Insulating a suspended timber floor

gnolan

I'm planning on insulating our suspended timber floor in a house from the 1930s in the next few weeks. Currently we've got carpet > underlay > pine boards > uninsulated floor joists @400 centres > a void of about 500mm. I'm planning on relaying a new engineered wood floor after insulating. I've got a decent idea of what my plan is. I'll pretty much be following this guide and the included PDF, but I'm looking to see if there's any issues with it before I proceed. This is the plan:

1. Pull up the old floorboards and clear out the void as best I can, making sure there's adequate airflow between the subfloor vents
2. Lag all pipe work. I'd love some recommendations for good pipe lagging
3. Level any uneven floor joists by installing plastic shims under the joists and above the sleeper walls
4. Hang a breathable membrane over the joists and fix in place
5. Install 100mm (the depth of the floor joists) rockwool or mineral wool between the joists. I'm not going for PIR / Celotex as I've read that there should be breathability to prevent moisture buildup around the floor joists
6. Install a vapour barrier on top of this
7. At this point I don't know if I can put back down the existing pine boards or if I should be putting down a thinner chipboard. Id love some advice on this
8. Install underlay. Recommendation on this would also be appreciated
9. Install engineered floor

Does this seem like a good approach?

Lumen

I've done this a couple of times but TBH if I was doing it again I'd probably use PIR. Those diagrams make the rockwool-on-membrane approach look really simple but I found it quite fiddly to get the membrane properly tensioned. As long as there's a vapour barrier above the insulation I don't think breathability of the floor timbers is a massive deal, there should be ventilation to the outside from the underside and you can always go a bit shallower e.g. 80mm to leave more of the joists exposed since PIR has higher insulation value.

That said, this might a case of "the thing I haven't tried yet will be easier".

Regardless, think about total finished floor thickness and specifically whether you need to match existing floor levels, architraves, thresholds etc anywhere on the ground floor. I assume you're ripping and replacing skirting as part of the job.

gnolan

Thanks @Lumen . Yeah, I'm removing skirting and architraves. When tensioning the membrane, did you staple the top and install battens at the bottom of the joists?

The PIR does look a lot easier, although I think cutting to fit wonky 90-year old joists might be just as annoying. I am also pretty paranoid about any damp forming; but I've been back and forth between going for PIR and rockwool for a while now.

Given your experience, do you think there's any problems if the underlay I choose also has a vapour barrier? So from the top I'd have; engineered wood floor > underlay with vapour barrier > old pine boards > vapour barrier > insulation > breathable membrane

loyatemu

How much extra benefit is the insulation giving you if you're putting an engineered floor, underlay and airtight membrane on top anyway? I'd have thought eliminating the drafts will give you 90% of the benefit. I know my suspended floors are really drafty (despite being T&G) but they don't feel that cold on the wood itself.

gnolan

@loyatemu I'd expect the benefit of the insulation to be significant. I'm sure I could work out the u-value / r-value. Air tightness to eliminate the drafts is all well and good but an uninsulated floor is a massive surface area to leach away heat, just like an uninsulated wall or a single glazed window.

Lumen

https://www.boards.ie/discussion/comment/122578836#Comment_122578836

I think as long as any vapour barriers are on the warm side of the insulation it's ok.

Another thing I considered was getting right under the subfloor and stapling mesh and/or membrane across the bottom of the joists.

dbas

https://www.boards.ie/discussion/2058367431/insulating-a-suspended-timber-floor

Great approach. The timbers will breathe properly and shouldn't rot thereafter due to trapped moisture.

Go hard with the pipe insulation. Rockwoll/ kingspan pipe insulation. It's outside and will be colder now due to far less heat from the floor.

I don't know about underlay to be honest. Ecological could advise.

Stick to their system exactly is all id say. Use those membranes/ tapes insulation in their order

loyatemu

https://www.boards.ie/discussion/comment/122579127#Comment_122579127

true, but wood is a better insulator than concrete or glass. I only had a quick google but it seems the u-value of a timber floor is about half that of an uninsulated wall and less than a quarter of a single-glazed window. But that's before you put down your underlay and engineered floor on top which should great improve that figure as well as eliminating drafts.

GreeBo

https://www.boards.ie/discussion/comment/122580555#Comment_122580555

Removing drafts will certainly help, but if the floor is uninsulated it will still be a cold floor.

I would go with PIR, fixing some battens to the joists to keep them in place, foam the gaps, tape the joints and then VCL layer on top.

all the bais

This was my attempt below. I listed in the comments where I bought everything, mostly from TJ O'Mahonys / My Building Supplies. I followed the same PDF as you mentioned, just went for cheaper products than the ones they listed.

https://www.reddit.com/r/DIYUK/comments/14qgxqo/insulating_suspended_floor/

gnolan

https://www.boards.ie/discussion/comment/122580914#Comment_122580914

This is really useful, thanks. If you were doing it again would you do anything differently?

gnolan

https://www.boards.ie/discussion/comment/122580301#Comment_122580301

Anywhere you can recommend getting Rockwool or Kingspan pipe insulation? Most vendors seem to be UK-based

dbas

Sig do something similar. There's loads of places in Dublin or thereabouts.

You're looking for foil backed insulation with thermal conductivity less than 0.036. ideally less than that. Tape the connections together.

Make sure it fits snugly to the pipe. You're doing it once, do it right.

Hse are doing pipe insulation works and saving 30-40% energy. It's really important, especially with heating pipework as moving hot water loses more heat

all the bais

https://www.boards.ie/discussion/comment/122581868#Comment_122581868

I'd have staggered the OSB sub-floor. Rookie mistake. Other than that I was happy enough.