Suspended Floor Insulation Options - Ground Floor

[GeddyMortgage]

Hi all,

Very recently moved into a house built in 1955. Previous owner made me aware of an access hatch to get into the crawl space beneath the suspended ground floor. On inspection, fortunately no dead bodies....but did immediately notice there is absolutely no insulation down there at all. I'm assuming this will be losing a notable amount of heat efficiency here with all the drafts from the airbricks coming up through the floor boards and carpet.

So naturally looking into the best bang for buck for suspended ground floor insulation options, and had a few questions for anyone in the know and would be happy to advise :-

:- As the crawl space is 2.5ft-3ft deep or so, im assuming in this case its generally better to do the insulation from below?
:- When going from below is it best to use insulation roll or board?
:- Ive read if using roll generally 200mm is needed, board generally 100mm, is this correct?
:- For either roll or board, should there be an air gap between the floorboards and insulation, if so how much?
:- If using board, do you seal the gaps either side to stop drafts etc, with tape? or expanding foam? or what?
:- I've read mixed things about a vapour layer, is this necessary given the situation above?

thanks so much, really appreciate any guidance.

Floor plan attached (X marks the crawl space access hatch)

 

[oldbutnotdead]

Best to use rockwool (roll or slab)- unless you lift all the floorboards, getting slabs of pir through the floor trap will be tricky plus you'll have to cut the slabs accurately to size, seal any gaps etc etc. 100mm slab used to be minimum for refurb, think that's up to 120 now so 240 soft (go for 300 if you have the room).
No you don't need air gap between floorboards and insulation.
Soft can support itself to an extent- working from below, wedge your lengths of soft between the joists then support it with chicken wire or strawberry net or lots of polypropylene string stapled under the joists or lengths of slating batten under the joists

 

[GeddyMortgage]

thanks for the response,

Yeah logistically that’s a really fair point. I’ve not yet measured the width between the joists. If that’s wider than the access hatch I guess that kills the board idea dead…

I’d initially wanted to entertain the board idea as it does seem to be more efficient heat and insulation wise (and way more expensive)… Id have preferred it despite the price..but not at the behest of having to lift the floor boards.

Re: logistical nightmare, I’d have the same issue with Rockwool slabs wouldn’t I? Or have I misunderstood you somewhere there?

Roll does seem cheaper. With roll do I need to be aware of any issues with damp? Or damp vapour layers etc?

 

[oldbutnotdead]

Rockwool slabs can be bent and folded a bit to wriggle them into a tight corner.
Cost benefit- the PIR has better thermal performance per millimetre thickness but (last time I priced it) the cost of 1 cubic metre of pir (which then would have done 10 square metres) was much higher than the cost of 2 cubic metres of either rockwool slabs or roll loft insulation (which would cover the 10 sq m). So if you have the space to use soft you get the same or better thermal performance for less capital cost (cutting PIR to size especially for older houses where nothing is actually square is a pain, you have to be millimetre accurate).
Damp- long as your subfloor is well ventilated it won't be an issue.

 

[Notch7]

actis hybris is easy to cut and fit and no dust or fibres.

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[JohnD]

There is not very much loss through floors, unless you have bare boards. It is mostly draughts, coming up through the gap under the skirting.

Obviously there is no loss through convection, and precious little through radiation or conduction

Mineral wool is very good for this as it is easy to stuff into irregular gaps with no precision cutting needed.

A thickness equal to, or a lttle greater than, joist depth is a convenient size

Clean it down there, remove any organic material, open up airbricks, and take care not to obstruct airflow.


BTW I strongly recommend mineral wool treated with Ecose, which does not shed irritant dust and fibres. It is marked on the packaging.

 

[GeddyMortgage]

Very good to read the different opinions and approaches. Thanks.

Im taking a look at the options for mineral/rock wool now and the prices do seem to vary massively even though the thermal resistance specs appear to be the same....

For example:-

ISOVER 200mm insulation roll - area per roll 6.03m2 / £5 per m2:- - ISO 200mm
KNAUF 200mm loft roll - area per roll 6.84m2 / £5 per m2 :- KNAUF 200mm
Rockwool thermal insulation roll - area per roll 3.3m2 / £16m2 - Rockwool 200mm

Why's there such a difference in price on those above? Am I looking at the right stuff here or have I missed something really obvious? As far as I can see they all have the same thermal properties advertised.


Also comparatively, Celotex GA4000 PIR insulation board comes out about £14m2 for the 100mm stuff. PIR 100mm Celotex

 

[JohnD]

I usually get Knauf, as they make the Ecose one, though it is also sold under other brand names at the big chains.


Knauf Insulation Super Top Up 200mm Loft Roll - 5.61m (1217) £25 £4.45 per m2

Knauf 200mm Insulation Super Top Up Loft Roll - 5.61m2 | Wickes.co.uk

Knauf 200mm Insulation Super Top Up Loft Roll - 5.61m2

www.wickes.co.uk

It's possible the high price you saw is due to shortages as winter comes and the cold weather starts. Check if the price includes VAT and if you can collect or have to pay delivery.

Summer is the best time to buy insulation.

 

[oldbutnotdead]

Yeah, so PIR is still nearly 3 times the price of loft roll per sq m of thermal equivalence. Have a look at the 2 cheaper roll types, check for that antidust agent, also see if the precut widths are different (probably not, think they're all 400/400/400 or 600/600, see which suits your joist centres best). Price differences- branding mostly.

 

[GeddyMortgage]

Yeah, thats what confused me actually..... the PIR is thrice the price of the cheaper of the loft rolls. But still cheaper than the rockwool loft roll.

Looking a little closer at it, the KNAUF and ISOVER are both labelled as glass mineral wool.... the Rockwool is just mineral wool. I wonder if thats got something to do with the huge difference in price point.

That said - the KNAUF was the one I was leaning towards anyway.

For installation from beneath, its pack it into the joist cavities and staple chicken wire to the base of the joists to hold it in? is it really that primitive ha-ha?

 

[oldbutnotdead]

The rockwool might be a bit denser- which is a bonus if you need sound insulation but not enough of a bonus to justify the price (bought some in the summer, think it was about £7 per sq m then). And yes that's the plan. Don't compress it vertically, having the lengths a bit fat so they fit snug left and right to the joists is fine. If joist depth is less than 200 then either make chicken wire baggy or put some blocks under the joists (again so you're not compressing the stuff too much).

 

[JohnD]

It won't matter if you compress it a bit

If the wool is nominally 200mm thick, and your joists are 7 inch, the chicken wire might compress it to a bit over 7 inches. It will block draughts and the difference in insulation will be imperceptible.

 

[GeddyMortgage]

Good to know - what do folks generally do for stopping drafts coming up through the skirting board gaps? Will this insulation method address this too or is this separately dealt with?

 

[JohnD]

They will be blocked if you take care to stuff the wool tightly into the gaps round the edges of the room, both between the joists and against the wall where end-on, and between the last joist and the wall where parallel. Brush out any dirt or rubble from the wall and joist, it may hold damp.

 

[GeddyMortgage]

Great, will go and buy some of that knauf stuff today.

Will any of the pipe work need lagging down there? It’s mainly copper and some JG speedfit.