Suspended timber floor with shallow joists: an indecent proposal

[Nickfromwales]

4 minutes ago, tenovus said:

Thanks, Nick! Just on the VCL - would there be an issue with using a polythene VCL sheet over PIR so that I don't need to get too exact with the foil tape or does that create issues by having two impermeable layers on top of each other?

I just don’t like the gaps, if there’s no voids then nothing can be interstitial. Foil stuck to foil of board, if you use PIR, and across the tops of the joists would be my go to. 
 

If you’re filling with mineral wool, seems to be plenty good enough and much less of a PITA, then the membrane as you show will suffice. Just seems a total

ballache and a significant undertaking? No probs is you’re DIY’ing it, just your own time / effort. 
 

The membrane ‘hammock’ will be fine on its own, I doubt there’s actually any need for the OSB, if using wool or fibreboard type stuff. Just push fit it gently and you’re done, using the wool at the bottom to absorb undulations and the fixed dimension stuff up top. 
 

Or, this is belt & braces: 

 

On 27/01/2025 at 23:06, Iceverge said:

 

My suggestion. U Value about 0.16. 

 

 

Rip some 9mm OSB, nail 2 x battens to either side of the bottom to create a flange. Nail it to the sides of the existing joists. 

 

Drop in a bottom piece of OSB to create a box. A/T tape the corners to make it completely wind proof. This is very important the performance as your insulation will get wind washed if it is not sealed.

 

Drop in the mineral wool/woodfiber insulation. 

 

Lay the Chipboard on top and fastidiously foam glue all the joints. This will be your internal airtight layer. 

 

 

This proposed arrangement can all be done without the bother of getting into the crawl space at all. If you add some glue to the OSB and the battens you would effectively be creating an I beam which would massively strengthen the floor too. 

 

 

I never like PIR between joists. Too many reports of it shrinking and coming loose over time. 

 

 

👌

[Iceverge]

As I'm avoiding doing real work I though it might be interesting to do a cost comparison for a PIR between the joists vs my above method. 

 

First lets look at the PIR wedged between the joists with foam and foil tape. 

 

Here's the normal arrangement. 

 

 

However with a little thinking and extra spend on PIR and just let it extend below the floor joists you can get a good improvement in U Value. Here's 150mm between the joists. Ubakus won't let me show the PIR as a single board)  Lets work from this.  

 

 

 

A 150mm board is £56. However because of the way you need to cut it you'll loose 0.36m2 from each board. Maybe you can use some of these offcuts so lets be generous and say you get 2.6m2 from every board £21.54/m2 for PIR but you have to account for the spaces taken up by the joists so the PIR will cost you £18.84 per m2 of your room. 

 

FM330 foam is £12 per can and lets say you make sure it's done well and use 1 can for every 3 boards. That comes to £1.5/m2.   

 

150mm Foil tape is £2.20/m to bridge the top of each joist and give 50mm coverage on each side overlapping the foil face of the boards. You'll use 2.5 linear m per m2 floor so £5.5/m2

 

 

Total cost per m2 for the above arrangement is £18.84+£1.50+£5.50= £25.84 per m2. Add a little more waste in the tape and call it £26 per m2 For a U-Value of 0.23W/m2K. 

 

 

Nicely done. ( until the foam shrinks in 5 years and the insulation falls into the ventilated floor that is!) 

 

 

 

 

I'll split the post here. 

 

 

[Iceverge]

Here's my idea. It's hard to show the OSB as described above in UBAKUS but it's useful for a U value comparison anyway. 

 

 

 

 

9mm OSB is £10.56 per board. Use 200mm as the verticals and 400mm as the horizontals, its 600mm per bay, and 2.5 bays per m2. It all cuts nicely so no waste. £5.50 per m2. 

 

Next use 19x38mm battens at 45p/m nailed to both sides of the web to create the flange. you'll need 5m per m2 so £2.25/m2. 

 

D4 glue will make the whole thing much stronger. Lets be generous and use 100ml/m2. say £1/m2. 

 

Nails will keep everything in place nicely. £4.26 per 1kg which is about 700 nails of 40mm x2.65mm galvanised ones. Say 40 nails per m2.  £0.25 pf nails per m2. ( nails are really cheap!) 

 

Next insulation. I have found in my experience loft roll doesn't quite expand to the desired thickness when installed in a timber frame. I have used it in walls but it was unsatisfactory. However in this situation where it is lying flat I would instead opt to install 150mm + 100mm and then compress it very slightly to fit the gap.  The cost of is it £6.72 per m2 and there won't be any wasted offcuts.  

 

Airtight tape the corners would make a really pucker job of it. No need for any fancy tapes. This stuff is fine. 5m per m2 again so  £3.16 per m2. 

 

 

 

 

 

Lets do some adding. 

 

OSB £5.50

Battens £2.25

Glue £1

Nails £0.25

Insulation £6.72

Airtight tape £3.16. 

 

£18.88 per m2. 

 

 

Labour of course is the variable but it depends on how good a result you want. Access to a small nail gun would make it immeasurably faster too.