Floor insulation ideas

[Montana]

Looking for ideas for insulating my suspended wooden floor. I’ve not got any room under the floorboards to insulate and I’m a bit stuck for a solution. I’ll probably over board the existing floorboards with 18mm P5 t&g boards, so guessing that will seal up the drafts better but won’t add much thermal benefit? The floor joists are 100mm deep. I’ll attach a photo. 
Thanks Richard 

[Gill]

We went mineral wool between the joists held in with netting. We have access so it could be done from below. 

Looks like you could get solid insulation  between the joists from above - are you trying to avoid taking the floor boards up? 

[Nickfromwales]

Are your internal walls sat on top of the floorboards? This was done with great wisdom by our forefarters  

2 stories of brick wall with the upstairs joists spanning it from 2 rooms, all on top of the floorboards and the ( now diminishing ) 4” timbers on dwarf walls ( with failed “damp proof” slates lol ) at mine ffs

What the hell were they thinking ?!  

[Montana]

No not worried about taking the floor boards up, just we only have the 100mm void between the joists for the air flow, so I can’t really put anything between the joists unfortunately, as it will stop or reduce the ventilation. 

[Nickfromwales]

2 minutes ago, Montana said:

No not worried about taking the floor boards up, just we only have the 100mm void between the joists for the air flow, so I can’t really put anything between the joists unfortunately, as it will stop or reduce the ventilation. 

The gaps UNDER the joists forms your ventilated space, and not the gaps in between them   

[Montana]

2 minutes ago, Nickfromwales said:

Are your internal walls sat on top of the floorboards? This was done with great wisdom by our forefarters  

2 stories of brick wall with the upstairs joists spanning it from 2 rooms, all on top of the floorboards and the ( now diminishing ) 4” timbers on dwarf walls ( with failed “damp proof” slates lol ). 

What the hell were they thinking ?!  

It’s only a bungalow, cable end front and back, all internal walls are stud work. The 4 inch timbers on top of the dwarf walls are in good condition, as are the bricks and mortar, they also have an old dpm which looks fine. It had some woodworm on the end of some joists, which I’ve cut out and just about to replace. 

[Montana]

18 minutes ago, Nickfromwales said:

The gaps UNDER the joists forms your ventilated space, and not the gaps in between them   

Can you show me the gaps under the joists please? It’s one course of continuous bricks, with a timber plate on top, joists sit on that. So the gap between the joists allows for ventilation? 
Thanks Richard. 

Edited February 25, 2023 by Montana

[Nickfromwales]

6 minutes ago, Montana said:

Can you show me the gaps under the joists please? It’s one course of continuous bricks, with a timber plate on top, joists sit on that. So the cap between the joists allows for ventilation? 
Thanks Richard. 

I can’t show you mine, I took one look, swore several times, and put the boards back down. Mine will get completely dug out, insulated, concreted with mesh and UFH pipes, and the internal walls rebuilt.

 

I’ve worked in the construction industry for 3 decades so am speaking about your job from experience. 
 

If each of your gaps between joists were the ventilated spaces, you’d need an air brick for each void, so the house would collapse.

 

Your air bricks will feed down into the void / underbelly and in-filling between the joists will be perfectly fine, as long as the insulation doesn’t completely foul the air bricks and their path to the remainder of the ventilated void. 

[Nickfromwales]

To add:

 

With your floors, you will of course not be able to fully fill the spaces. but you’ll be able to install 50-60mm of fixed rigid PIR without issue. Use the highest performing product you can. 
 

If you wish to go further, then you can remove the floorboards and put strips of 20mm Compacfoam on top of each joist, to arrest the repeat cold bridging. Then use 75mm rigid insulation, with the top of that being left flush with the too of the Compacfoam. That will leave you a 45mm void underneath, where each supporting pier is, for through ventilation.

[Iceverge]

Assuming you have enough space for the P5(18mm) and floorboards (25mm) I would propose something like this. 

 

Notch out the wall plate as shown for ventilation. (Disclaimer------ I don't "think" the house will fall down!!) or drill a couple of 25mm holes. 

 

 

 

 

Follow ecological building systems approach with the two membranes keeping the drafts at bay. Both from an windtightness approach( thermal bypass) and airtightness. Return the membrane to the internal walls. 

 

 

 

Lay 25mm battens on top of the existing joists in lieu of the thickness of the floor boards ( you could even rip down some floor boards to do this). 

 

 

This will give you a insulative value of about 0.3W/m2K using a high performing mineral wool batt like 

 

 

 

If you want to substitute the mineral wool for the more painful and less robust process of PIR then the U value could drop to 0.25W/m2K. 

[Iceverge]

Good thinking @Nickfromwales but the compacfoam doesn't make much difference unfortunately vs timber at such insulative values. 

 

 

WITHOUT COMPACFOAM

 

 

WITH COMPACFOAM

 

 

 

 

 

 

 

 

Edited February 25, 2023 by Iceverge

[Nickfromwales]

22 minutes ago, Iceverge said:

Good thinking @Nickfromwales but the compacfoam doesn't make much difference unfortunately vs timber at such insulative values. 

I’ll get my coat. 😭

 

I won’t start cladding my clients exposed structural steelwork with timber any time soon though