t be helped.

8. Inspection

As early as possible. Early checks, or benchmark inspections make sure the installation is right from the beginning and prevents having to redo work.

9. Insulation is easy to cut, but difficult to cut well

It’s so soft that the blade quickly wanders and you end up with a wonky edge joining another wonky edge. Its worse for thicker materials. Set up a cutting area, and find a reliable method. There’s some suggestions on how to cut material in the full guide.

10. Work from the edges inward

For rigid boards its much better to use the whole boards with a straight (most of the time) side at the edge of the floor slab or wall where a straight side is useful. This is the weakest point thermally, the infill sections can go in the centre of the floor.

11. Take photographs of everything

If you are the builder it will show the client the quality of workmanship. The expectation of photographs and hidden work being visible can change the approach to installing.

12. Protection

Insulation is damaged easily. Protect it from the weather and from people and machines on site.

13. Check

Get signoff from the site manager before covering insulation materials. This goes especially for floor slab insulation and cavity wall construction.


There is none! The beauty of fabric first construction.

What does good look like?

See below for some good practice photographs from various sites: