I am currently planning the construction of a 15m2 ground floor extension to our lounge and am torn between having the insulation above or below the concrete slab.

From what I can gather the most significant and real-world difference between the two is the temperature response with insulation-above-slab being quicker to heat up (and cool) down given the effective reduction in thermal mass.

Even considering this factor alone, and acknowledging that thermal performance is a very important criteria for me with this extension, I still don't know which way to go. As the extension is for the lounge I am assuming that floor temperature will likely be an important component affecting comfort. For what's it's worth, the floor covering will likely be engineered wood with rugs.

Any comments or advice that might help me make the decision?

On Friday, April 6, 2018 at 8:03:21 PM UTC+1, Mathew Newton wrote:
I am currently planning the construction of a 15m2 ground floor extension to our lounge and am torn between having the insulation above or below the concrete slab.

From what I can gather the most significant and real-world difference between the two is the temperature response with insulation-above-slab being quicker to heat up (and cool) down given the effective reduction in thermal mass.

Even considering this factor alone, and acknowledging that thermal performance is a very important criteria for me with this extension, I still don't know which way to go. As the extension is for the lounge I am assuming that floor temperature will likely be an important component affecting comfort. For what's it's worth, the floor covering will likely be engineered wood with rugs.

Any comments or advice that might help me make the decision?

Personally I would go for the low thermal mass. As you say, it warms up faster, but also when you stand on it with bare feet or socks it will feel less cold (because your feet warm it).

if I lived in a very hot climate, I would make the opposite choice.

Robert

On Fri, 06 Apr 2018 12:03:18 -0700, Mathew Newton wrote:

I am currently planning the construction of a 15m2 ground floor
extension to our lounge and am torn between having the insulation above
or below the concrete slab.

From what I can gather the most significant and real-world difference
between the two is the temperature response with insulation-above-slab
being quicker to heat up (and cool) down given the effective reduction
in thermal mass.

Even considering this factor alone, and acknowledging that thermal
performance is a very important criteria for me with this extension, I
still don't know which way to go. As the extension is for the lounge I
am assuming that floor temperature will likely be an important component
affecting comfort. For what's it's worth, the floor covering will likely
be engineered wood with rugs.

Any comments or advice that might help me make the decision?

We went with insulation over the slab.

In the lounge/diner/kitchen extension.

Laminate flooring on top.

It seems O.K. so far.


Cheers


Dave R


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On Friday, April 6, 2018 at 8:03:21 PM UTC+1, Mathew Newton wrote:
I am currently planning the construction of a 15m2 ground floor extension to our lounge and am torn between having the insulation above or below the concrete slab.

From what I can gather the most significant and real-world difference between the two is the temperature response with insulation-above-slab being quicker to heat up (and cool) down given the effective reduction in thermal mass.

Even considering this factor alone, and acknowledging that thermal performance is a very important criteria for me with this extension, I still don't know which way to go. As the extension is for the lounge I am assuming that floor temperature will likely be an important component affecting comfort. For what's it's worth, the floor covering will likely be engineered wood with rugs.

Any comments or advice that might help me make the decision?

I went for insulation below the slab as I thought insulation with just screed over would be a bit "wobbly" with a spinning washing machine on it (wait for someone to say should-a bought a Miele). This option just seems more solid to me.

Simon.

Mathew Newton wrote:
I am currently planning the construction of a 15m2 ground floor extension to our lounge and am torn between having the insulation above or below the concrete slab.

From what I can gather the most significant and real-world difference between the two is the temperature response with insulation-above-slab being quicker to heat up (and cool) down given the effective reduction in thermal mass.

Even considering this factor alone, and acknowledging that thermal performance is a very important criteria for me with this extension, I still don't know which way to go. As the extension is for the lounge I am assuming that floor temperature will likely be an important component affecting comfort. For what's it's worth, the floor covering will likely be engineered wood with rugs.

Any comments or advice that might help me make the decision?

Why not both ?

On Friday, 6 April 2018 20:03:21 UTC+1, Mathew Newton wrote:
I am currently planning the construction of a 15m2 ground floor extension to our lounge and am torn between having the insulation above or below the concrete slab.

From what I can gather the most significant and real-world difference between the two is the temperature response with insulation-above-slab being quicker to heat up (and cool) down given the effective reduction in thermal mass.

Even considering this factor alone, and acknowledging that thermal performance is a very important criteria for me with this extension, I still don't know which way to go. As the extension is for the lounge I am assuming that floor temperature will likely be an important component affecting comfort. For what's it's worth, the floor covering will likely be engineered wood with rugs.

Any comments or advice that might help me make the decision?

It depends on your lifestyle.
If the house is continuously heated, then the insulation should be below the slab.
If the house is intermittently heated the insulation should be above the slab.
The technical term is "thermal mass". BTW

https://en.wikipedia.org/wiki/Thermal_mass

harry Wrote in message:
On Friday, 6 April 2018 20:03:21 UTC+1, Mathew Newton wrote:
I am currently planning the construction of a 15m2 ground floor extension to our lounge and am torn between having the insulation above or below the concrete slab.

From what I can gather the most significant and real-world difference between the two is the temperature response with insulation-above-slab being quicker to heat up (and cool) down given the effective reduction in thermal mass.

Even considering this factor alone, and acknowledging that thermal performance is a very important criteria for me with this extension, I still don't know which way to go. As the extension is for the lounge I am assuming that floor temperature will likely be an important component affecting comfort. For what's it's worth, the floor covering will likely be engineered wood with rugs.

Any comments or advice that might help me make the decision?

It depends on your lifestyle.
If the house is continuously heated, then the insulation should be below the slab.
If the house is intermittently heated the insulation should be above the slab.
The technical term is "thermal mass". BTW

https://en.wikipedia.org/wiki/Thermal_mass


Which was the term that he correctly used if you'd read it properly...
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Jim K


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Thanks everyone for the input; there seems to be a slight preference for insulation-over-slab and the potential for this feeling slightly warmer underfoot appeals. Points noted regarding the potential downsides though.

I went for insulation below the slab as I thought insulation with just screed over would be a bit "wobbly" with a spinning washing machine on it (wait for someone to say should-a bought a Miele). This option just seems more solid to me.

With ours being a lounge I am assuming that loadings should be fairly static and spread out. It will be certainly something I'll make sure are factored into the design.

Speaking of design, or rather considerations for installation, I note from the Kingspan brochure (https://www.kingspan.com/irl/en-ie/p...-tf70-brochure, page 8 specifically) that insulation-above-slab requires the slab to have dried out completely before insulation boards are laid. That sounds reasonable, but it also says that this will typically take 1 day per mm. With a 100mm slab that means 100 days which seems rather a long pause in an installation?!

On Saturday, 7 April 2018 14:52:07 UTC+1, Mathew Newton wrote:

Thanks everyone for the input; there seems to be a slight preference for insulation-over-slab and the potential for this feeling slightly warmer underfoot appeals. Points noted regarding the potential downsides though.

I went for insulation below the slab as I thought insulation with just screed over would be a bit "wobbly" with a spinning washing machine on it (wait for someone to say should-a bought a Miele). This option just seems more solid to me.

With ours being a lounge I am assuming that loadings should be fairly static and spread out. It will be certainly something I'll make sure are factored into the design.

Speaking of design, or rather considerations for installation, I note from the Kingspan brochure (https://www.kingspan.com/irl/en-ie/p...-tf70-brochure, page 8 specifically) that insulation-above-slab requires the slab to have dried out completely before insulation boards are laid. That sounds reasonable, but it also says that this will typically take 1 day per mm. With a 100mm slab that means 100 days which seems rather a long pause in an installation?!

yet another reason to insulate under.


NT

FMurtz Wrote in message:
Mathew Newton wrote:
I am currently planning the construction of a 15m2 ground floor extension to our lounge and am torn between having the insulation above or below the concrete slab.

From what I can gather the most significant and real-world difference between the two is the temperature response with insulation-above-slab being quicker to heat up (and cool) down given the effective reduction in thermal mass.

Even considering this factor alone, and acknowledging that thermal performance is a very important criteria for me with this extension, I still don't know which way to go. As the extension is for the lounge I am assuming that floor temperature will likely be an important component affecting comfort. For what's it's worth, the floor covering will likely be engineered wood with rugs.

Any comments or advice that might help me make the decision?

Why not both ?


Why both?
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On 07/04/2018 15:01, wrote:
On Saturday, 7 April 2018 14:52:07 UTC+1, Mathew Newton wrote:

Thanks everyone for the input; there seems to be a slight preference for insulation-over-slab and the potential for this feeling slightly warmer underfoot appeals. Points noted regarding the potential downsides though.

I went for insulation below the slab as I thought insulation with just screed over would be a bit "wobbly" with a spinning washing machine on it (wait for someone to say should-a bought a Miele). This option just seems more solid to me.

With ours being a lounge I am assuming that loadings should be fairly static and spread out. It will be certainly something I'll make sure are factored into the design.

Speaking of design, or rather considerations for installation, I note from the Kingspan brochure (https://www.kingspan.com/irl/en-ie/p...-tf70-brochure, page 8 specifically) that insulation-above-slab requires the slab to have dried out completely before insulation boards are laid. That sounds reasonable, but it also says that this will typically take 1 day per mm. With a 100mm slab that means 100 days which seems rather a long pause in an installation?!

yet another reason to insulate under.


NT

But you still need to wait 100 days before putting down any sort of wood
flooring, unless you use 2nd dpc.

My 1976 house has solid internal walls (3 inch cinder blocks). On the
ground floor these have a line of thin bricks on the slab, then a
bitumen dpc, which would have been just above the screed, but hidden
by the skirting board, then the internal wall.

I think this was to stop moisture from the slab and screed wicking
up the internal walls and damaging the plaster and paintwork.

On Saturday, 7 April 2018 19:01:27 UTC+1, Andrew wrote:

But you still need to wait 100 days before putting down any sort of wood
flooring, unless you use 2nd dpc.

Ah, okay,

To be honest my planning hadn't got as far as the details regarding laying the wood floor as that seems like such a long way off. That's only a few stops shy of soft furnishings!

I think I need to a bit more learning...

On 06/04/18 20:03, Mathew Newton wrote:
I am currently planning the construction of a 15m2 ground floor extension to our lounge and am torn between having the insulation above or below the concrete slab.

From what I can gather the most significant and real-world difference between the two is the temperature response with insulation-above-slab being quicker to heat up (and cool) down given the effective reduction in thermal mass.

Even considering this factor alone, and acknowledging that thermal performance is a very important criteria for me with this extension, I still don't know which way to go. As the extension is for the lounge I am assuming that floor temperature will likely be an important component affecting comfort. For what's it's worth, the floor covering will likely be engineered wood with rugs.

Any comments or advice that might help me make the decision?


Under. The slab will not compress in random areas like doorways as it
may be prone to if the top is made of insulation.

replying to Mathew Newton, Matt carpenter wrote:
Have you considered putting 2 layers in?
Not sure on your total thickness of insulation lets say 100mm
Put 75mm under slab and 25mm above slab. Youll need a perimeter 25mm
upstand on external walls to stop the cold bridge anyway cost wise apart from
a little labour is the same just make sure slab is smooth

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On Sunday, 29 April 2018 16:44:04 UTC+1, Matt carpenter wrote:
replying to Mathew Newton, Matt carpenter wrote:
Have you considered putting 2 layers in?
Not sure on your total thickness of insulation lets say 100mm
Put 75mm under slab and 25mm above slab. Youll need a perimeter 25mm
upstand on external walls to stop the cold bridge anyway cost wise apart from
a little labour is the same just make sure slab is smooth

Cool, you can have the worst of both worlds. Not a hoh poster is it?

On 06/04/2018 20:03, Mathew Newton wrote:
I am currently planning the construction of a 15m2 ground floor extension to our lounge and am torn between having the insulation above or below the concrete slab.

From what I can gather the most significant and real-world difference between the two is the temperature response with insulation-above-slab being quicker to heat up (and cool) down given the effective reduction in thermal mass.

Even considering this factor alone, and acknowledging that thermal performance is a very important criteria for me with this extension, I still don't know which way to go. As the extension is for the lounge I am assuming that floor temperature will likely be an important component affecting comfort. For what's it's worth, the floor covering will likely be engineered wood with rugs.

Any comments or advice that might help me make the decision?

Speaking from experience of both.... The new extension was done by
builders with insulation under slab and the old garage which was
converted into gym (internal/habitable space) had insulation over
original slab.

I fitted the same UFH pipes to both. New room and entrance
hall/reception were done with a 7 layer multi-foil insulation under
egg-crate pipe trays and old garage was egg-tray obviously sat directly
on 4" insualtion.
Both were screeded to the same depth and 2 new rooms are 2
zones/circuits and old room is 2 circuits but 1 zone.

The egg-tray UFH with multi-foil keeps what appears to be a very good
degree of isolation between floor slab and certainly heats up quickly so
although it sounds correct to stick the insulation on the slab I've not
noticed any difference from both variants.