Suspended timber floor insulation - type?

dannim12345

Hello,

At the moment all our floor boards are up in my living room (timber suspended floor), it’s a Victorian terrace house. Approx 75mm joists. While they are up I thought I may as well insulate the floor. I’m not expectecting it to meet current standards but I figured anything is better than nothing...

The house has suffered with damp (hopefully we have done various things to improve this) but i’m cautious to reduce ventilation under the floor boards too much so looking for something thin.

I have came across multi foil insulation (which has mixed reviews) or Jabfloor70 (50mm).

Any suggestions of which would be better or different product completely?

Thanks

Heedtheadvice

You are right to be cautious especially if you have dampness.
I cannot recommend a specific product but, if fitted, being thin is not the best criterium! Thickness often goes with more insulation but the overriding one needs to be material that does not prevent moisture evaporation or you will be trapping it and just creating a problem.

FreeBear

The problem you will have (the same as me...) is the 3" joists will be resting on sleeper walls at regular intervals - This places a serious restriction on how much insulation can be stuffed under the floor. Ideally, you need to leave 2" (50mm) gap between the sleeper wall timbers and the insulation to allow air to flow around. This will limit you to around 1" (25mm) of insulation.

Looking at the specs for Jabfloor, it has a thermal conductivity of 0.038W/mK - Compare it to Celotex/Kingspan PIR boards which come in around 0.023W/mK (much better). If you shop around the builders merchants, you can probably get the PIR boards cheaper than Jabfloor.

Solid boards would need to be carefully cut to fit between the joists and small battens added to stop them from going down too far. Once in place, the gaps between the boards<->boards<-> joists would need to be filled with squirty foam. As would any gaps around the walls.

The thin laminated insulation rolls could be rolled out across the joists (paying attention to the joints), and then the floorboards nailed down through the foil - Much quicker and simpler, but at a greater cost.

Once the floorboards are relaid, I'd suggest putting down a layer of woodfibre underlay followed by a good quality foam underlay and then a nice carpet- Assuming of course you want carpet...

dannim12345

Thank you both, especially freebear. So much mixed information on the internet so glad to have some additional options. I will look into 25mm and again at the foil.

martinsurrey

dannim12345 wrote: »

Thank you both, especially freebear. So much mixed information on the internet so glad to have some additional options. I will look into 25mm and again at the foil.

a cheaper option that can give good results is to suspend mineral wool between joists.

something like this.

https://www.permaculture.co.uk/articles/planning-lay-new-carpet-save-energy-insulate-first

done well it allows ventilation and is a lot more forgiving on condensation issues.

FreeBear

martinsurrey wrote: »

a cheaper option that can give good results is to suspend mineral wool between joists.

done well it allows ventilation and is a lot more forgiving on condensation issues.

Problem with rockwool/glassfibre is you need quite a bit more than PIR boards to get the same thermal insulation. For example, the Knauf Earthwool is quoted at 0.044W/mK, so you'd need twice as thick a layer. With only 3" joists, it going to be difficult... With mineral wool, 150-200mm is typically the target thickness. That would wreck any underfloor ventilation.

dannim12345

I think it’s between the multi foil and 25mm PIR. I don’t think there is much different in the cost, the foil would be easier to install. A lot of the info about the foil is on the brand websites so harder to research...

martinsurrey

FreeBear wrote: »

Problem with rockwool/glassfibre is you need quite a bit more than PIR boards to get the same thermal insulation. For example, the Knauf Earthwool is quoted at 0.044W/mK, so you'd need twice as thick a layer. With only 3" joists, it going to be difficult... With mineral wool, 150-200mm is typically the target thickness. That would wreck any underfloor ventilation.

ohh I dont disagree at all, but PIR boards badly installed can cause havoc.

Mineral wool is relatively safe and cheap