Insulating Suspended Ground Floor between joists

WildCardDoW

So had a wood worm problem in a house I bought as well as some leaks that were hidden.

The result is the bottom floor carpet etc was all lifted as well as the boards and we've sprayed or replaced the joists.

We're now considering insulating this because, better now than having to have a huge cost and repeat lifting everything.

Any reccomendations on the process we've considered some options:

1.
Rigid Insulated Foam Board ~100mm
polyethylene slip sheet
Flooring

When we eventually do the walls they can extend over this floor insulation. The air vapour would be installed such that the skirting holds it in.

The underfloor has vents and will have a vent to front and back so should avoid damp/rot issues.

Potentially spray the edges if there's gaps but otherwise fit it as best as possible.

2. Rock wool suspended by chicken wire or netting. Potentially both in between joists and running under under.

3. Sprayfoam option.


Any recommendations or missing items on these?

listermint

Tbh I'd go with rigid. Relatively easy install. Good values obtained for the depth. And then membrane over it. Try not skimp out on the membrane the good stuff doesn't cut easily and has directional threads in it for strength.

Make sure tape all the edges rather than foam for gaps.

And make sure vents are open below

Water John

Depends on your budget but as you'll now have insulation under it, consider UFH.
You can hang metal heat spreaders between the joists and the UFH pipes run in grooves. Not cheap but it is a once off opportunity. Yeah go with the rigid insulation.

WildCardDoW

Water John wrote: »

Depends on your budget but as you'll now have insulation under it, consider UFH.
You can hang metal heat spreaders between the joists and the UFH pipes run in grooves. Not cheap but it is a once off opportunity. Yeah go with the rigid insulation.

Thanks John, we are also thinking of Air to Water (so another reason for good insulation) but my understanding is it can be tricky to meet the grant approval process for that. (Proving to the assessor that insulation meets required u values especially as doing self build so no architect working out u values and signing off on them etc.)

As part of doing this we will likely do walls at same time but was thinking of just straightforward aluminium rads. Cost is a factor (even self doing insulation is going to hit high amounts) and was able to get suitable ones for extremely cheap.

I'll look into underfloor costs but likely not a flier. Despite the nice clean look it offers.

WildCardDoW

listermint wrote: »

Tbh I'd go with rigid. Relatively easy install. Good values obtained for the depth. And then membrane over it. Try not skimp out on the membrane the good stuff doesn't cut easily and has directional threads in it for strength.

Make sure tape all the edges rather than foam for gaps.

And make sure vents are open below

Thanks lister, seems like the best option. Just perfecting it is the problem.

Any links to the membrane stuff? And the tapes to use - guessing it's special stuff to help avoid thermal bridges or escape points for heat?

Lumen

The membrane and tapes are for air tightness.

Typically you'd put the membrane over the joists/insulation and then tape or bond the perimeter to the wall, and then install an 18mm OSB subfloor before the final finished flooring.

The detailing of the membrane perimeter depends somewhat on the state of the wall. If it's bare block you can use primer or sealant to get a better bond.

Rows Grower

Lumen wrote: »

The membrane and tapes are for air tightness.

Typically you'd put the membrane over the joists/insulation and then tape or bond the perimeter to the wall, and then install an 18mm OSB subfloor before the final finished flooring.

The detailing of the membrane perimeter depends somewhat on the state of the wall. If it's bare block you can use primer or sealant to get a better bond.

You could staple the membrane to the joists too, you might have to add some noggins to the joists as well to eliminate movement on the floor which can be an issue even when they are covered afterwards with sheets of plywood or osb.

Water John

I thought your engineer, not architect, would be visiting regularly and inspecting also you'd be forwarding pics. He has to be able to sign that it meets building guidelines.

WildCardDoW

Water John wrote: »

I thought your engineer, not architect, would be visiting regularly and inspecting also you'd be forwarding pics. He has to be able to sign that it meets building guidelines.

We haven't started yet John, it's a technical assessor as part of the SEAI process to show improved BER. I'd record everything for him (u values as per product specification offered by installed material for the given floor area ratio) so he can confirm it meets the guidelines.

I'll see what he says with this - he might say he can't sign it off.

Biker1

You will also need delivery dockets for the insulation showing the house address, manufacturer, type and thickness of insulation supplied.

Water John

Be sure to take plenty pics at every stage, a bit of insurance.

WildCardDoW

Any recomendations on the material to use between insulation and 18mm flooring?

And as a breather membrane underneath the rigid / insulation?

For underneath was thinking this even though its for rooves:
https://www.goodwins.ie/products/Vent-3-Pro-Breather-Felt-50x1.5mtr-165gm-Heavy-Duty.html?filter_set[]=16,98,594

And something like this for over the top of insulation and joists:

https://www.goodwins.ie/products/siga-majpell-5-air-tightness-membrane-interior-50x1-5mt.html?filter_set[]=1413,1440

Lumen

I don't think you need a membrane under foil-faced solid insulation, that's more for rockwool or low density woodfibre that wind can blow through. You just need to keep it from dropping out the bottom of the joists, e.g. using battens.

Robin Clevett has just done a video on Gapotape which can be used to friction-fit solid foam insulation boards. It comes across a bit like a JML ad, and I'm quite dubious about the possible sponsorship, but it's nonetheless interesting.

I'm particularly dubious about it being airtight, so I'd also use a Siga/Intello/whatever membrane over the top, taped to the wall.

https://www.youtube.com/watch?v=Omk68Z-tE7U

listermint

Air tight is key here. Had parents do a garage renovation around 16 years back used all the top products related to insulation. But air tightness wasn't adhered to as I suppose it wasn't really the rage as such. The room was never really warm the suspended floor let cold air in via the vent . Kinda crap all round

Water John

Suspended floor has a poor rating on BER.

WildCardDoW

listermint wrote: »

Air tight is key here. Had parents do a garage renovation around 16 years back used all the top products related to insulation. But air tightness wasn't adhered to as I suppose it wasn't really the rage as such. The room was never really warm the suspended floor let cold air in via the vent . Kinda crap all round

This does seem to be huge which is why an airtight layer secured over the insulation seems key.

I do have someone pushing for spray foam to guarantee the airtightness (and it would be in these trays so not secured only to joists or bottom of OSB) but I'm concerned about the thermal performance this would offer. There's absolutely no info on u values offered by it at all.

My preference is actually METAC right now for cost effectiveness and the u value performance of it with an airtight membrane for my floor area is well within the guidelines: 0.23 v 0.34.

I've got a whole spreadsheet done on some different options if anyone wants a look haha. Basi ally just merged the u value info for a few different product types at different sizes into one but you can also update the area and permiter values so might be handy for others.

Lumen

WildCardDoW wrote: »

I do have someone pushing for spray foam to guarantee the airtightness

Spray foam insulation does NOT provide air tightness.

Ask the person for the NSAI or BBA certs proving air tightness specifically, in this application. They won't be able to.