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The air in my attic gets superheated during summer, from the sun baking the (clay? terracotta?) roof tiles (I live in Sydney, Australia).

I have good ceiling insulation, and even put in some sarking (foil insulation just under the tiles) but some of the heat still penetrates into the house.

Also, we now have a ducted (central) AC unit in the attic which absorbs a bit of the heat and so cools a bit less effectively.

So I need to cool the attic (area about 100m2). I have 2 vents under the eaves (opposite sides of the house) and one small whirlybird (wind/convection powered thermal chimney).

Should I get more whirlybirds? They are about $100 each and I'm concerned they might not move enough air.

Has anyone used a solar or mains powered roof extracter fan? Can I get one for less than $300 or so?

How about heat-reflective roof paints, are they effective and cheap? (My roof is black and the house will lose a lot of resale value with a light colour)

Any other good solutions?

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Regarding heat reflective paints... In at least some parts of the USA you get an energy tax credit for installing a metal roof of certain colors that reflect heat. So at least the government thinks it's a good idea! This style of roof is more popular in certain areas than others based on aesthetics, local customs and typical weather patterns. I have such a roof and have been pleased with its performance. –  techkilljoy Mar 20 at 14:15
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1 Answer

up vote 3 down vote accepted

Given that hot air raises, have you considered a Passive Stack Ventilation system. Basically:

  • A tall pipe coming out of the top of your loft so the hot air can raise
  • You must not have bends in the pipe or it does not work as well
  • The topping on the pipe must not add much to the flow resistance
  • The sizing of the pipe is important (sorry can’t help on this)
    • Too wide you don’t get fast enough upwards air movement.
    • Not wide enough, you get to much air resistance
  • The pipe must come out of the top of the loft and be high enough above the roof
  • Soffit venting of some type so the colder air can get in at the bottom

The system will only tent to pull air though the loft when the outside air is colder than the air in the loft (just what you need).

Given the correct conditions and design, a Passive Stack Ventilation system will work forever with no running cost or maintenance. Get anything wrong and it will not work at all.

Also check that your ceilings are well sealed, otherwise all the expensive cold air from your AC will just go up into the loft and be vented out.

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Thanks, this is an interesting idea. To make it practical in my particular situation, though, it'd have to be cheaper or more effective than a $100 whirlybird (which also requires no running costs or maintenance). This sounds like you need a skilled installer to design it to the right size for you, which is surely far more expensive than all the options I mentioned. –  MGOwen Feb 8 '11 at 4:08
    
@MGOwen, as I understand it a whirlybird only works when there is wind, will you always have wind when you need to vent? –  Walker Feb 8 '11 at 9:06
    
actually, the same principle that moves the air in the passive stack (or any other thermal chimney) moves air through the whirlybird too. When there's wind, it's a fan, when there isn't, it's still a thermal chimney. –  MGOwen Feb 10 '11 at 5:59
    
@MGOwen, a whirlybird is not a very good thermal chimney as it is not high enough above the top of the loft to generate lot of convetion currents. –  Walker Feb 10 '11 at 9:23
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