21 October 2014

Concrete slab: Normal vs Waffle Pod recommendations

| amar1223
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Hi All, I am looking to build a house in Canberra, and I was given the option to either go with a waffle pod slab with single glazed windows or a normal slab with double glazed windows to achieve the 6 star rating. Do any of you out there have any suggestions as to which option would be best suitable for the Canberra climate (although they are both going to give me the same star rating). I also need to compare the cost of the 2 different varieties of slabs, i.e. normal inground slab vs a waffle pod slabs. Any referrals for a concrete slab company which you have had experience with would be highly appreciated.

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Thankyou all. That really helps. I have decided to go strip footing with double glazing.

Appreciate your input.

Under floor slab insulation will give little benefit compared to double glazed window. If you are looking for added energy efficiency, the answer is definitely strip footing/double glazing.

Generally waffle pods are only used by builders who prefer to use them, sites where you have a flat grade and don’t want to build the slab up, or where you want to minimise excavation of strip/raft footings.

There are no structural benefits between the two. The reactivity of the site will be controlled locally by deepening your edge beams/strip footings for the footing/slab system you chose (waffle, strip footing or raft). However as 90% of Canberra is classified as Slightly/Moderately reactive, there would be little differences between the construction costs of a waffle or strip footings system. It is not until you hit class H, H1/H2 sites where costs benefits come into play to go with a waffle system (current Australian standards do not allow conventional strip footings/in fill slab systems on Class H sites anyway..).

If it were my own home, I would go with a conventional strip footings system and infill slab (only for a class S site can infill slabs be used, ties may be required between the footings and slab in a class M site, or a raft system adopted). I would also suggest if you wanted any added benefit from under slab insulation you look at installing it around the perimeter of the slab only for a width of 1metre. You may also look into thermal blocks being installed between the base brickwork and slab. This will assist with preventing the cold transferring from the ground>footing>base brickwork>slab.

I hope this helps.

Waffle pod (or raft) slabs only really need to be used where there is highly reactive soil or areas of different reactivity. They allow the building to move as one and avoid cracking. Differential movement can occur where normal strip footings and slab on ground are used on reactive foundations. Waffle slabs are, of course, more expensive.

If it’s for thermal reasons definitely go for double glazing. If you want extra thermal protection for the floor, you can specify rigid underslab insulation (polystyrene) for 1 to 1.5m wide around the perimeter of the slab.

And waffle slabs are cheaper not more expensive. Your comment is filled with many errors and bad advice.

I have a standard concrete insulated slab, concrete walls, double glazing and a correctly orientated house. Last winter, most days I needed no supplementary heating.

Waffle slabs are an aid to the builder. They don’t help you at all. Go double glazing and a standard slab. Get heating and solar orientation that will get the thermal mass of that slab nice and warm and it’s comfort all winter long.

Double-glazed windows should reduce the temperature variation within a room, which I imagine would be more beneficial for your comfort.

I think generally waffle pod slabs are likely to be cheaper than a conventional slab, however there have been some instances where waffle pod slabs have had subsidence problems which you should probably check out.

Thermally, a normal slab with double glazing should be better, particularly if the slab is insulated. If you can, you should orient and design your house in line with solar passive principles.

The energy star rating system in Canberra is a bit of a joke.

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