Insulating Suspended Wooden Floor or not advice needed!

dublinbay34 · 27-01-2021 11:19AM #1

Hi All,

I have gone down a rabbit hole of insulation and gotten lost and need some advice!

I have purchased a 1950's Semi-D BER Rating F. The rating is a little surprising as the house is in liveable condition, and has a new gas boiler (<5 years). We are doing mostly cosmetic renovations before moving in and considering any major works. These works involve replacing the carpet in the hall and 2 reception rooms with either a laminate or engineered wood floor.

My issue is the level of insulation that I should put in now as I am cognizant that once an engineered floor goes down it can't be lifted without damage. I have lifted the carpets and taken the skirting off and as expected there are a lot of draughts between the meeting of the walls.

My plan was to lay a small piece of plastic sheet between the floor and wall then lay the floor on top and place the skirting over and caulk which should block drafts coming from the skirting.

However I am wondering should I go to the effort of lifting all the floorboards and put in insulation between the joists? How much benefit it would give? The existing floor is in good condition and if I lay a tongue n groove engineered floor over it should be insulated enough? I wouldn't be able to do the lifting of the floorboards myself due to time so any idea of companies that do the full service and costs involved?

Lastly, if I am not undertaken works now to insulate should I just lay a laminate with a good underlay on top of the floor?

I also have a slight concern about laying a thick engineered floor ontop of the existing floor creating issues with levels into rooms that do not have the floor installed and doors etc.

Any advice/experience much appreciated

Calahonda52 · 27-01-2021 01:16PM

https://www.ecologicalbuildingsystems.com/post/best-practice-approach-insulating-suspended-timber-floors

do it once and right

https://www.nsai.ie/about/news/publication-of-sr-542014-code-of-practice

WildCardDoW · 27-01-2021 01:29PM

I had the same issue. I eventually went with the below:

Spray foam suspended on breathable vent cards, all supplied by the spray foam company. We also underlined the joists with the rigid boards at the recommendation of the SEAI Technical Advisor who said the spray foam will be a good choice (mentioning that to pre empt the doubters on it).

It probably isn't as good as doing the same with actual rigid insulation as per the link from ecological above.

Then you should buy the sealing tape or foam and complete your current plan:

https://www.mcmahons.ie/siga-rissan-100-adhesive-tape-100mm-x-25m-green

If you can do it you could also insulate the external facing walls or otherwise check if external or internal insulation is best for your house.

I'd assume it's solid block so I think an external wrap down the line should be good.

dublinbay34 · 27-01-2021 02:39PM

WildCardDoW wrote: »

I had the same issue. I eventually went with the below:

Spray foam suspended on breathable vent cards, all supplied by the spray foam company. We also underlined the joists with the rigid boards at the recommendation of the SEAI Technical Advisor who said the spray foam will be a good choice (mentioning that to pre empt the doubters on it).

It probably isn't as good as doing the same with actual rigid insulation as per the link from ecological above.

Then you should buy the sealing tape or foam and complete your current plan:

If you can do it you could also insulate the external facing walls or otherwise check if external or internal insulation is best for your house.

I'd assume it's solid block so I think an external wrap down the line should be good.

Thank you, I assume you had to lift all the floorboards to do the spray foam? I am struggling to find an insulating contractor in Dublin who will also lift and replace floorboards.

When you say complete my current plan with the tape, do you mean use is to tape the small plastic sheet to the wall and the floor?

dublinbay34 · 27-01-2021 02:46PM

Calahonda52 wrote: »

do it once and right

Thanks, I am struggling to find anycontractors that would do this in Dublin including removing and reinstalling the floors.

Calahonda52 · 27-01-2021 03:18PM

even harder to get someone who
a: understand the task
b: will execute according to plan and do it right

dublinbay34 · 27-01-2021 04:58PM

Calahonda52 wrote: »

even harder to get someone who
a: understand the task
b: will execute according to plan and do it right

Thanks for the links above that NSAI document is very useful. The eco homes looks a little complex to execute. I am seeing the benefit in doing it now, and taking the time to get it done.

I think roughly probably what I will do is lift the floors myself, get somebody in with experience to do the insulation involving a material between the joist plus a membrane above the top and relay the floors then put a laminate on top to keep the costs down.

WildCardDoW · 27-01-2021 05:02PM

dublinbay34 wrote: »

Thanks for the links above that NSAI document is very useful. The eco homes looks a little complex to execute. I am seeing the benefit in doing it now, and taking the time to get it done.

I think roughly probably what I will do is lift the floors myself, get somebody in with experience to do the insulation involving a material between the joist plus a membrane above the top and relay the floors then put a laminate on top to keep the costs down.

Yes, you'd need someone to lift the boards you could look at putting 18mm OSB down after and see if someone will do that if you do the lifting!

The general advice is a layer below the insulation (that's what the cards do in the system I had) and then another layer above. The layer above should be acting as the airtightness layer and the layer below should be a breathable membrane.

With the OSB you'd get away with your plan, with floorboards you would need to go over the full floor. That's just my opinion though so check with an expert if you are concerned.

mloc123 · 27-01-2021 07:49PM

I would consider cost versus return... in an F rated house, I think wall and roof insulation along with new glazing will have a much better return

Calahonda52 · 27-01-2021 07:53PM

mloc123 wrote: »

I would consider cost versus return... in an F rated house, I think wall and roof insulation along with new glazing will have a much better return

Not unless the floors are are least airtight

Pete_Cavan · 27-01-2021 07:57PM

Would you consider cutting out the joists and putting in a proper floor slab with insulation, radon barrier, etc? Could even have under floor heating.

listermint · 27-01-2021 08:03PM

mloc123 wrote: »

I would consider cost versus return... in an F rated house, I think wall and roof insulation along with new glazing will have a much better return

BER sucks as a system. Incremental changes like the OP is doing massively make a difference to comfort levels.

I'd take comfort levels over arbitrary numbers on a spreadsheet any day.

dublinbay34 · 27-01-2021 09:12PM

WildCardDoW wrote: »

Yes, you'd need someone to lift the boards you could look at putting 18mm OSB down after and see if someone will do that if you do the lifting!

The general advice is a layer below the insulation (that's what the cards do in the system I had) and then another layer above. The layer above should be acting as the airtightness layer and the layer below should be a breathable membrane.

With the OSB you'd get away with your plan, with floorboards you would need to go over the full floor. That's just my opinion though so check with an expert if you are concerned.

I have seen the bottom layer as netting with soft material on top or the kingspan rigid boards.

I think relaying the floorboards plus an underlay and the laminate should be very good as for some of the floors I am considering just using the original floor boards!

karlitob · 27-01-2021 11:49PM

listermint wrote: »

BER sucks as a system. Incremental changes like the OP is doing massively make a difference to comfort levels.

I'd take comfort levels over arbitrary numbers on a spreadsheet any day.

I’ve a similar set up as OP. Our builders put in foil backed insulation between the joists when we renovated 10 years ago, I presume OSB on top of that. I didn’t think at the time it was a great job and I’m not sure how good they were and putting it right up to exterior walls. Certainly no tape at the edges of the room.

I’ve notions of updating the insulation when I redo the carpet. But just wondering if there’s any point. Is a lot of money and effort for very little return?

Quiet Achiever · 22-07-2025 10:25AM

https://www.boards.ie/discussion/comment/116067418#Comment_116067418

Hi WildCardDow,

SoSorry for resurrecting an old thread but have found somone willing to insulate under suspended timber floors and he wants to do spray foam

There are many horror sories online but am i right in thining that as long as a breathable membrane is put under the joists that rotting should not be an issue? Or does it need to go over the joists and pushed down the cavity between?

WildCardDoW · 22-07-2025 10:18PM

The membrane card should act like a support for spray foam between joists.

This of it as a wide u and the upper edges nailed to joist, foam is sprayed into it.

The membrane is effectively a black card with air gaps if I remember right.

The rigid insulation is secured to the bottom would be similar what's attached to walls internally (the version with no plasterboard obviously).

The foam would need to be open cell, so you might want to get proof of what they're using such as a NSAI certificate for the product purchased.

Quiet Achiever · 23-07-2025 09:36AM

Thanks for taking the time to explain that, appreciate it and will pay it forward

GreeBo · 12-08-2025 12:50PM

https://www.boards.ie/discussion/comment/123661173#Comment_123661173

Do you have a link/picture that shows this?
I am struggling to see how it works as typically vent cards would be to ventilate the attic space while you insulate the ceiling below.

In this case if you ventilate the flooring, how are you also insulating the same flooring?

Grassy Knoll · 12-08-2025 01:07PM

Interesting thread, please OP post pics as the project continues, informative and interesting for non experts such as myself

GlennaMaddy · 13-08-2025 02:14PM

I got my suspended floor insulated like shown in this picture, albeit with foam rather than mineral wool. The work was part of a larger project by Crystal Complete Builders. It has made a big difference. The airtightness alone probably would have made a significant difference but the floor also feels warm on bare feet now compared to how it was. Sadly, my T&G floor boards ended up as firewood. Q-Bot UK make trips to Ireland once a year to do several jobs together, that's the only way to preserve existing floorboards IMO, unless you want to crawl under your own floor😀

WildCardDoW · 13-08-2025 08:36PM

https://www.boards.ie/discussion/comment/123716091#Comment_123716091

Here's one.

circadian · 13-08-2025 09:48PM

I done this a few years ago, I also ended up replacing the floorboards because they weren't in fantastic condition anyway.

https://rhwoodfloors.ie/suspended-floor-insulation/

Earthwool with membranes. I didn't want spray foam in the house, partly because I do like to be able to get into the floor to get to wiring/plumbing and I feel foam would be a pain to work with and also the horror stories of jobs done incorrectly with it.

GreeBo · 18-08-2025 02:52PM

https://www.boards.ie/discussion/comment/123720422#Comment_123720422

Ah ok, I get it now. It will significantly reduce the effectiveness of the insulation though.

WildCardDoW · 19-08-2025 01:01AM

https://www.boards.ie/discussion/comment/123733131#Comment_123733131

I've never noticed, the heating rarely goes on anyway.

To be clear the insulation sits on that, I suppose you're saying it's not air tight which makes sense.

But I'd imagine very little cold rises up through the floors. Would be interesting to compare to circadian's version which I guess I could have done.

Live and learn, won't be fixed for 20+ years now anyway.

GreeBo · 21-08-2025 11:34AM

https://www.boards.ie/discussion/comment/123734867#Comment_123734867

Its more that more of the joists are exposed to the cold, so you get more cold bridging.
Here is the equivalent showing it with studs in a timber framed house.

They will actually do a lot to reduce drafts (and wind washing), which is the primary source of heat loss anyway, hence you are still noticing the benefits!

WildCardDoW · 21-08-2025 10:45PM

https://www.boards.ie/discussion/comment/123741400#Comment_123741400

The joists had insulated board attached to the bottom, may help with cold bridging.

GreeBo · 22-08-2025 11:45AM

https://www.boards.ie/discussion/comment/123743274#Comment_123743274

Hmm, that would help alright, but would seem to negate the ventilation?

loyatemu · 22-08-2025 11:54AM

the method described above is what's recommended by the SEAI and the UK equivalent. Wood is a fairly good insulator and joists are 6 inches deep plus the depth of the floorboards and anything that's on top. I can't imagine it's a huge issue.

GreeBo · 28-08-2025 10:23PM

https://www.boards.ie/discussion/comment/123744299#Comment_123744299

Normally (or at least what I have seen) is that the cavity between the joists is fully insulated, either with PIR or fibre held in a net.
The concern was that in adding the vent cards, you reduce the overall insulation and extend the uninsulated part of the wood, essentially part of the floor is uninsulated (between the joist and the actual insulation), since by design they are adding ventilation to the area.

Wood is certainly better than concrete etc. but its still going to be worse that insulation and the vent cards increase the likelihood of drafts in the room above.

Insulating Suspended Wooden Floor or not advice needed!

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