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Concrete under-slab insulation

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Started 9 years ago by sib
Latest reply from AndyG

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sib
Member

Hi there. this may have been answered elsewhere so please feel free to refer me to other threads.

we are in Melbourne. building approx 80sqm2 concrete slab for renovation. 150mm, slab-on-compacted sand on-ground. we will have minimum 50mm slab edge insulation to approx R1.5. internal heating is hydronic, but there will not be in-slab heating. may put timber flooring over slab. slab will get moderate/much northern daylight in winter.

however, seeking views on benefits of under-slab insulation. considering substituting sand substrate for some form of polystyrene insulation product to approx R1.2. given slab is contacting to earth, are there actually passive properties to be gained to argue against under-slab insulation? i am not sure given Melbourne cooler climate.

have also been recommended this product: http://www.greenspecsolutions.com.au/flooring.php.

thanks

Posted Tuesday 31 Jul 2012 @ 5:27:42 am from IP #
Brendan C
Member

hello sib,
From what I understand it will depend on the ground temperatures at your location as to whether you will want to insulate the underside of the slab.
You can use the slab as a thermal mass, that is to store heat energy, only floor coverings such as wood will reduce the effectiveness by effectively insulating the concrete from the rest of the house spaces.
If the ground is around the desirable internal temperatures that you want to achieve then leave the slab bare/un-insulated under-slab. Seeing as it's Melbourne, I would reckon that the ground temperatures will be a little too low, so it's probably best to insulate. Don't forget to isolate the slab from the ground with some plastic sheeting in addition to the polystyrene, unless the polystyrene is totally waterproof it will allow some transfer of moisture, and hence heat.
Lastly, if you're going for thermal mass, using it to stabilize the internal temperatures, the key factors are mass and exposure. The more mass the merrier as a larger mass will store a greater thermal energy. More exposure means that the rest of the house (mostly the air) can come into contact with the mass, reducing the air temperature when its hot down to the mass's, and raising the air temperature when it's cold, up to the masses. Insulative coverings will reduce the exposure - timbers, carpets, linoleums. Non-insulative, or thermally "heavy" coverings in their own right will assist the temperature transfer - tiles, bare concrete (polished?), stone.
Remember that the thermal mass will have to come up to/down to temperature which may take some time depending on the mass size and exposure. In other words that a very large thermal mass may take several months to reach a stable temperature, though most domestic masses will be much quicker. Until then it may make energy use higher but will pay you back large dividends over time

Posted Friday 3 Aug 2012 @ 3:28:11 am from IP #
Greg
Member

Opinions please.
Thinking of doing slab edge insulation with either Compressed Cement Sheet or preferably ModakBoard. I was thinking this as it will make termite protection easy.

CC Sheet properties 0.50W/m ºK so if my conversion is right that is R2.0
ModakBoard 0.44W/m ºK R2.3
although there is a reference here (http://modakboard.com.au/productinfo/ )that states that it is 0.152W/m ºK which would be R6.6 ????? I find that hard to believe.

Termite resistant, basically waterproof, high strength and durability and fire resistance (compared to polystyrene).

Have I got the numbers wrong or would it be OK ?

Posted Tuesday 28 Apr 2015 @ 6:43:24 am from IP #
Bushwalker
Member

Compressed cement sheet is R2.0 ????????????

Doesn't sound right to me, maybe you're using the US scale which is 10x (I think) = R0.2

This might help:

http://www.thegreenage.co.uk/article/thermal-conductivity-r-values-and-u-values-simplified/

Posted Tuesday 28 Apr 2015 @ 7:32:45 am from IP #
Benny
Member

Not US as the units are metric.
As your units are W/(mK) this is not the U value but the conductivity. See here...

http://en.wikipedia.org/wiki/Thermal_conductivity

To get U value (conductance) U = kA/d - where A is area and d thickness.
So if A is 1sqm and d say 20mm (0.02m) then U = 0.5*1/.02 = 25 W/K (per sqm as its 1sqm)
So R is 1/25 = .04.

Posted Tuesday 28 Apr 2015 @ 9:26:32 am from IP #
Chip
Member

Whatever you use for slab edge insulation, you will probably need a termite shield like Kordon glued across the top of the join with the concrete to stop termites getting up that join and into your frame.

I'm currently attaching 30mm Foamular extruded polystyrene foam sheets with compressed fibre cement sheets outside for protection and I think that adds up to about R1.2 . The edge beam of our slab is 450mm deep and has nice straight sides since formwork went all the way to bottom. The slab is on rocky fill which drains very well so we don't have underslab insulation as the dry fill will not conduct much heat away from underneath.

Posted Tuesday 28 Apr 2015 @ 12:08:34 pm from IP #
Greg
Member

Thanks Benny. That looks much better.
Chip, I won't use kordon no matter what. I'd rather use aluminum angle and termite silicone ( like an ant cap ) those chemical barriers scare me.

Posted Tuesday 28 Apr 2015 @ 12:35:23 pm from IP #
Bushwalker
Member

Chip, how does dry fill 'not conduct much heat away'?

Without underslab insulation, the wet poured concrete makes a 100% contact with the fill, putting in place excellent conditions for heat transfer. I agree that wet fill would conduct better than dry fill, but won't dry fill still conduct well?

Posted Tuesday 28 Apr 2015 @ 9:52:54 pm from IP #
Chip
Member

I agree that close coupling of the slab and the ground will allow heat transfer but I'm hopeful that the slab and adjacent fill underneath will stabilise at a higher temperature than it would if it was damp. Maybe at 18 degrees where sun hits the slab and 16 on the southside. We have an insulated 4m by 4m compressed rock mass 600mm deep under the centre of the slab and that will be heated by a solar air heater system in autumn and winter and we figure that will help.

Re Kordon - yes we'd prefer to avoid it as well. Is aluminium an acceptable barrier or is stainless steel sheet required? I was told that we need a certificate from an authorised termite control company to show council before they would give us final occupancy permit but I need to follow up. Is that correct in NSW and if so, would they certify aluminium and could I install that myself or would I have to pay them the exorbitant rates to do it for me in return for that certificate?

Posted Tuesday 28 Apr 2015 @ 11:27:14 pm from IP #
Johnnojack
Member

Chip, Tell more about the "insulated 4m by 4m compressed rock mass" do you have ducts running under/through it? how and where and what size?
Not sure what is so bad about Kordon, it has the insecticide trapped in a felt layer between plastic layers. The only way it is going to release any toxicity is by something chewing it, ie a termite. I have it in my house but only around pipe entrys. Hope to insulate slab in new house so want to know the best way to do it.

Posted Wednesday 29 Apr 2015 @ 1:34:02 am from IP #
Chip
Member

Johnnojack - I have been very impressed with northern US and Canadian houses which build whole house on a couple of metres of super insulated compacted (not compressed!)rock. They have hydronic heating pipes in the bottom and they use solar heated water to store heat in the hundreds of tonnes of mass during the summer. The heat gradually rises and it keeps their slab warm all year.

I didn't want to do anything that elaborate and in any case our subsoil temperature is about 14 and theirs is about 7 so we don't need it as much.

Anyway, I used 30mm XPS foam on base then laid 150mm manifold each end and ran 8 lengths of 90mm aluminium irrigation pipes between the manifolds. Then I filled it all up with a weak concrete made from road base and cement so there is very little entrapped air. At this stage I just have the inlet and outlet 150mm pvc coming out of the slab where built in cupboards are planned.

Eventually there will be a solar air heater (about 20 sq m) on an adjacent garage and air will be ducted all along southern wall for distribution to rooms and/or the under slab heater. Air won't be recovered for entry back into the solar heater as it's not cost effective and having a little positive air pressure inside will reduce cold air ingress plus give more distribution options. On winter days of good solar gain, direct sunshine through northern windows should mean that a lot of surplus heat can be directed under slab to be enjoyed after dark.

The under slab mass could also be heated in autumn to help through the cooler nights that are common near Canberra. It is easy to get 40 degree air out of a solar air heater so the slab could get up to 25 degrees with a bit of luck. If so, we have the option of using some water filled seats to harvest and store even more heat in that area.

It might not be very effective but it's pretty cheap fun!

Cheers

Posted Wednesday 29 Apr 2015 @ 7:16:18 am from IP #
Greg
Member

Chip, I was told by a builder that it was acceptable to use in this fashion. In a warmer climate you could just leave 70mm ( I think that was the number he mentioned) of exposed concrete as a visual inspection. There is also a liquid that can be sprayed on (applied by licenced company) that provides a termite barrier. It is like the silicone in that there are no insecticides present.
Anyway at this stage I'll go back to looking at xps in some form but with an elevated slab the xps would be well above ground and with bushfire potential I'm not happy with it exposed. It all gets complicated again...

Posted Wednesday 29 Apr 2015 @ 11:12:55 pm from IP #
Chip
Member

Thanks Greg - We have 6mm compressed fibre cement sheet on the outside of the XPS foam which will be plenty to stop bushfire damage to the foam which is against the above ground part of the slab. So we have a total of 35mm outside the slab. Flashing will be vertical against timber bottom plate then horizontal over the foam and cfc then be vertical for 30mm down the outside face of cfc sheet with a little kick at the bottom to shed water. So we'll need 70mm of exposed cfc sheet below that flashing for termite observation. Our colorbond cladding will be sitting above the flashing that's above the foam.

I'll contact a local termite company to see what the options are though I suspect that as long as there is an exposed perimeter, council won't think to ask for certification that says there is a termite proof barrier to force termites which come up between the concrete and slab edge to go across to the outside where they will be spotted. Whether we use aluminium or kordon, it will need to be adhered to the slab then across the foam and down the side a bit and then the metal flashing will need to be adhered on top of that with termi proof silicone to stop the little buggers coming out between the kordon and the slab edge insulation and then going back between the kordon and the underside of the flashing rather than up the outside of the flashing where they'll be spotted.

Posted Thursday 30 Apr 2015 @ 3:21:12 am from IP #
LopoJr
Member

Hi all,
What is the thickest EPS/XPS foam insulation that you can have under and around a slab? I usually specify 25-50mm, but anything over this seems to give structural engineers the heeby-geebys. They tell me it's due to EPS and XPS compressive strength.
What does your experience tell you?
Cheers,
LopoJr

Posted Wednesday 21 Sep 2016 @ 5:31:21 am from IP #
pulpo
Member

In North America they do super insulated slabs with 300mm of eps and or xps.
The most I know of in Canberra was 150mm of xps.
Most xps is rated 250 to 350 kpa, I have also used 500 kpa xps.
Although this is 10 % compression rating it's still no problems under concrete slabs.
I use eps H grade (135 kpa) for under concrete slabs for non load bearing areas and xps all load bearing areas.
You have to get engineers familiar with insulated concrete slabs.
If having in slab hydronic heating you should insulated.
The building code requires edge slab if having in slab heating for climate zone 7.
However climate zone 8, the bca requires under slab insulation if in slab heating.

Posted Wednesday 21 Sep 2016 @ 8:43:41 pm from IP #
Catopsilia
Member

Hi pulpo
There seems to be a lot of valuable information in your post. Would you mind de-coding it for me please? Or perhaps give a source for the information?
I am fairly sure that eps and xps are expanded polystyrene and extruded polystyrene, but which one is which? Both names begin with "ex". It is like the old puzzle in telecommunications: RT and WT were different, but the difference was not between the "R" and the "W", but between the "T" and the "T"!
How is a 10% compression rating defined? What is the risk involved in building with that?
Are any of the engineers "familiar with insulated concrete slabs" affordable to home-builders?

Posted Wednesday 21 Sep 2016 @ 10:58:39 pm from IP #
pulpo
Member

XPS extruded, supposedly better for moisture resistance but certainly higher kpa rating.
I know of a few engineers who charge no more for an insulated slab.

Concrete is heaviest when wet, so any compression if not load bearing will happen then.
My biggest problem is getting builders and concreters to do it at reasonable pricing.

My under standing for kpa rating (300kpa) if placed approx 30 tonnes over 1 square metre the XPS will compress 10%.

So 50mm will compress 5mm.

H grade eps has 135kpa at 10% and 58 kpa at 1%.

I'm no expert in this area.

Posted Friday 23 Sep 2016 @ 8:50:36 am from IP #
axess
Member

Pulpo,
The issue is getting a supplier to guarantee the performance specification of the xps? so the engineer and builder won't carry the liability if something shifts later.

Posted Saturday 24 Sep 2016 @ 4:59:32 am from IP #
LopoJr
Member

Interesting. I suppose once the slab is laid not much further compression happens that will effect the slabs integrity when the XPS/EPS moves.

Also, does foil backed insulation work under a slab? My understanding is that it would be so minimal that it wouldn't be worth it.

Posted Saturday 24 Sep 2016 @ 6:24:17 am from IP #
Catopsilia
Member

The strength of a layer of extruded polystyrene under a floor slab could be a critical weakness in the event of an earthquake.
I know severe earthquakes are very rare, but the shallow ones can happen anywhere at all. (Like in Newcastle!)
I would expect building regulations to take earthquakes into account. Perhaps private houses are exempt because there is little danger to the public when they collapse?

Posted Saturday 24 Sep 2016 @ 11:06:46 am from IP #
Bushwalker
Member

I think if we have a significant earthquake, a bit of hard EPS under a slab is going to be the least to worry about.

Have a look here, it would seem that EPS under a building is not a big problem:

http://www.carnationconstruction.com/Techniques/03-03-Techniques-Structure-PolystyreneFoundations.html

Starts about halfway down with:

"Lateral movement in an earthquake
Footings resist lateral movement

One potential concern is whether the house might slip sideways if it's built on a slope and an earthquake happens. The fact that the footings go a couple of feet further into the ground than the slab makes lateral movement unlikely as there is a big mound of trapped undisturbed soil within and surrounded by the footings. There is also undisturbed soil on the outside of the footings."

Posted Saturday 24 Sep 2016 @ 1:59:09 pm from IP #
Johnnojack
Member

Reviving this very informative thread. We are currently organising laying of the slab for our new home and I wish to add edge insulation in conjunction with hydronic underfloor heating. Chip you put a lot of effort into your design, can you update us on how it all went? I'm most interested in the edge insulation but we would all love an update on the under slab mass air heating. Termite treatment, edge insulation and Concreters, unfortunately it is very difficult to have all of them at once.

Posted Thursday 31 Aug 2017 @ 10:58:33 pm from IP #
Chip
Member

Our house is still under construction Jonnojack. Life and other stuff has been slowing us down! As mentioned above, we retrofitted the slab edge insulation. It was going to be a complication to attach it inside the formwork prior to the pour. It was easy to do it later as our trenches for edge beams extended about 150mm outside the finished neat vertical slab edge and right to the bottom of the slab so access was good. Fibre cement sheet outside the foam then trimmed level with top of slab and pest control guy fixed termite barrier starting from 50mm on slab then over the insulation with 50mm overhang which will fold down over the outside edge and be covered by flashing under the colorbond wall cladding. Cheers

Posted Friday 1 Sep 2017 @ 10:25:20 pm from IP #
JB
Member

Chip,

Would you have any photos?

I'm also insulating the slab but having trouble getting my head around how to properly insulate the slab edge for hydronic heating.

Another issue I also have is I'm on a bit of a slope, so one side of the house will have the slab about 500mm off the natural ground level, i.e. drop edge beam. I'm not sure if the drop edge beam is a good or bad thing for slab edge insulation.

Maybe I'll try to fill the gap between the brick and slab with expanding foam or something to help insulate the slab.

Posted Saturday 2 Sep 2017 @ 9:18:45 pm from IP #
Chip
Member

I didn't take any photos of ours JB but if you google "slab edge insulation " and look at images then there are heaps of good diagrams. If you have some slab edge beams very exposed then you'll get all the more benefit from insulation if your night temperatures are cold.

Posted Tuesday 5 Sep 2017 @ 3:38:35 am from IP #
AndyG
Member

Catopsilia said:
The strength of a layer of extruded polystyrene under a floor slab could be a critical weakness in the event of an earthquake.

I insulated under my concrete pool with XPS and was never concerned about either the insulation crushing or any other kind of movement (including earthquakes) as I had piers that went down into the rock below the pool and tied those to the rest of the pool reo.
Just left 400 diameter cutouts in the XPS for the piers.

Any potential crushing of the XPS while the concrete was wet in nothing compared to the load the piers will take once the pool is full

Posted Tuesday 5 Sep 2017 @ 8:16:45 am from IP #

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