Underfloor insulation

OxfordO1

Hi,
I would like to insulate under my floorboards to save heating costs and be more efficient but I don’t know what to use? As we are retro fitting I need something that goes in between the joists. Should I use Thermawrap (silver backed bubble wrap) or Diall loft insulation (made from plastic bottles). I’m Worried about causing issues with water Vapor and subsequent damp/rot if I use the plastic bubble wrap. Should I be using something that is breathable?

FreeBear

Don't use Thermawrap or any of the other multifoil products  Whilst they may be OK for reflecting back radiant heat, they don't do will with conducted heat.

Best insulation for performance/thickness/cost is Celotex/Kingspan type foam boards. How thick a board you can use depends very much on the depth of your joists - You must avoid compromising ventilation. If your joists are 3x2, fixing 2x1 battens either side and then resting 25mm of insulation on top would be OK. With thicker joists (say 4x2) you could get away with 50mm of insulation.

Using loft insulation would mean having to fix a mash support between the joists, and it would sag. Getting a consistent layer is also going to be tricky - In addition, you'd need twice the thickness to achieve the same u-value compared to Celotex/Kingspan boards. Oh, and mice & rats love the stuff to nest in.

OxfordO1

Due to finances, I’ll be doing this myself. Do those boards have to be held in place? Would nails be suitable, or would I have to batton them into place?

kipsterno1

Are you lifting the floor boards or looking to fit it via a crawl space?

FreeBear

OxfordO1 said:

Due to finances, I’ll be doing this myself. Do those boards have to be held in place? Would nails be suitable, or would I have to batton them into place?

May I refer you to a thread over on a different forum (complete with picture) - https://www.periodproperty.co.uk/forum/threads/underfloor-insulation.17758/

Cut the insulation board to be a snug fit between the joists. Fix the 2x1 battens to the joists either with nails or screws every 600mm or so. The insulation will then push down on top of the battens.

Tip - A sharp serrated kitchen cuts the boards cleanly and without any mess, unlike a hand saw.. I use a Kitchen Devil like this one - https://www.amazon.co.uk/Kitchen-Devils-Lifestyle-Knife/dp/B0012B80HG (available elsewhere for less).