Green Building Forum - U value of uninsulated concrete slab and options to improve

U value of uninsulated concrete slab and options to improve

Kenny_M — Thu, 25 Jul 2019 15:28:16 +0100

Hi all,
Ahead of finalising on the triple glazed doors I mentioned in a previous post I wanted to think about the floor in case I need to raise the floor to insulate, and thus change the height of the doors.

As a starting point I wanted to try and understand the u value of the existing floor. The part of the extension I am concerned with is the jutting out part of the L and raised above the internal level of the rest of the extension and the external level of the patio. It is currently tiled and I have removed an inconspicuous tile and drilled down with 8mm bit, and I get concrete for about 90-100mm before breaking through into what appears to be earth.

It surprised me that this was so shallow because compared to the top of the tiles the step outside is 150mm lower, and the foundations for the wall below that step are around 250mm lower than the tiles. Thatâ€™s on the East elevation, on the west elevation the foundations start at around 210mm below the level of the tiles. So, it seems that the ground level inside is higher than outside, as though the foundations and walls have been built, then earth piled inside, and then concrete poured over the top, maybe to reduce the amount of concrete needed. I donâ€™t know enough about construction (circa 1989), to know if this is normal?

Anyway, back to the U value. From reading a few previous posts and such I understand that the U value for floors is a bit more complicated than other elements. I presume this is because the heat transfer is mostly to the ground rather than the air. Using the changeplan calculator, and a P/A = 0.833, the current floor (concrete and tiles) works out at about 0.78 W/m2K.

This is better than I had expected as this floor â€˜seemsâ€™ to be very cold. My presumption is that the P/A ratio is needed because heat transfer will be greater at the edges close to outside air, and lower in the centre, and so I wonder if the fact that this part of the extension is raised might make Perimeter more of a negative factor?
In addition, where the current door is there is no cavity wall between the concrete floor and outside -

https://www.dropbox.com/s/lqls3i5munm3f5r/2019-07-24%2020.04.38.jpg?dl=0.

Doing a calculation for 100mm of concrete wall gives a u value of 3.9 W/m2K so I presume that the u value of this section at the door must be approaching this. Would I be right to suspect that with all these factors the U value of the current floor is likely to be worse than the calculated 0.78?

In terms of what to do with this I suspect there are probably two options
1. Rip up the concrete floor and start again with a different floor structure. Is it possible/desirable/straightforward to do this? Does the slab affect the foundations?
2. Lift the tiles (getting replaced with wood floors anyway) and lay insulation on top of the slab, and try to fit some external insulation around the perimeter at ground level (restricted by the fact one wall borders into a neighbourâ€™s garden. Also very restricted as room height as 2230mm and I was also hoping to put some internal insulation on the roof!

As always the voices of experience on here are most welcome!

Thanks,
Kenny.

U value of uninsulated concrete slab and options to improve

WillInAberdeen — Thu, 25 Jul 2019 20:29:36 +0100

We dug ours out and laid DPM, insulation and new screed, very pleased with results. Messy job of course. we were replacing all the wall linings at the same time. The dig is adjusted such that you end up with whatever final floor level you want. The finished floor is at room temperature so is pleasant to walk/sit/play on.

U value of uninsulated concrete slab and options to improve

Peter_in_Hungary — Thu, 25 Jul 2019 23:27:36 +0100

Posted By: Kenny_MSo, it seems that the ground level inside is higher than outside, as though the foundations and walls have been built, then earth piled inside, and then concrete poured over the top, maybe to reduce the amount of concrete needed. I donâ€™t know enough about construction (circa 1989), to know if this is normal?

Sounds about right. In '89 people weren't so enthusiastic about insulation as they are today and building up the internal floor level with foundation spoil is a cheap fix. 100mm of concrete done properly would be OK and there may be rebar in it but probably not.

Posted By: WillInAberdeenWe dug ours out and laid DPM, insulation and new screed, very pleased with results. Messy job of course. we were replacing all the wall linings at the same time. The dig is adjusted such that you end up with whatever final floor level you want. The finished floor is at room temperature so is pleasant to walk/sit/play on.

This is the best fix but the most disruptive and expensive.

Posted By: Kenny_M2. Lift the tiles (getting replaced with wood floors anyway) and lay insulation on top of the slab, and try to fit some external insulation around the perimeter at ground level (restricted by the fact one wall borders into a neighbourâ€™s garden. Also very restricted as room height as 2230mm and I was also hoping to put some internal insulation on the roof!

A cheaper option and perfectly feasible. Even 20mm of insulation with 12mm of engineered wood over will make a tremendous difference. And if you are going for this option - why lift the tiles, just put the insulation on the tiles.

External insulation down to the foundations will also help and whilst I understand you have boundary issues on one side, doing the sides available will help. Have you spoken to the neighbour about getting their permission to insulate the boundary wall?

U value of uninsulated concrete slab and options to improve

gyrogear — Fri, 26 Jul 2019 07:19:43 +0100

Posted By: Peter_in_Hungarywhy lift the tiles, just put the insulation on the tiles.

+1

gg

U value of uninsulated concrete slab and options to improve

Kenny_M — Fri, 26 Jul 2019 09:57:20 +0100

Thanks folks.

The only reason I was considering digging up and not just laying over existing tiles is that the ceiling height is already low in that raised part (2230mm). 30mm for the floor takes me down to 2200mm and I had been planning internal insulation on ceiling, which brings me down to say 2170mm.

I can't find any regulations on minimum ceiling heights but have found guidelines stating a minimum of 2150mm so might work but could feel very low.

If I insulate over the tiles can I run the insulation under the door? I think I read somewhere that spaces under doors need to be raised with treated wood when raising the floor level.

U value of uninsulated concrete slab and options to improve

djh — Fri, 26 Jul 2019 11:18:44 +0100

Posted By: Peter_in_HungaryA cheaper option and perfectly feasible. Even 20mm of insulation with 12mm of engineered wood over will make a tremendous difference.

Cheaper but not feasible. It won't meet Building Regs.

U value of uninsulated concrete slab and options to improve

djh — Fri, 26 Jul 2019 11:23:52 +0100

Posted By: Kenny_MI can't find any regulations on minimum ceiling heights but have found guidelines stating a minimum of 2150mm so might work but could feel very low.

IIRC, they were abolished some time ago. But low ceilings feel bad - a constant reminder of pennypinching. They also make the room much hotter, because there's nowhere for the hot air to escape up to.

It's not that expensive to take up a floor. Just hire a jackhammer to break up the concrete, dig out the earth, lay a DPM and some insulation (oh after sand blinding) and then the most tricky bit is putting a new concrete slab on top.

U value of uninsulated concrete slab and options to improve

Kenny_M — Fri, 26 Jul 2019 16:38:45 +0100

Thanks djh.

Regarding the building regs, is that something I need to concern myself with as that part I would probably DIY? Don't mean that as a dismissal of importance of the regs, just that if the choice ends up being between improving energy efficiency or not doing anything because it won't meet the regs then the former would seem better.

You reaffirm my concern about the low ceiling. Although as this is a raised area with steps up from the rest of the extension, it will probably feel more deliberate. And there is also a 1.2m2 skylight tunnel which add some feeling of height.

Thanks for the outline for removing floor. I'd be comfortable enough myself doing this part even, but I wasn't confident about whether the concrete floor had any connection to the structure, and whether there was any risk in removing it. I suppose that the fact that I have discovered that the walls are lower than the slab makes this unlikely.

U value of uninsulated concrete slab and options to improve

djh — Fri, 26 Jul 2019 17:18:31 +0100

Posted By: Kenny_MRegarding the building regs, is that something I need to concern myself with as that part I would probably DIY?

I believe you do need to concern yourself, yes. It doesn't matter whether you DIY or not. If you do work affecting a thermal element, which I believe the floor of a room is, then it needs to meet current building regs after completion, with some exceptions.

The point is that just making marginal improvements to energy efficiency isn't going to save the planet. Many on here argue that the building regs aren't good enough either, but that's a different debate.

U value of uninsulated concrete slab and options to improve

Kenny_M — Fri, 26 Jul 2019 20:43:53 +0100

Posted By: djh
The point is that just making marginal improvements to energy efficiency isn't going to save the planet.

I understand what you are saying, but I also have to work within a budget and/or within the time I have available. I may not be going anywhere near what others are achieving on here but I am dealing with a 200 year old house and a 30 year old extension, still reeling from a Â£40k bill for stone masonry, and various other outlays, and I don't have the money to meet modern building regs on every element in the house. I'm trying to focus my time and energy on the things that give me the best energy saving return for the money I have.

Off topic also, but I have neighbours who have modern houses that meet building regs and use more energy to heat the house than I do because they insist on every room being 20+ degrees, while I throw a blanket over me when I am sitting down to watch a movie. Right or wrong, my feeling is that my energy consumption is more important than whether I meet building regs.

U value of uninsulated concrete slab and options to improve

Nick Parsons — Fri, 26 Jul 2019 21:19:52 +0100

But there is a legal obligation to comply with (or seek a waiver relating to) Bldg Regs, and non-compliance can cause problems when selling. Yes, the fee can be galling (Â£150 per thermal element (or sometimes two) in my neck of the woods) but a buyer sniffing an 'irregularity' (for which read 'a chance of a discount') will try to beat you down by far more than that.

U value of uninsulated concrete slab and options to improve

djh — Fri, 26 Jul 2019 22:46:05 +0100

Exactly, Building Regs are the absolute minimum that society expects. Personally, I wouldn't go anywhere near a 200 year old property for that reason, but that's just me.

U value of uninsulated concrete slab and options to improve

WillInAberdeen — Fri, 26 Jul 2019 23:11:16 +0100

The over riding criteria in Building Regs part L1B (for renovations) is that improvements are only required if they can achieve a simple payback within 15 years.

Per the OP, the existing floor is U=0.78 so is losing ca 0.78x10degCx24x365/1000 = 70kWh/m2/a, costing ca 70xÂ£0.05 = Â£3.50/m2/a.

Over 15 years there is no way that would justify spending Â£1000s on pouring a new floor slab, so Building Regs do not require this to be done. It's the homeowners choice, based on comfort and enthusiasm vs cost and inconvenience.

Pretty much any improvement would exceed Building Regs in this regard.

As Kenny said in the OP, tiles on concrete do feel particularly cold underfoot due to the combination of high thermal conductivity with high heat capacity. A thin layer of surface insulation will fix the 'feel' nicely.

If Kenny reduces his floor U by say 0.3 W/m2K, he will have achieved a lot more for society and the UK housing stock, than anyone who takes a plot that was already destined for a Building Regs compliant floor U<0.25, and blings it up to PH standard, thus making a 'marginal improvement' of 0.1 or 0.2.

(Edited for clarity 27/7/19 and assuming the house is in England)

U value of uninsulated concrete slab and options to improve

Kenny_M — Sat, 27 Jul 2019 11:18:14 +0100

Posted By: Nick ParsonsBut there is a legal obligation to comply with (or seek a waiver relating to) Bldg Regs, and non-compliance can cause problems when selling.

I appreciate the warning, but I expect to be carried out of this house in a box so I won't be too concerned at people trying to get discounts at that stage! :) But seriously, I'm not convinced that a buyer is going to be interested in or aware about what is under the wood floor, and from what I have seen of Surveyors they don't go into that much detail.

Posted By: djhPersonally, I wouldn't go anywhere near a 200 year old property for that reason, but that's just me.

Fair enough, but someone will buy it, and although I have seen some on here suggest that these houses should be knocked down and rebuilt, the reality is that it is in a conservation area and that is not going to happen. I would argue that it is better me doing this than most people, because at least I am trying to improve the situation. The previous owner was very wealthy and hadn't improved the house for about 30 years. I've replaced the 30 year old boiler, fitted double glazing units into existing wood window frames, reduced dampness in the walls, insulated the attic, insulated various walls as the opportunity arises, and I am about to spend a lot of money on triple glazed wood windows and doors and continue to do what I can to improve.

Posted By: WillInAberdeenThe over riding criteria in Building Regs part L1B (for renovations) is that the improvement should achieve a simple payback within 15 years.

I didn't realise that, but in fact I doubt that any of the improvements I am doing will ever give me payback, unless energy costs increase dramatically (which may well be the case). I worked this out for the boiler and given the lifespan of modern boilers I doubt I will ever make a financial return.

Posted By: WillInAberdeenAs Kenny said in the OP, tiles on concrete do feel particularly cold underfoot due to the combination of high thermal conductivity with high heat capacity. A thin layer of surface insulation will fix the 'feel' nicely.

If Kenny reduces his floor U by say 0.3 W/m2K, he will have achieved a lot more for society and the UK housing stock, than anyone who takes a plot that was already destined for a Building Regs compliant floor U<0.25, and blings it up to PH standard, thus making a 'marginal improvement of 0.1 or 0.2.

This is exactly my feeling. There is a cold draught from the tiles in winter, and as an experiment last winter we put down two large rugs that covered most of the floor and the difference was incredible, so even 10mm insulation under wood would have a similar effect.

Don't get me wrong I am still considering all options. I would really love (and probably enjoy) going to full way and digging this out, but I really have to consider my time and finances, and the disruption. I have a 4 year old and work full time, so I work on this after 8 when she has gone to bed and the few hours I get at the weekend With the cavity wall insulation already done, the triple glazing about to be ordered and the roof insulation to follow the heat loss of the extension will be a fraction of what it was before.

I do appreciate the advice and don't want to appear to be diminishing this by arguing about building regulations.

U value of uninsulated concrete slab and options to improve

Sprocket — Sat, 27 Jul 2019 13:12:38 +0100

We have just (last couple of months) dug up all our (installed in 1996) underfloor heating slab (70-90 mm sand-cement screed) and insulation below (25mm thickness of foil faced PUR foam sheets). ie. stripped it back to bare slab.

After we level it all with a bonded screed (some areas were 40-50mm low) We will be putting back 100mm of PIR insulation and a new 65mm UFH pipe in screed above.

It has been pretty disruptive (huge amounts of dust). I cannot imagine trying to live in it whilst this was done. The old screed had to come out anyway as it was 1996 spec polybutylene pipe and plastic manifolds in the floor so was a bit of a liability.
I really did not want to raise floor levels by so much but 50mm wasn't going to get us to Building regs so we went to 100mm and we're going to have to raise a few doorways.

I don't particularly like the PUR or PIR foam panel insulation for floors. I would much rather use Foamglas but it's thermal properties are not quite as good so it would have taken almost double the thickness and our rooms are not high enough to get away with that.

We took the opportunity to add radon sumps below the slab whilst we had the floor up. We've no idea yet how well that will work but it seemed worth a shot.

U value of uninsulated concrete slab and options to improve

WillInAberdeen — Sat, 27 Jul 2019 17:07:06 +0100

Sprocket, don't know if it's any help, building regs part L1B for renovations don't require you to add extra insulation, if it will cause problems with heights, or if it won't payback within 15years. Good on you for adding lots of insulation, but don't feel compelled by regs.

Instead of that depth of screed over the insulation, did you consider a floating floor, using the chipboard or fermacell or polystyrene panel systems that come with channels cut in them for the ufh pipes?

Certainly is a messy job! We only did half the ground floor (other half was suspended due to hillside), sealed it off from the rest of the house with works access through a window, and went on holiday while someone else did the worst bit.

U value of uninsulated concrete slab and options to improve

Kenny_M — Sun, 28 Jul 2019 18:39:12 +0100

If I go the route of insulation over the tiles, would I stick the PIR board to the tiles with sticky foam or grab adhesive or something else? And would the wood flooring also be bonded or just laid on top?

Also is a PIR board strong enough to be laid under the new door, or does the door need to be raised with a strip of treated wood?

U value of uninsulated concrete slab and options to improve

tony — Sun, 28 Jul 2019 18:50:12 +0100

For me wood in the doorways, poly on top of insulation, floating floor on top of that gaps to all edges say 10mm

U value of uninsulated concrete slab and options to improve

CWatters — Sun, 28 Jul 2019 19:18:32 +0100

As a starting point I wanted to try and understand the u value of the existing floor.

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=8965

Posted By: Nick ParsonsA simplified method of calculating the U-Value of an uninsulated floor has been described in BRE Information Paper IP 3/90.
This simplified formula can be used for all types of ground floor including; ground bearing, suspended concrete and suspended timber. It can also be used with relative ease for irregularly shaped floor plans, not just simple rectangular shapes.
The IP 3/90 formula is:
U = 0.05 + 1.65(P/A) - 0.6(P/A)Â²
Where:
U = U-Value of the uninsulated floor (W/mÂ²K).
P = Length of the exposed perimeter (m).
A = Area of the floor (mÂ²) ''

There is another thread on this somewhere, but I have no idea where. Anyone?

U value of uninsulated concrete slab and options to improve

Kenny_M — Mon, 29 Jul 2019 09:50:16 +0100

Thanks CWatters. I saw this post before, and this formula is part of the calculation on the changeplan calculator I was using, but what I was wondering was whether the height of the floor relative to the ground outside would affect this calculation.

For example, if an inside floor is level with the ground then most of the perimeter is conducting heat to the ground, while if the level of the inside floor is high in relation to the ground then more of the perimeter area will have a short path to outside air, presumingly increasing the overall u value.

As the part of the extension with the concrete floor is quite above ground level I wondered if this explained why my floor 'feels' colder than the calculated value of 0.8.

U value of uninsulated concrete slab and options to improve

Kenny_M — Mon, 29 Jul 2019 09:55:08 +0100

Posted By: tonyFor me wood in the doorways, poly on top of insulation, floating floor on top of that gaps to all edges say 10mm

Thanks Tony, does the insulation need to be bonded down to the tiles or just laid over, and likewise with the wood flooring?

Sorry if this is a stupid question but the only time I have put down floor insulation before was under tiles and the directions were to use tile adhesive under and over the insulation. I thought maybe that sticky foam I have seen recommended on here for wall insulation might be the thing, or maybe just a grab adhesive

U value of uninsulated concrete slab and options to improve

Peter_in_Hungary — Mon, 29 Jul 2019 12:35:16 +0100

When I insulated a concrete floor some 3 years ago I loose laid the insulation (50mm) to a good fit at the walls and laid a floating 'click fit' engineered wood floor on top of that with the recommended gaps at the walls. So far no problems.

U value of uninsulated concrete slab and options to improve

Kenny_M — Mon, 29 Jul 2019 13:26:55 +0100

Thanks Peter, sounds good. Did you not bother with the vapour barrier then?

U value of uninsulated concrete slab and options to improve

Peter_in_Hungary — Mon, 29 Jul 2019 13:46:38 +0100

I didn't bother with a VCL. The insulation was XPS

U value of uninsulated concrete slab and options to improve

tony — Mon, 29 Jul 2019 15:11:19 +0100

No need to bond anything, gravity does a nice job. Wood flooring, I was assuming sheets, if boards, I would lay them on top of sheets, the sheets are t&g glued together but not bonded down, leave gap round the edge for expansion.

U value of uninsulated concrete slab and options to improve

djh — Mon, 29 Jul 2019 16:41:56 +0100

Posted By: Kenny_MI was wondering was whether the height of the floor relative to the ground outside would affect this calculation.

I expect it does, but as Nick's post says, it is a 'simplified formula'. I expect there are more comprehensive formulae available. The official reference was BR 443 https://www.bre.co.uk/filelibrary/pdf/rpts/BR_443_(2006_Edition).pdf but I believe that is now obsolete, though that doesn't make it less accurate. I don't know what has replaced it.

U value of uninsulated concrete slab and options to improve

Jeff B — Mon, 29 Jul 2019 17:53:20 +0100

Posted By: Kenny_MThanks Peter, sounds good. Did you not bother with the vapour barrier then?

I did a floating floor arrangement with T&G chipboard (sheets glued together with PVA) over EPS. As Tony says gravity does the "fixing" for you. We left a gap around the edges for expansion. We might have used those cork strips for the gap - I can't remember for certain. However we did use a VCL because we were advised that the floor might squeak if you lay chipboard directly onto EPS. Must say we have never had a problem with squeaking as a result!

U value of uninsulated concrete slab and options to improve

WillInAberdeen — Mon, 29 Jul 2019 23:11:22 +0100

I laid click-lock laminate floating on a chipboard floor, with ~10mm gap all round the edges, covered with ~15mm skirting. Over time, some but not all the planks managed to wriggle to one side of the room, leaving a visible gap beneath the skirting on the other side. The planks came from a national DIY chain and were laid with their thin foamy plastic sheet to cushion them from the chipboard. I swore in public (well on GBF) never to lay another floating floor!

The floor U value formula has lots of approximations, it doesn't account for thicker/thin walls, foundation types, soil types, underground moisture, surface coverings inside/outside. It's a bit dubious for L shaped houses like yours/mine, you get different results depending how much of the floor plan you include. But it's very easy and quick, so use accordingly!

U value of uninsulated concrete slab and options to improve

Jeff B — Tue, 30 Jul 2019 10:47:50 +0100

Posted By: Jeff B
Posted By: Kenny_MThanks Peter, sounds good. Did you not bother with the vapour barrier then?

I did a floating floor arrangement with T&G chipboard (sheets glued together with PVA) over EPS. As Tony says gravity does the "fixing" for you. We left a gap around the edges for expansion. We might have used those cork strips for the gap - I can't remember for certain. However we did use a VCL because we were advised that the floor might squeak if you lay chipboard directly onto EPS. Must say we have never had a problem with squeaking as a result!

Perhaps I should have added that the final floor cover was that very heavy insulating underlay* and carpet, so no danger of that moving like Will-in-Aberdeen's laminate flooring!

*Something like this:

https://www.carpet-underlay-shop.co.uk/products/42oz-wool-felt-carpet-underlay?utm_medium=cpc&utm_source=googlepla&gclid=CjwKCAjw1f_pBRAEEiwApp0JKAm8qJN80m-AC5Gq4cwlEr084zLJtv_O1Jsne9xN4dfbczgIr7ZqZhoCQb0QAvD_BwE

U value of uninsulated concrete slab and options to improve

Peter_in_Hungary — Tue, 30 Jul 2019 11:50:10 +0100

Posted By: WillInAberdeenI laid click-lock laminate floating on a chipboard floor, with ~10mm gap all round the edges, covered with ~15mm skirting. Over time, some but not all the planks managed to wriggle to one side of the room, leaving a visible gap beneath the skirting on the other side.

The idea of the 10mm gap at the edges is to allow for movement. If a e.g. 22mm skirting was used then no gaps would ever be visible. Over here a beading is sold the go at the edges, attached to the skirting board, to prevent any gaps showing.