Green Building Forum - insulated slab, UFH and perimeter insulation Tue, 19 Dec 2023 07:05:52 +0000 http://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 insulated slab, UFH and perimeter insulation http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17887&Focus=302287#Comment_302287 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17887&Focus=302287#Comment_302287 Tue, 04 Jul 2023 19:59:57 +0100 number_thirty_three
Have read a lot of different threads on here and elsewhere and if money was no object I'd be going for a limecrete/glasscrete floor, we have to be a bit leaner on the spend here so looking now at XPS, topped with underfloor heating and a thin concrete slab.

Some questions I'm looking for guidance on are:

1) The main part of the house has a suspended floor, which I've already insulated - I need to make sure it's ventilated to the rear - at the moment there are vents under the existing slab so we'll need to lay some new very long pipes to provide ventilation through the slab, which I am conscious will be thermal bridges through the insulation layer. Any suggestions on how this should be achieved? Is it best for the pipe to be as low down as possible in the insulation layer?

2) We're going to have quite a few pipes running around the place to the WC under the stairs, the new HW cylinder to the heat pump, and obvs the usual plumbing as well - all of which I guess presents a bit of a thermal bridge to the insulation layer. Other than making sure these penetrations are well sealed is there anything else to consider?

3) Do we put the XPS under the concrete slab or slab first, then XPS, then screed? Seen advocates for both options with no obvious winner...

4) What would you do for UFH in this context?


For further context, the finished floor is going to be marmoleum (Forbo's lino).

Thanks in advance :)]]> insulated slab, UFH and perimeter insulation http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17887&Focus=302292#Comment_302292 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17887&Focus=302292#Comment_302292 Tue, 04 Jul 2023 20:55:17 +0100 Mike1 Posted By: number_thirty_three1)...Is it best for the pipe to be as low down as possible in the insulation layer?Yes, as long as there's no risk of it filling with water due to flooding or the like.

Posted By: number_thirty_three2) We're going to have quite a few pipes running around the place
Difficult to envisage the issue without a sketch...

Posted By: number_thirty_three3) Do we put the XPS under the concrete slab or slab first, then XPS, then screed? Seen advocates for both options with no obvious winner...
You' may find the same again. Personally I'd skip the screed and add additional insulation if I could. You could plan to add a thin self-levelling compound over the top before laying flooring (as long as it's one suitable for use over heated floors) if you're concerned that there may be a problem.

Posted By: number_thirty_three4) What would you do for UFH in this context?
Pipes? I guess that's not what you're asking?]]> insulated slab, UFH and perimeter insulation http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17887&Focus=302327#Comment_302327 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17887&Focus=302327#Comment_302327 Fri, 07 Jul 2023 06:33:27 +0100 tony Do you have a U-value in mind?]]> insulated slab, UFH and perimeter insulation http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17887&Focus=302328#Comment_302328 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17887&Focus=302328#Comment_302328 Fri, 07 Jul 2023 09:25:37 +0100 GreenPaddy
Standard answer is to apply a liquid DPM over the concrete, prior to the finished floor going being laid. That's what I did on on commercial projects, and what I do now on domestic projects. As Mike1 says, allow a £100 for someone to fix any low bits with self level, as insurance, but hopefully won't need if the concrete guys do their job.

If you go for say 2 layers of 100mm XPS, that shd get you to around Uval of 0.13 W/m2K (need to calc with area/perimeter ratios etc.)

Air ducts for u/f ventilation, being below the new solid floor insulation (dug into the solum), will likely be below external ground level. To get back above ground level when you get beyond the new solid slab, you could connect the ducts to periscopic vents, which you build in to the block/brick underbuild, here's an image for info....

https://sydenhams.s3.amazonaws.com/Building%20Materials/Roofing%20and%20Ventilation/Building%20Ventilation/g960swl.jpg]]> insulated slab, UFH and perimeter insulation http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17887&Focus=304029#Comment_304029 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17887&Focus=304029#Comment_304029 Tue, 24 Oct 2023 13:43:51 +0100 wookey
Doing the same in the rest of the house would have saved you having to put in annoying vent pipes, but a bit late for that now :-) Yes vent as low as possible then a periscope vent back up to air-brick height.

As for pipes being thermal bridges. Not much to be done about that except use plastic pipe, and grommet every entry to the airtightness layer (top of the insulation? top of the concrete/screedboard?]]> insulated slab, UFH and perimeter insulation http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17887&Focus=304030#Comment_304030 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17887&Focus=304030#Comment_304030 Tue, 24 Oct 2023 18:05:35 +0100 tony