Floor Insulation

The most common form of under-floor insulation is probably rigid foam (PIR) boards. These are cut to size to fit between the joists before the edges are sealed with expanding spray foam or tape.

With quality workmanship this is an effective method. However, there are pitfalls to be aware of. Cutting the boards accurately enough to form a tight seal takes skill. If the edges aren’t tight or properly sealed then cold air can simply flow around the boards rendering them almost useless. Fitting the boards around any under-floor pipes also adds complexity. Overall we wouldn’t advise this approach for a DIY project.

Instead, we recommend the use of wool-type insulation as explained below. This approach has several benefits over PIR boards -

The wool is “squishy” making it easy to fit into irregular spaces without leaving gaps.
The airtightness membrane and vapour control layer ensure good levels of airtightness.
Some diffusion of moisture vapour can occur through the breather membrane, reducing the risk of rotting of the joists.

Rigid PIR boards used to insulate a suspended timber floor. Note that the gaps around the edges which will need to be sealed for the insulation to be effective.

Step 1 - Preparation

Strip off floor coverings, floorboards, skirting boards etc. to reveal the joists.

Before progressing with the installation this is a good time to tidy up under the floor. Secure any loose electrical cables, insulate any water pipes, and seal any gaps around the joist ends with expanding foam. Check that the ventilation is free of obstructions.

Step 2 - Install breather membrane

This serves two purposes. It supports the insulation, but crucially also prevents airflow through the floor. The breather membrane should be pinned to each joist using timber battens in the lower corners to maintain the shape. To complete the seal the membrane should be continuous, with joints between the sheets fully taped. It should be wrapped up the walls to be pinned in place by the skirting boards at the final step.

It’s important to use a breathable membrane because these have some vapour permeability meaning any dampness that gets into the insulation can harmlessly evaporate.

Floor insulation - step 2 breather membrane

Step 3 - install insulation

Install the insulation between the joists. Any wool type insulation will work, with rockwool being typical. Purchase the insulation in widths just larger than the joist gap so it squashes into the space with no gaps. There may be a smaller gap between the final joist and the wall - a thinner strip of insulation should be inserted here too.

Step 4 - install vapour barrier (optional)

Install vapour control membrane on top of the insulation. This prevents any humidity from the room getting into the insulation system. Any gaps in the vapour control membrane should also be taped, as well as the joint with the wall. Any holes for pipes or cables should be sealed with airtight grommets.