Insulation Requirement .. R ? & Sarking ?

hi people .. we're getting the insulation as R1.5 for walls and R3.0 for roof .. i was wondering if this is ample enough .. we're not getting any sarking under the roof .. (i know sarking is mostly to keep moisture out) but it does help in insulation as well .. and do we really need sarking?

Btw, we're in Sydney North-West .. where the temperatures are 2-3 degrees higher or lower depending on the season ..

Wasn't an energy rating report done? That will tell you the recommended R ratings.

Not yet.. we were just at tender stage .. the plans are with the architect, so hopefully all the reports and diagrams will come back and state the required .. i was being ready for it ...

Add sarking and you should be OK. Sometimes, you get offered R3.5 ceiling insulation for the same price as R3. If so, go for it too.

Technically any vapour barrier is called sarking. However, in Oz applications it usually has a foil coating. The foil has important thermal prpperties. It acts as a radiant barrier which blocks >99% of solar radiation. The heat from direct sunlight is equivalent to the energy of one 1 kW bar radiator per square metre beaming onto your roof.

In the sub-tropics it can reduce air con energy consumption up to 20%.

It costs 3-5m2 and it is the most cost effective material to help improve summer cooling. Consider also using it on sun exposed walls e.g. west/east wall.

It's usefullness in keeping the house warm in winter is much more limited as most heat loss is not from radiation but by conduction. Foil easily conducts heat outward. That is why you also need batts which slows this form of heat transfer.

Does make any sense to use Air-Cell Glareshield or Insulbreak instead ??

Chris

Composite materials like Air cell and Etherm have some bulk insulation effects but no where as much as batts or cellulose.

Whilst radiation protection is purely based on the surface composition of the material, bulk insulation effectiveness is dependent on the thickness (or more correctly on the number of air spaces that are isolated from one another).

Air cell is relatively thin compared to an R 3.5 batt. The probably are about R 1.0 equivalent (which is inadequate for most climates)

I'm considering using Air-Cell instead of sarking AND the bulk insulation (R2.5 batts) in my stud walls. I've asked around if Air-Cell makes any sense but haven't got a definite reply.

Chris

National Association of Steel-Framed Housing Inc. (NASH) recommend Air-Cell Permishield Insulation for the walls. Appearently it gives a R rating of 1.8 (for the 6.5mm one) or 1.9 (the 8mm thick one).
Would you? Or wouldn't you?

There is lots of options, but what sort of roof do you have (metal / tile / other) - is it pitched or flat or skillion?
dynomite is right, you will get a big benefit from the relective properties of sarking, and anticon or air-cell provide bulk insulation benefits as well. If you have a flat roof I think sarking is required by building code, but steeply picthed metal you don't have to (but you should!). The pay back in reduced heating costs will be very short.

Sorry to hijack the thread

We'll have a tile roof (25 degree pitch). What would you recommend?

I am assuming it is a new house and you are located in adelaide ... it is pretty easy, I would just use any reflective foil sarking (www.bradfordinsulation.com.au, www.air-cell.com.au) under the tiles, ensuring its 'sags' ie has an airspace between it and the tiles and then use well installed (no gaps) bulk insulation (batts) directly on the ceiling between the joists. This should give you the right balance. Sarking improtant under metal roofs for condensation less so under tiles, but essential because of its reflective properties - ie keeping out summer heat.