insulation under screed - best insulation ?

[edward]

Hi one and all, I built an extension and now need to screed the floor.

There is 135mm to play with between the old floor line and the current concrete slab top.

I was planning on insulating above the slab and under the screed. (No underfloor heating, just hoping to improve insulation and maybe gain a bit of heat from passive solar gain) The room is a kitchen and measures about 4.7m by 5m. The walls are block with a 70mm plus cavity (fullfill eventually) and there is a floor above - ie two storey extension).

I was planning on putting 70mm of insulation in and a 65mm screed but have just had a crisis of indecision since getting quotes from the local builder.

They recommend Kingspan TP10. I thought that this was for roofing and having used it in the roof I know that it is pretty weak and I would have thought not ideal in compression.

They have given me prices for the following:

Kingspan Kooltherm K3 (50mm) Jabfloor 70 (50mm) Kinspan TP10 (70mm)

I haven't seen the K3 but from the online literature from KSpan it looks like I can use K3 or TF70.

Is TP10 the wrong material ?

Whatever insulation I end up using I was hoping to avoid cracking by using mesh. I was going to add some wrapping mesh (D49) or chicken wire at the bottom of the screed.

However I read that it is a good idea to stop cold bridging by using

20 to 25mm of perimeter insulation.

I would have thought that this will lead to more compression of the insulation ? THen you have a floating layer of screed with insulation all around.

I want to lay tiles and don't want to find a floor cracking and sinking after a few years....

Any thoughts on this ?! Help ! What is the standard way of doing this ?

Thanks for any help. Ed

[mail]

Been there done this TP10 is exactly the same material as the floor stuff They just market it with a different name I have put 75 mm of screed on mine no problems If you work out the fact that it is not a point load and look at the compressive strength you will cease to worry

I would put > Hi one and all,

Tables say you can get away with 75 mm here I personally put in 100 mm

Use Fibromesh in the screed

Not in compression - bending maybe

and I would have thought not ideal in

Look it up it is quite good

no

Good idea as well as the fibromesh just make sure that it is well covered and does not rust and put in a decent water proofer

yes see above

how so?? there is very little load sideways

THen you have a floating layer of screed with insulation

that's the ticket

It wont

[dg]

You need at least 70mm screed, with chicken wire.

For insulation, any PIR type (ie celotex or Kingspan) with a silver foil backing will do. They will perform more or less the same.

I'm not sure that the Jablite product is as thermally good as the PIR foams

They are designed to take the weight of floors, no problem

dg

[edward]

Thanks for the speedy responses - much appreciated.

So I have a max of 135mm between floors and you are recommending a

70mm min screed which leaves me with the following options:

70mm screed --> 65mm insulation

75mm screed --> 60mm insulation 80mm screed --> 55mm insulation 85mm screed --> 50mm insulation

I guess I can look for some 65mm or 60mm insulation but doubt I will find it.

Am I ok with 50mm insulation ? (Regs are a bit different here in Channel Islands to UK so I am not required to put any in - I just wanted some to take the chill off a bit).

I'll use some offcuts to go round the perimeter but just cut them to

20mm and go full screed depth. Is there a max on this perimeter insulation? ie would you gain by having it 50mm? Or is this determined by the plaster and skirting board width/depth? ie a limitation of about 20mm.

As for the fibromesh - haven't heard of this so will have to investigate locally.

Chicken mesh or a thicker gauge mesh? Local builders don't sell D49 (2.5mm bar with 50mm squares) but do sell either chicken mesh rolls or A142 (6mm bar and 200mm squares)

Was in two minds whether to screed with a mate (and get mix delivered by big mixer or dumped into the back of his truck - he is a stone mason and has done floors before). Or to pay some plasterers to do it.

I understand the wood and bay approach but am not convinced I can get a decent floor with no dips or hills. Did the slab and blocking previously myself.

Thanks for any help. Ed

[dg]

Chicken wire is fine, anything more is overkill for this situation.

Also 50mm insulation is fine. On the basis that heat goes up, and there is quite a bit of mass below the floor, heat loss downwards is not that great.

Is a bit of a placebo, in that you will 'feel' warmer if you know some insulation is in, and warmer still the more you put in. But in real terms the difference is not that much.

dg

[mail]

Dear Ed About 10% of the heat loss of an average uninsulated house goes through the floor so the other poster is quite right that the gain is small but as we find the cost of energy going up and up and you are not going to be re doing this and the building is to last at least 100 years design life it would be good for you and those that follow you to put in more rather than less. This is a personal view and dependant on the prediction that energy costs will continue to rise exponentially - any normal payback calculation would probably opt for lesser amounts. Fibromesh is a white cottonwool-like / fluffy material that you mix with the dry pack and comes in 900g bags and acts like the horse hair in old fasioned coarse stuff (lime putty/sand mixtures for plastering)

- it is a binding agent providing tensile strength. It is not that expensive but you may have to get it posted over. Keep the edge insulation to 20/25mm - just covered by the plaster (but leave a gap vertically! - and the skirting. Any more is a waste of time and has practical problems you have spotted. Pre mixed screeds come with the properly formualted fibromesh but normally come in minimum quantities of 4 tonnes I doubt you could do that yourself in a day! remember you have to get it off the lorry onto a platform/tarpaulin etc outside your house, barrow it in, lay screeds (dont forget the roof battens for this) to correct levels - fill in between and it is quite a big area. I reckon you will need the two of you as plasterers and a couple of guys to barrow it in for you to do it in a day Chris G

[Tony Bryer]

If the Jablite product is expanded polystyrene, it's not: you need 80mm of EPS to do the same as 50mm of PIR/polyurethane board.

[edward]

Thanks for all the info - much appreciated.

I found out the cost of screed from the one and only supplier here.

=A366 a ton and an extra fiver a ton for adding fibromesh. Delivery is =A326 an hour with no minimum order.

I was thinking of putting in 50mm of Kingspan K3 (will cost me =A3139 for enough to do 25m square - this is a quid more than jablite and the same price as TP10 but K3 seems to have a slightly better U-value than both these. Says it is a different material than TP10 in the Kingspan details) which will give me a screed of 85mm (fibromesh and chicken wire).

Not sure how many tons to order ?!

25m square by 0.085m is 2.125m cubed.

Not sure how heavy screed is per metre cubed... bit over a ton ? So I need 2.5 tons ?

I ordered concrete for the foundations and slab last year but have lost all the paper work. Doh.

THink I shifted 8 tons of concrete for the slab in 45 mins with a mate last time but know that screeding is a bit more precise so will try and get at least three of us.

How long do you have from unloading to the screed going off? THink they use LaFarge cement over here which goes off a bit faster than Portland blue circle.

Cheers for any advice - think I am getting there ! Ed

[The Natural Philosopher]

about 2.5ton a cube..

At this time of year outside, 24 hours.