Insulating a suspended timber floor

MichaelR

Hello,

In a two-storey house built in the 60s, I have a ground floor room that has a suspended timber floor. (The rest of the ground floor apparently has concrete floors).

It was originally a sitting room, complete with fireplace. I use it as a bedroom and study. The room is rather cold, and there are drafts from the floor around the walls which I was unable to seal fully.

Currently the following layers are on this floor above the floorboards, from bottom to top: a thin bubbly plastic underlay, old engineered wood (or maybe laminate), vinyl. They were all put here by the previous owner, but I like having a vinyl floor, because of its resistance to dirt and spillages (also stuff dropped on vinyl has less of a chance of breaking).

So the question is how to insulate the floor and also stop the drafts:

- I could get an insulation company. But all the companies I can find online in Limerick seem to be offering spray forma insulation for suspended timber floors. I have my doubts about this. The ventilation of the floor makes sure the boards can't get moist and rot, and the foam would block this air. And as rot would be invisible under the vinyl, I would never know the floor is rotting until it's way too late.

- I'm told a local builder can remove the suspended floor and pour a concrete floor instead, but I am not sure if it is a good idea to alter a house design like that? Besides, it might be expensive and, even more importantly, the concrete and then the screed would take weeks to set?

- Finally, there is the DIY-ish option. Remove everything over the floorboards but do not take up the floorboards, and put something new over them The question is what to put there.

I was thinking of circa 10 mm XPS boards, and did find them, though a bit expensive, at https://www.screwfix.ie/p/klima-thermal-board-insulation-5-pack/93806 . ALso there seem to be far cheaper insulation floor boards at https://www.woodfloorwarehouse.ie/techni-board-insulating-underlay - wonder if the difference is big at all, if I am not doing an underfloor heating system?

Anyway - the issue is that these boards seem to require a hard flooring (laminate, tiles) or else a screed, because they can't take high load in one point (like a bed post). And I want to be able to spill and drop stuff on the floor without consequences so I'm not sure about laminate. (I will not have enough height for board plus laminate plus vinyl.)

Of course I can also just use soft underlay for vinyl, but I would not really trust a soft underlay to seal all the drafts (the current one doesn't after all). So what then? A non-insulating but rigid board, sealed around to stop the drafts, something like https://www.screwfix.ie/p/diall-wood-fibre-underlay-boards-7m-15-pack/7824R or even like mere 6mm plywood, and then a thick soft insulating underlay on top, and then vinyl?

Anyway - of ALL these options, what should I be doing? Nor really "wedded" to anything at all at this point.

WildCardDoW

All I can tell you is that I went for spray foam.

I think the main concern you highlighted isn't a problem with closed cell foam.

If you don't go for that I believe the rock wool or insulated boards are needed. There's a formula you'll need to check for a decent size:

Length of Exposed Walls / Area of Room = P/A ratio and then pick a material that meets this value:
Product Thickness (mm) 0.1 0.2 0.3 0.4 0.5
Quinn Therm 90 0.15 0.19 0.22 0.23 0.24
Quinn Therm 100 0.14 0.18 0.2 0.22 0.22
Quinn Therm 110 0.14 0.17 0.19 0.2 0.21
Quinn Therm 120 0.13 0.16 0.18 0.19 0.2
Ballytherm 100 None None None 0.21 None
Ballytherm 90 None None 0.21 None None
Ballytherm 70 None 0.21 None None None
Ballytherm 25 0.21 None None None None
Kingspan 90 None 0.17 0.19 0.2 0.2
Kingspan 100 None 0.16 0.18 0.18 0.19
Kingspan 110 None 0.16 0.17 0.18 0.18
Kingspan 120 None 0.15 0.16 0.17 0.17
Isover METAC 100 0.15 0.2 0.22 0.23 0.24
Isover METAC 150 0.13 0.16 0.18 0.18 0.19
Isover METAC 180 0.12 0.15 0.16 0.16 0.17
Isover Spacesaver 100 0.16 0.21 0.24 0.26 0.27
Isover Spacesaver 150 0.14 0.18 0.2 0.21 0.21
E-Zero E500 Spray 150 N/A 0.18 N/A 0.2 0.22

Can't get that to format well, I got those values from each suppliers data sheets so you should read into that as it could be different already.

If the main issue is drafts and you know where they're from you could consider the air tightness tape used to seal windows around the edge , at least one system I looked up online recommended that or similar materials.

Basically put it onto the subfloor and under skirting. Others might say if that is good or not.

Lumen

Loads of previous threads on this but the authoritative guidance is...

S.R. 54:2014&A1:2019 Code of practice for the energy efficient retrofit of dwellings
https://www.nsai.ie/about/news/publication-of-sr-542014-code-of-practice/

Page 144 onwards.

It's a free order but you have to jump through absurd registration/watermarking hoops and then get emailed a link.