Insulating under slab, much benefit in this situation?

Hi,

I'm getting fairly close to having my builder start on the slab for a new house. I had the building designer specify 50 mm of under slab polystyrene insulation as that added about 1 star to the energy rating. I don't know what specific type of under slab insulation was assumed in the rating software (I will ask the assessor).

I'm in Tassie, so likely cold enough where it's beneficial to insulate both under the slab and on the edge. However the whole right side of my slab is a carport (40 m²) and drawn by the engineer as one continuous piece of concrete just with a 40 mm lower floor level, so it's all thermally linked.

No point in insulating the carport part of the slab of course, but is there much point in insulating the rest of it given the large amount of heat that may be lost anyway? Maybe I should just compromise and insulate under the 100 mm thick sections of concrete and leave the beams alone?

I guess separating the slabs with a thermal break in between could be the best way to go but I'm not sure how that would work. Might ask the engineer. Are there people who specialise in this sort of thing?

Cheers

There is always someone willing to take your money, but you can consider much of this yourself if you are inclined and have the time.

http://www.yourhome.gov.au/materials/concrete-slab-floors

http://www.yourhome.gov.au/passive-design/insulation

In the grand scheme of things insulation is cheap, and you will never get another chance to insulate the slab again (unlike most walls and ceilings). Personally I would cost out the 40m2 of insulation and, unless your budget is really tight, would consider even insulating under the garage. At least do the edges all round. If you don’t insulate the garage, then a thermal break is a good idea. The garage will not be heated (I presume) so you don't want the raw cold in that thermal mass creeping into the slab under the living area (or vice versa with the heat from the house).

Here's a picture of insulation being installed during our build (yeah, not a very good one I'm afraid). The insulation is 50mm foam and very high density.

We had 50mm XPS insulation installed under our slab and on the sides; we haven't moved in yet, but I'm already feeling slightly regretful about not going to 100mm (this is in Canberra). Having experienced houses with and without under-slab insulation, I'd strongly recommend it for anyone in a cool climate - it makes a big difference in winter in particular

After a lifetime of being conditioned to "tiles are hard and cold", it was quite an experience to walk barefoot on tiles, in winter, on an insulated slab.

I agree, though, that the thermal leak that is your carport is a big concern, and I personally would be looking at what thermal separation can be provided (we avoided the issue entirely by having our garage physically separate from the house, connected by a breezeway - thermal performance was one of the reasons for doing this). As it's a carport you should have several options available for separation.

For the rest of the slab, even comparatively small thermal bridges can significantly impair the performance of insulation, so do it all (including beams). To my eye based on what you've posted the total earth-contact area of the beams (base and sides) looks like it's greater than the cross-sectional area of the carport 'join', in which case it'd definitely be a false economy to leave the beams exposed.

arcadelt, it's an exposed carport so even insulating under that part of the slab won't help unfortunately. I've got highly reactive soil hence all of the beams, I wish my slab was flat like yours!

algernon, great to hear of your experience with insulated versus uninsulated slabs. I was tossing up whether to go to the extra expense or not but you've helped make up my mind. That is if I can get a design for the thermal break in time before the slab prep starts.

Do you have any pics of how the insulation was put down? Is it just laid at right angles on the sides of the beams? Does it matter if the ground isn't totally flat under the foam boards?

Thanks!

We had a waffle slab (class M soil on a flat site), and the insulation was just laid as a flat base under the entire slab (with the waffle pods then placed on top). There's a photo of our insulation going down in this post on our build thread (second photo); you can see the slab edge insulation on the inside of the perimeter formwork. The base was levelled with standard concrete sub-base gravel; I imagine that you want it as level as possible to avoid gaps in the insulation (and to make sure the insulation evenly carries the load of the house to the ground across the entire slab base).

There's other ways to do slab-edge insulation by attaching it later, but pouring the concrete directly against it avoids gaps and cavities. The insulation sheets are just placed at right-angles at the edges, and my understanding is that this is also what's done for insulating beams; also see page 6 of this brochure about Knauf ClimaFoam for angled profiles.

Thanks algernon.


Have been doing a bit of visualising in a 3D program today and come up with the below idea for a thermal break between the two slabs. Waiting to hear what the engineer thinks.

I found this resource very useful when it gets to slab insulation and thermal breaks between external and internal slabs:

https://www.laros.com.au/resources-solu ... -drawings/

They are offering some cool thermal break products too.

Could you not just use the same insulation product between the slabs?

Thanks for the Laros link, those diagrams are helpful and those thermal blocks could be part of the solution.

-

It would be using the same insulation product between the slabs, would just need to figure out how to keep the slabs structurally connected without too much messing around/increased cost.

Hi there

Just wondering if you’ve moved in to your house yet and had a chance to feel the difference in the floor temp with the slab insulation? We are building in vic on a stifn raft slab and putting in 50mm under slab and around the perimeter. Would be good to get some real world feedback.

Under slab insulation itself won't help much in regulating floor temp. The floor will be felt a couple of degrees warmer though, but surely not as warm as the inside walls (esp. tiles, so I would still advice to use warmer timber floors if it is a concern).

Concrete slab has a thermal mass effect and the ground temperature is close to constant, so it will be smoothing out significant differences with the flywheel effect, so it will be always something average between the ground temperature and inside temperature.

Insulation works better when there is a difference in temperatures (it slows down the effect), so in your scenario it will be working well towards preventing energy escape through the floor, so overall energy efficiency of your house will be high, but that won't necessarily be projected into the "warm" floors, just "slightly warmer".

thanks for the reply, yes i'm across all that. I was interested to know if the insulation takes the chill off a tiled floor in winter