Author Topic: Insulating the under floor (Read 1744 times)

Russell · « **on:** 24 January, 2024, 06:27:44 pm »

We have an ideal opportuntiy in my daughter's 1930s maisonette to insulate the suspended floor in one room as we are lifting the floorboards to investigate/repair historical worm damage. I have decided to use breathable membrane to support Rockwool bats between the joists as the most straight forward and cheapest solution.* I saw a YouTuber do this and he then put damp proof membrane (in his case a pond liner) on top of the joists and insulation. Is this necessary and/or wise? I know that just because he has a YouTube channel does not make him an expert hence I am asking for opinions from the worldly wise.

* an alternative would be solid PIR foamboard but this needs battons to support them.

Wowbagger · « **Reply #1 on:** 24 January, 2024, 06:32:27 pm »

My pal Jon Fuller did this a while back and I think he made a series of videos about it. It wasn't his that you watched, was it?

Russell · « **Reply #2 on:** 24 January, 2024, 06:35:16 pm »

No, don't think so, it was an Ali someone. I'll do a search, thanks.

Wowbagger · « **Reply #3 on:** 24 January, 2024, 06:37:31 pm »

I thnk he probably put them on facebook, which means that they are now at the bottom of a midden heap of cat photos. He also has two cats.

Canardly · « **Reply #4 on:** 24 January, 2024, 07:20:18 pm »

Polar Bear did this some time ago. Not sure he filmed it though. Perhaps some photos and issues?

Asterix, the former Gaul. · « **Reply #5 on:** 24 January, 2024, 08:01:32 pm »

When I did a similar job in France I didn't bother with damp proof membranes. I thought that sub-floor ventilation was more important. It worked ok and the Limousin can be a very damp place!

With a concrete floor I certainly did use a membrane.

Russell · « **Reply #6 on:** 24 January, 2024, 09:00:48 pm »

Quote from: Asterix, the former Gaul. on 24 January, 2024, 08:01:32 pm

...... and the Limousin can be a very damp place!

Oh we know all about the weather in Limousin!

Russell · « **Reply #7 on:** 24 January, 2024, 09:03:10 pm »

Found a thread from 2021 and Aidan posted a link which I'll read through.

mrcharly-YHT · « **Reply #8 on:** 25 January, 2024, 07:46:18 am »

I've insulated a wooden floor from underneath, and did a lot of research first.

Any DPM in contact with wood is a bad idea. Apart from the risk of condensation, imagine what happens to liquid spills on the floor - they will just pool on the DPM, causing rot.

I don't think you need a (breathable) membrane above the rockwool, but one below is wise. There is a sticky tape stuff you can use to join the strips of breathable membrane.

De Sisti · « **Reply #9 on:** 25 January, 2024, 08:31:29 am »

Quote from: Russell on 24 January, 2024, 09:00:48 pm

Oh we know all about the weather in Limousin!

Do we?

Polar Bear · « **Reply #10 on:** 25 January, 2024, 09:31:11 am »

Beneath the ground floor of this 1870's terrace is a void of more than 1 metre with air bricks front and back of the property. I have insulated beneath 2/3rds of this by using battens supporting PIR. The PIR has foil on the main surfaces, not the edges. I actually didn't he first room from beneath the floor working in the void. Saved pulling up a perfectly good floor.

I spoke to a number of people and did quite a bit of research and decided to not use vapour barriers. So far as I know after 8 years there are no problems, yet.

The biggest issue was cutting the PIR for fitting between joists on such an old property. They are as you might imagine spaced irregularly and not one is actually straight.

When I do the final area I shall cut leaving a small gap and use foam expanding tape designed for this job which you stick around the edge, slide the slab into place and within minutes the tape expands to fill the nooks and crannies. It was quite a job making the slabs a tight fit when we last did the job.

paton · « **Reply #11 on:** 25 January, 2024, 09:43:28 am »

Someone such as Roger Bisby might be able to give some advice
https://skill-builder.uk/send

https://www.youtube.com/watch?v=SkF1U4vrlhU

https://www.youtube.com/watch?v=cqZYMxE0ETI

Robin Clevett like PIR / PUR and Gapotape

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

tom_e · « **Reply #12 on:** 25 January, 2024, 10:06:21 am »

My understanding is that you want to minimise the leakage of warm moist air from the living space into the colder back part of the insulation. Otherwise if the lower part of the insulation encounters room air with a dew point higher than the temperature of that outer part of insulation, then you will get dew.

You definitely do want the space under the floor ventilated though. If this is decent and the leakage of air from above is minimal, I think it will be fine whatever. Because any moisture will be transported out from below - that is, after all, the point of your breathable membrane.

I'm planning to do similarly for one floor in ours, but at it is probably a project for 2025 rather than 2024.

paton · « **Reply #13 on:** 25 January, 2024, 10:13:00 am »

multifoil insulation has some advantages, being easy to fit.

https://www.superfoil.co.uk/floor/

https://www.youtube.com/watch?v=ql_I3m-xbNU

HTFB · « **Reply #14 on:** 25 January, 2024, 10:47:59 am »

Our buildings engineers are quite keen on the QBot which trundles about under your joists spraying insulation in, without lifting the boards. The implication is that there isn't any membrane involved.

mrcharly-YHT · « **Reply #15 on:** 25 January, 2024, 11:24:42 am »

Quote from: HTFB on 25 January, 2024, 10:47:59 am

Our buildings engineers are quite keen on the QBot which trundles about under your joists spraying insulation in, without lifting the boards. The implication is that there isn't any membrane involved.

That was touted as good roofing insulation solution, but now is frowned upon as it encourages rot.

paton · « **Reply #16 on:** 25 January, 2024, 01:37:54 pm »

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/898872/suspended-timber-floors-underfloor-insulation-best-practice.pdf

Joe.B · « **Reply #17 on:** 08 February, 2024, 03:02:01 pm »

I’ve insulated most of our downstairs floors, just one room left to do.
When I started at the back of the house I used 150mm Celotex boards tightly fitted between the joists and sealed with foil tape. When doing the Hall and Living Room a couple of years later I hung a breathable membrane between the joists, filled it with RockWool, and then put another layer of membrane over the top. Much better breathability than my earlier method.

Russell · « **Reply #18 on:** 02 April, 2024, 05:48:30 pm »

You might or might not like to see how we've been getting on with the insulation.

First issue to deal with was an airbrick 1 course of bricks higher than the others so it was between the joists rather than under them. So I had to make a duct to divert the air behind the well perforated sleeper wall so that the insulation could be laid over it

IMG_20240321_122158817 by Russell Wiles, on Flickr

Wood fibre insulation on top of breathable membrane. Central heating pipes not insulated here as at this point they transition from under the joists to between them so will be inside the insulation. They are actually sitting on the sleeper wall here.

IMG_20240322_124456592 by Russell Wiles, on Flickr

Finished laying insultion. Bulging joists show where we twinned up new joists after removing wormy old ones. Rough looking area of insulation was using off cuts and to pack around the central heating pipes.

IMG_20240323_171011852 by Russell Wiles, on Flickr

Finished with vapour control membrane. Next job new floorboards.

IMG_20240324_115315964 by Russell Wiles, on Flickr

mrcharly-YHT · « **Reply #19 on:** 03 April, 2024, 08:08:56 am »

Looks very tidy

Polar Bear · « **Reply #20 on:** 03 April, 2024, 12:30:54 pm »

Excellent job.

What are you using for floorboards? Just interested.

Joe.B · « **Reply #21 on:** 03 April, 2024, 06:14:10 pm »

Nice job, looks exactly like the way I did our living room & hall

Russell · « **Reply #22 on:** 04 April, 2024, 07:42:23 pm »

Quote from: Polar Bear on 03 April, 2024, 12:30:54 pm

Excellent job.

What are you using for floorboards? Just interested.

PAR Redwood Boards 150 x 25mm (6" x 1") NOM PEFC from Selco.

The existing boards are square edged so am copying that as the will meet in the hallway.

Russell · « **Reply #23 on:** 17 April, 2024, 07:34:38 pm »

Floor now finished, planed and sanded to 120 ready for staining and varnish.

IMG_20240417_092817557 by Russell Wiles, on Flickr

IMG_20240417_092805999 by Russell Wiles, on Flickr

Canardly · « **Reply #24 on:** 17 April, 2024, 07:49:36 pm »

Nicely done.