I’m sure I’ve seen an advert for a product (metal/plastic things?) to raise the joists to make it easy to board over thick insulation, but I can’t find it now I need it. Is it expensive? I’m getting a builder to do the work so would it be just as easy/cheap to get him to raise the joists with wood? He suggested screwing down tongue and groove to squash the insulation, but I assume that woud reduce the effectiveness of the insulation? Can anyone help please?

  • Jonathan Hetreed

    Hi Christine,

    If you use a rigid insulation such as Celotex then you can apply floorboards directly on top as you builder suggests. However if you are using a compressible insulation such as mineral wool then its performance relies on the air spaces and you will need to increase the depth of the joists. Metal channels will create cold bridges through the structure which can result in damp patches on the ceiling where condensation occurs, timber is a better thermal insulation so less of a risk. Whichever approach you take make sure that either you have a completely unbroken vapour control layer below the insulation or that the boards you apply are vapour permeable, otherwise you risk interstitial condensation on the underside of the boards which can lead to rot in the timber.

    Make sure there are no cold bridges around the insulation as this is a cause of damp, eaves are a common place where this occurs.

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