Slab partial insulation

Kym068mac · Oct 03, 2020 11:36 am

I am planning to insulate under the slab with waffle pod and the East and West sides to reduce heat loss and gain in winter and summer. The house is oriented for maximum Northern exposure with verandah on north south and west on the house.
I’m thinking the pad in winter will heat the slab and the heat will move to the south of the house heating the back rooms. And in summer the opposite.
Does anyone have any experience with this sort of design?

alexp79 · Oct 03, 2020 6:04 pm

You should expect heat to only dissipate from the local area being heated and may be a "circle" around that area, also a lot will depend on what will be your flooring cover. E.g. timber is insulator, so even if slab heats up a bit, you won't be able to feel much through the timber floor.

The level of heat will decrease with the distance from the heat point in squared relationship.

Waffle pods are insulators on their own and I don't think it will be a good idea to insulate under the footing beams from structural standpoint, even though you should be able to find XPS which is thick enough to handle the load, you never know who it will behave eventually in soil. In Europe, they apply at least 100 mm of special 250 kpa+ XPS for insulation under footing beams.

bashworth · Oct 04, 2020 5:35 pm

Extra insulation under the slab is really a bit of a waste.

If you are going to look at slab insulation the edges are more important; (see http://anewhouse.com.au/2013/01/slab-insulation/)

alexp79 · Oct 04, 2020 6:48 pm

The main purpose for under slab insulation is to protect the water in the soil under the slab from freezing (and, hence, from the expansion) in the cold countries. For warmer floors slab edge insulation should be more than enough.

ddarroch · Oct 24, 2020 10:43 am

Firstly, unless you're in a very warm climate, I wouldn't be putting a verandah on the northern side of the home. This will result in too much shade in winter. Ideally you want winter sunshine to enter the home, not be shaded.

An appropriate sized eave is a better option. Shading the home from summer sun, while allowing winter sun into the home.

Read more here.

https://www.yourhome.gov.au/passive-design/shading

As for your slab, is very dependent on your location, importantly, the ground temperature.

In places like Sydney the ground temperature is ideal. So it's a good idea to ground couple the slab. This will result in a warmer slab (& home) in winter & a cooler slab (& home in summer).

In cooler climates like Melbourne it's a good idea to insulate the slab edges. As the ground temperature near the surface fluctuates a lot, with the ambient temperature. The temperature below the slab will not fluctuate anywhere near as much.

In cold climates, like the alpine, it's a good idea to also insulate under the slab, as the ground temperature, even at depth, will be too cold.

Under-slab insulation should also be used if underfloor heating is used.