Green Building Forum - UFH - thick insulation and no screed?

UFH - thick insulation and no screed?

dnfh — Tue, 19 Feb 2008 10:07:37 +0000

This may be daft - if so apologies!

We plan to fit low temp UFH in our house which has a screeded floor, but minimal insulation. SO normally we would dig up the screed, fit insulation, lay the pipes and put new screed back on top. I am told we have to have the recommended 50mm screed as a minimum to prevent cracking of the screed, but why not do without it?

We will be having a solid wood floor (reclaimed planks). Is there any reason we actually have to have the screed? Could we just have much thicker insulation and embed the pipes in that, possibly with some thinish layer on top? Heat loss would be minimised and response time improved as far as I can see.
People's weight would be distributed over quite a large area by the planks, and the UFH tubes are full of water anyway. Wouldn't it just feel like a sprung dance floor?

[I appreciate there can be merits to thermal mass in the floor, but for controllability etc I lean towards speed of response.]

Crazy, wrong or just weird?

UFH - thick insulation and no screed?

Mike George — Tue, 19 Feb 2008 10:24:20 +0000

Not an expert on UFH at all, though I have screeded over it many times. The minimum thickness I have seen specified is 65mm, not 50mm. For some systems the spec is 75mm, and often reinforced [fibamix]. I have also seen cracking in screeds which are too thin.

Can you do without it? I don't know but I imagine you can get weyrock flooring with PUR or EPS laminated to it. Others here will no doubt clarify.

UFH - thick insulation and no screed?

mark_s — Tue, 19 Feb 2008 11:19:46 +0000

I thought the point was to give even heat across the surface. Without that you are directly heating your floor with the loops of the pipe. Does that not cause problems with expansion/splitting or whatever?

UFH - thick insulation and no screed?

Ed Davies — Tue, 19 Feb 2008 11:22:39 +0000

I thought UFH under wooden floors on joists was quite common, using aluminium heat spreaders but without screed and with insulation between (any maybe below) the joists. Am I wrong?

UFH - thick insulation and no screed?

TheDoctor — Tue, 19 Feb 2008 11:40:38 +0000

it is quite common - more often used upstairs between existing joists

yes, it will heat up quicker
it will cool down quicker

the screed gives you the thermal mass which then regulates heat more evenly over the day.

look at the systems designed for suspended timber floors, and modify it accordingly.

I am going for the full 140mm buildup

70 insulation
70 screed

UFH - thick insulation and no screed?

caliwag — Tue, 19 Feb 2008 11:52:42 +0000

http://www.ebuild.co.uk/forums/messages/3322/3322.html?1203364881

Pretty good ongoing thread on Ebuild

UFH - thick insulation and no screed?

dnfh — Tue, 19 Feb 2008 17:10:13 +0000

My problem is that the current floor is (probably, not sure if they did what they had permission to do 10 years ago) only 75mm of screen on a DPC, and it will cause problems if we go higher (and cost a fortune to go lower). So I was trying to see if I could put more insulation under to maximise efficiency, at the cost of sacrificing some thermal mass.

If I'm not worried about screed cracking (should I be?), are there any data on how much (or rather how little) screed would provide useful evening of the heating? I suspect that if run @ 35oC the gradients shouldn't be that large?

UFH - thick insulation and no screed?

dnfh — Tue, 19 Feb 2008 17:31:11 +0000

caliwag:
href="http://www.ebuild.co.uk/forums/messages/3322/3322.html?1203364881"
Pretty good ongoing thread on Ebuild

That link takes me to the main page. Could you point me to which of the subpages is talking about this (they're all interesting, but I haven't found it yet!)

UFH - thick insulation and no screed?

tony — Tue, 19 Feb 2008 18:32:51 +0000

The bottom line answer to you question is, yes it will work without screed as you are not concerned about cracking because you are putting down a wood floor.

You need to sure of the dryness and quality of the wood and how you are going to fix it as these will be your big problems.

I would strongly recommend perimeter insulation and the more the better.

UFH - thick insulation and no screed?

CWatters — Tue, 19 Feb 2008 22:39:10 +0000

I have the following floor condsruction on my new build...

21mm T&G Engineered oak
Insulation with UFH between joists/battens
Beam and block floor
Void.

We used Osma profile insulation with metal heat spreader plates. Works well.

The joists/battens are same depth as the insulation and are for two purposes as I understand it...

1) To fix the wood floor to (eg secret nail)
2) To stop the insulation carrying point loads. I gather this is important in high traffic areas shuch as doorways or when thinner floor boards are used. Thin floor boards flex and don't spread the load as expected. The repeated compression can cause some insulation to breakdown.

UFH - thick insulation and no screed?

CWatters — Tue, 19 Feb 2008 22:40:58 +0000

I forgot to add that we also have areas that are screeded and tiled. The above applies only where we have a wood floor.

UFH - thick insulation and no screed?

caliwag — Wed, 20 Feb 2008 10:21:05 +0000

Sorry, about as far as I can narrow it down...a longrunning thread.

http://www.ebuild.co.uk/forums/messages/3322/3324.html?1203500185

UFH - thick insulation and no screed?

dnfh — Wed, 20 Feb 2008 16:52:33 +0000

tony
I would strongly recommend perimeter insulation and the more the better.

I have read the perimeter insulation should always come up above the screed. Should it be level with the final floor / above it / just under?
If using OSMA it seems to be c. 40mm XPS (with pipes buried in it). Are you suggesting it should be more than that? I believe the reduction in heat loss with insulation is a decreasing exponential?

UFH - thick insulation and no screed?

tony — Wed, 20 Feb 2008 17:11:10 +0000

It is but is not the same for all places on the floor. Huge losses from the perimeter reducing on something like an exponential inverse cube law towards the middle.

Most peoples idea of perimeter insulation is a diddly bit round the walls 10, 12 15 or 20mm stopping at floor level. insignificant?

My ideas are far more grandiose -- 200 or 300mm in the cavity going down below ground 1m or 2m ! or hefty wing insulation.

UFH - thick insulation and no screed?

mybarnconversion — Sat, 15 Mar 2008 10:23:47 +0000

Perhaps I've missed something in the thread (apologies if I have) but one of the functions of the screed is to act as a thermal store to retain and release radiant heat, without that layer the performance of the UFH will be compromised (to what degree?) - if that compromise is one you accept then go for it.

UFH - thick insulation and no screed?

Henry Sears — Sat, 15 Mar 2008 13:59:58 +0000

Hi - I have suspended timber floors over which I want to fit UFH. I read about a method of filling the void around the pipes with dry sand:

http://www.wideboards.com/underfloor_heating.php

Does anyone see any problems with this?

I can certainly see the advantages.

My plan was to screw and glue a subfloor of 20mm ply onto the existing 150 x 50 joists (which have rockwool fitted to full depth). Then polythene sheet, then 50mm XPS and 50mm floating counter batten to create void for UFH pipes in lightweight screed or dry sand, then finish with reclaimed 28mm yellow pine boards screwed and plugged to counter battens.

Not certain if the insulation board will compress over time with the weight of the counter battens and floor, especially where heavily used or supporting heavy furniture. Maybe needs to go between counter battens? (obviously creating thermal bridge).

Cheers for any feedback.

UFH - thick insulation and no screed?

tony — Sat, 15 Mar 2008 14:11:00 +0000

I would say that you will not need screws and plugs! I would put the ufh pipes on top of the rock-wool and then lay the pine boards. Forget the rest?