Jon Weaver wrote:

Well, 25mm of insulation would be infinitely better than none. That would
allow 75mm of screed.

Christian.


Interesting.. I just spoke to my local(ish) supplier of insulation and
they can supply 40mm boards.. This will give me 60mm of screed, and if
I follow the next posters advise of using chicken wire as re-enforcing
material, its should give me a base which is still strong enough.

With regards to installation, is it as simple as cutting it to shape
and placing it on the floor (ontop of the DPC) before they lay the
screed? Should it go right upto the wall, or should I leave a gap??

I was surprised how much the insulation costs though (£16+vat for
1.2x24m boards).. I will make some enquiries over the next few days.


Make sure you are gettng polystrne foam - blue or pink generally - not
Cleotex. That price sounds about right for a 5 pack of 4x2 shhets.

Just slap it down, and you may, if its a huge area, want to lay some up
the sides of the screed as well to allow for expansion, but it shouldn't
be necessary on normal installations.


What I was quoted for was 40mm 'Polyurathane'. They quoted me £16+vat
for a 1.2 x 2.4 Mtr sheet, which seemed excessive.


Sounds like they are trying to sell you polyisocyanurate - Celotex.

The best value for money for screeds is blue or pink polystyrene. Its
hugely more dnes than teh whites tuff and is rally very strong - won't
compress as long as you wire the screed.




However, I am starting to think that this might be a bad idea.. One
thing which has bothered me from the start of this discussion is
strength.. A 50mm sand/cement screed ontop a soft (foam) base, doesn't
sound like it will have much local strength. I am worried that a few
months down the road, the screed could end up cracking, and it will
all be ruined.. Even if I added some re-enforcement in the screed, its
fact is that its on a soft, flexible foam base which worries me.



It may well crack a little as it shrinks anyway. I poured gallons of
thinned PVA down my cracks and they all shrank and glueed themsleves
back togeher. It wasnpt the fom so much as inconsistent mixes in teh
screed and doing it a little at a time.



I guess that I should have got the insulation put it weeks ago, when
the slab was being laid, but its too late to worry about that now.

I am not sure how much difference the lack of insulation will make,
but for peace of mind, I am thinking that it might be better to have a
cold but solid floor then a slightly warmer, but weaker one!


Put in teh insulation. Its gione with 75mm screed, Hobenets.