Insulation above or below slab?

[Mathew Newton]

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

From what I can gather the most significant and real-world difference betwe en the two is the temperature response with insulation-above-slab being qui cker to heat up (and cool) down given the effective reduction in thermal ma ss.

Even considering this factor alone, and acknowledging that thermal performa nce 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 w ith rugs.

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

[rmlaws54]

to our lounge and am torn between having the insulation above or below the concrete slab.

ween the two is the temperature response with insulation-above-slab being q uicker to heat up (and cool) down given the effective reduction in thermal mass.

mance 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 th at floor temperature will likely be an important component affecting comfor t. For what's it's worth, the floor covering will likely be engineered wood with rugs.

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

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

Robert

[David]

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

[sm_jamieson]

to our lounge and am torn between having the insulation above or below the concrete slab.

ween the two is the temperature response with insulation-above-slab being q uicker to heat up (and cool) down given the effective reduction in thermal mass.

mance 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 th at floor temperature will likely be an important component affecting comfor t. For what's it's worth, the floor covering will likely be engineered wood with rugs.

I went for insulation below the slab as I thought insulation with just scre ed 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 s olid to me.

Simon.

[FMurtz]

Why not both ?

[harry]

to our lounge and am torn between having the insulation above or below the concrete slab.

ween the two is the temperature response with insulation-above-slab being q uicker to heat up (and cool) down given the effective reduction in thermal mass.

mance 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 th at floor temperature will likely be an important component affecting comfor t. For what's it's worth, the floor covering will likely be engineered wood with rugs.

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

[Mathew Newton]

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

reed over would be a bit "wobbly" with a spinning washing machine on it (wa it 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 stati c and spread out. It will be certainly something I'll make sure are factore d into the design.

Speaking of design, or rather considerations for installation, I note from the Kingspan brochure

page 8 specifically) that insulation-above-slab requires the slab to have dried out completely b efore 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 10

0 days which seems rather a long pause in an installation?!

[tabbypurr]

insulation-over-slab and the potential for this feeling slightly warmer und erfoot appeals. Points noted regarding the potential downsides though.

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 mo re solid to me.

tic and spread out. It will be certainly something I'll make sure are facto red into the design.

m the Kingspan 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 say s 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

[Andrew]

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.

[Mathew Newton]

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...

[Tim Watts]

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

[Matt carpenter]

replying to Mathew Newton, Matt carpenter wrote: Have you considered putting 2 layers in? Not sure on your total thickness of insulation let?s say 100mm Put 75mm under slab and 25mm above slab. You?ll 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

[tabbypurr]

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

[www.GymRatZ.co.uk]

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.

[right3rdtime]

I am currently considering this issue and have questioned my architect on this and been advised to put the insulation on top of the slab and then embed the UFH in 55mm screed. This is a garage and concernes over weight from vehicles were dismissed, the correct screed will take the weight of whatever you need it to, in this case an EV.

Thermal mass was discussed and the idea from the UFH engineer was that we were spending more money by screeding on top of a slab when we could have just used the concrete mass to embed the pipes with the insulation under the slab. The best reason I was given for not embediing in the slab and insulating under was that the increased thermal mass of the slab also comes with an increased edge surface are and this loses heat even when insulated. If you have a 125mm slab versus 55mm screed losing heat at the perimeter that will have an effect and more so when your main heating sytem is feeding all its energy into it. This is a different prospect when your heating using wall radiators and only a very small proportion of your heat will end up in the floor.

I'm going to go with the insulation on top and UFH in screed and send as much of the energy into the room as possible, the heating sytem will back off quicker and I hope save some energy.

[Andrew]

Complete nonsense. A garage is an unheated space and is normally outside the thermal 'envelope'. Insulation is pointless if you are only ever *intending* to use it as a garage.

[ARW]

I suppose it depends on your income and what cars you own....

I have worked at houses that have UFH in the garage.

[Theo]

Some garages are integral and it gets complicated how it relates to the thermal envelope of the house. For example it could only be single skin brick between the garage and the house. Perhaps it would be possible to add internal wall insulation, or the alternative is to insulate the garage.

Plus the garage could be a nice cosy workshop or a home office, rather than a place to store your motor or a few tins of oil.

Theo

[The Natural Philosopher]

How insulating a garage with a door wide enough to admit a car, works I do not know.

I've never seen a double glazed draughtproof insulated garage door yet.

[Andrew]

There are a couple near me, sealed with expanding foam which over the years has gone all brown and dusty and looks awful ((from the outside)