Underfloor insulation

ccluedo

Which is the best wool type insulation for under suspended timber flooring pls? Google suggests the same stuff as for lofts?  Is 200mm the recommended thickness? Is there a recommended type of netting or is garden type netting adequate.  I have read that a vapour control layer is recommended if insulating from above but again there seems to be so many different options on that too leaving me unsure which to choose.
Thanks for any help.

Jeepers_Creepers

Do you have good access from below? Or will you be lifting the floor?

FreeBear

Not much to choose from between mineral wool or fibreglass - Both will be cheaper than hemp or sheeps wool.

If you have good access from below, ordinary chicken wire netting will be fine. Use a staple gun to fix it to the joists.

You might want to consider using Celotex/Kingspan type boards between the joists (depending on ventilation) - 100mm will give you the same thermal performance as 200mm of fibreglass.

ccluedo

Jeepers_Creepers said:

Do you have good access from below? Or will you be lifting the floor?

Will be doing it from above as replacing chipboard flooring.  Not a lot of crawl space underneath anyway so probably just as well.

Jeepers_Creepers

That's ideal then!

I would personally go 'rigid' as it's ultimately a lot easier to work with, and also has far better U value.

How much of it to fit is up to you. As long as you also keep your floor draught-proof (the main reason why floors make your house cold), then the actual heat loss through floors is far less than that through walls and ceilings.

This is what I would personally do - your call whether to ignore it

50mm Celotex/Kingspan-type rigid insulation. Two reasons why this thickness; one, it's easy to cut using a snap-off bladed craft knife or similar (you don't want to be sawing it as it's dusty and 'orrible), and - two - going any thicker is a relative waste of time. Ie, 100mm will make only the teeniest improvement over 50mm in the floor.

Trim the sheets to fit fairly snugly so's they (a) stay in place and (b) make a decent draught-proof seal - very important with T&G floorboards, but pretty important for chipboard flooring too. But also run a bead of polyurethane adhesive (foaming/gap-filling type) along each edge before fitting them. You can get large squeezy bottles of the adhesive used to join chipboard flooring together, for example (which it sounds like you'll be using anyway) - this is ideal. Tamp the insulation strips down from above using a long batten across the joists that'll get them down there until they are flush with the joist tops. The adhesive will 'foam' and fill any remaining voids - and the stuff used for chipboard flooring is easy to trim off afterwards, should it rise above joist height (which some of it will...).

Finally, either cut teeny strips to fit between the joists and walls, or else fire expanding foam down there - draughts coming up around the floor edges and under the skirtings are notorious.

ccluedo

Thank you JC and FB, much appreciated info.
Would we still need a VCL if we went down the rigid route? What holds the rigid up -  I'm sure a perfect fit will be a pipe dream with 1970's joists   Batons seem like more work and expense than netting.

grumbler

Jeepers_Creepers said:

 100mm will make only the teeniest improvement over 50mm in the floor.

You keep saying this again and again, but I don't understand what it means.

For 100mm the heat resistance is twice as much as of 50mm. The conductivity is twice as low. The heat flow/loss for the same temperature drop is twice as low.

ccluedo

grumbler said:

Jeepers_Creepers said:

 100mm will make only the teeniest improvement over 50mm in the floor.

You keep saying this again and again, but I don't understand what it means.

For 100mm the heat resistance is twice as much as of 50mm. The conductivity is twice as low. The heat flow/loss for the same temperature drop is twice as low.

I read JC comment as it's not so much the insulation per se that helps, more the tight fitting of it and the filling of any gaps no matter how thin. 

Jeepers_Creepers

ccluedo said:

Thank you JC and FB, much appreciated info.
Would we still need a VCL if we went down the rigid route? What holds the rigid up -  I'm sure a perfect fit will be a pipe dream with 1970's joists   Batons seem like more work and expense than netting.

Tsk... You cut them as snugly as possible whilst still making life easy for yourself - ie don't be fixated in getting it perfectly tight - and then run a bead of foaming poly adhesive along the edges that butt the joists, and butt the next sheet. The adhesive will foam, gap-fill, hold it securely in place, and insulate any teeny gaps.

My comments on the amount of insulation required is based on two things - common sense and practice.

Think it through - a single block wall will feel very cold and will draw heat out of a room. An example is the attached garage on our house where I am now sitting. Adding even one inch of high-quality insulation - eg Celotex - will transform the walls; they will now actually feel warm to the touch, and also dramatically reduce heat flow. How much better than a bare wall? I dunno - let's call it 'ten times better'. (It'll be that at least...)

Ok, now add a further inch of Celotex. How much better is this? Twice? No. Why? Because the first inch has reduced the heat loss by (say) that factor of 10 we mentioned. Ie, with ONE inch, only a tenth of the heat is now escaping through that wall. So, now tell me how much more heat loss the second inch is providing? Yup - a tenth of a tenth. A oneth extra.

The garage I mentioned has 2" of Jablite (that's the cheap, low U-value, white expanded polystyrene sheet) insulation under the floorboards, 1" Jablite on the walls, and is over-boarded with 35mm insulated plasterboard - so only 25mm of decent insulation over the cheap Jablite on the walls. I also covered the ceiling with Celotex - can't remember how much, but it was around 2" I think.

Why so little on the walls - 1" Jab and 1" Celo? Because garage internal width was an issue. Does it work? Yes - I can only wear a T-shirt and shorts (I am not kidding) due to the heat given off by just the plasma screen I use as a monitor. That's around 200 watts - and it's too warm.

It is, in fact, the warmest room in the house (primarily because it also ain't draughty...)

How much better would 4" of Celotex have been on my walls? Dunno - but I know I wouldn't notice the difference.

Anyhoo, floors are the least source of actual heat transfer, simply because it's at the coldest part of the room. Ergo, even 50mm Celo would transform this.

Will you need a VCL? I don't think so - the Celo itself is foil-backed on both sides. Moisture ain't going to want to diffuse down that way.

grumbler

Jeepers_Creepers said:

ccluedo said:

Thank you JC and FB, much appreciated info.
Would we still need a VCL if we went down the rigid route? What holds the rigid up -  I'm sure a perfect fit will be a pipe dream with 1970's joists   Batons seem like more work and expense than netting.

Think it through - a single block wall will feel very cold and will draw heat out of a room. An example is the attached garage on our house where I am now sitting. Adding even one inch of high-quality insulation - eg Celotex - will transform the walls; they will now actually feel warm to the touch, and also dramatically reduce heat flow. How much better than a bare wall? I dunno - let's call it 'ten times better'. (It'll be that at least...)

 AFAIK, about 8 - still dramatically.

Ok, now add a further inch of Celotex. How much better is this? Twice? No. Why? Because the first inch has reduced the heat loss by (say) that factor of 10 we mentioned. Ie, with ONE inch, only a tenth of the heat is now escaping through that wall. So, now tell me how much more heat loss the second inch is providing? Yup - a tenth of a tenth. A oneth extra.

Yes, this I agree with.  Extra 1 inch will reduce the loss 'just' by a factor of ~2 compared to the first 1 inch. Not as dramatical, but not "teeniest improvement" in terms of the heating bill.

Now a see what you mean.

theoretica

If you are going for a solid insulation be careful that you don't set up a microclimate with trapped moisture against your wood floor.  A relative had the wrong sort or wrong installation of insulation lead to rotting.