Ben1
Member
I am interested in fitting a mechanical roof ventilator (solar powered) such as the Solarvent (http://solarvent.com.au/). Has anyone installed one of these, and if so, how well does it work in practice? Any estimates on reduction of indoor temperatures?
I know the passive roof ventilators ("whirlygigs") are next to useless!
Thanks.
cool at home
Member
There are loads of posts/threads in this forum on the Solarventi, Maestro, Powervent... and probably others too. Do a search and you're bound to come up with a whole host of opinions.
Mine is that solar powered roof ventilators are not the way to go... in most instances you'll get best effect by venting your house and roof at night ('night purging'), replacing the warm air within by the coolest possible air available in each 24 hour cycle... at night... and then starting off the next day with the house as cool as possible. A well insulated house will then heat up, but slowly.
Power venting during the day will only replace hot air with hot air... perhaps a bit less hot than that in the ceiling. While not useless it will almost certainly not give appreciable comfort, leading to other strategies being used to cool living spaces.
Solar powered roof ventilators won't have power when you really need it - at night. The cost of running mains powered ventilators is low.
There is also a view that roof/attic spaces should be very well insulated (bulk and thermal insulation), sealed and left alone. That is, roof ventilation is best avoided totally.
As 'cool at home' correctly says, opinions vary on roof ventilation, so here's another 2c worth.
A 'one liner' that I often use is that '...in Summer your roof should work like an umbrella, and in Winter it should work like a beanie' - yet the current look in many residential buildings is to build homes with black beanies and then wonder why they get hot in Summer.
IF your roof and its insulation was a near perfect barrier to heat, you wouldn't need it to be ventilated - like if it was a metre thick slab of white styrofoam or something like that.
BUT conventional ceiling insulation is generally used in thicknesses that have R ratings of 2.5 - 4.0 (most I've ever used is R6.0 in Tumbarumba) which do an excellent job of SLOWING heat transfer depending on how much heat they are subjected to. R4.0 batt insulation lets heat through at half the rate of R2.0, but it doesn't stop it.
Imagine a typical Summer day with a 20 C overnight temperature and a 35 C mid afternoon temperature. First thing in the morning the roof that has been cooling all night will probably be about 20 C - the house has all the windows and doors open and it's very comfortable.
Assuming it's designed correctly so no Summer sun is coming in through any glass the home will heat up as the surrounding air and the structure both get hotter. (If you have a black roof it just happens faster.)
By mid morning it's already about 26-28 C outside and the house is closed up to keep the interior cool for as long as possible. The warmest air inside the home is up at the ceiling, so if that is 24 C and the air in the roofspace (above the insulation) is only 26 - 28 like the outside ambient air temperature the flow across the insulation will be minimal. If the roofspace is 10 C hotter than that, obviously the heat flow through the insulation will be greater.
In the hot part of the day there will be HEAPS less heat flow to the interior if the roofspace air is 35 C than if it achieves something like 50 - 60 C.
Also remember that air temps are measured in the shade, so if you are comfortable sitting under a shady tree in 35 C - it's not really very hot - still lower than body temperature after all. Try sitting in a closed up car in the sun beside the same tree - that can be fatal. We get used to equating hot car in carpark and hot sun at the beach and hot closed up black roofed house with 35 - 40 C days, when really a place designed for it shouldn't be uncomfortable - like sitting under a shady tree out of the hot wind.
The best Summer roof isn't just one that's ventilated, it's totally open to the ambient air on all sides like a tent fly - completely separating the solar load from the structure it's protecting - but it's an expensive thing to do for 2-3 months out of 12.
If a 'fly roof' is impractical then a ventilation system that is cost effective and can be opened and closed for the seasons makes sense.
JUST ridge vents of ANY type won't do much if there isn't somewhere for cooler air to enter the roof cavity. Even hot daytime outside air is better in your roof than really hot trapped air, and if you make enough openings natural flow will move the air for you, so you don't need any electric or PV powered contraptions.
I tend to use big gable end vents on all the roof ends and eave vents around the sides as well, and only ever specify light coloured roof materials - and in Winter you close them all up.
hamish05
Member
Another 2cents worth.
I installed a “Roof space ventilator” (Solar Wiz) to our tiled roof last summer and I am sure it is invaluable in keeping my house cooler on hot days.
Our double story home faces west, and is badly affected by the afternoon sun. Our living area/s are all on the upper level, we always use sun blinds over the west facing windows.
After the instillation, the temp build-up inside the house was noticeably less than what we had become to expect on hot days. Of course we have "No-Science" to quantify the difference and that is what Ben1 is asking for. However both my wife and I independently formed the opinion that there was distinct difference.
Furthermore and even more importantly I believe the temperature of the air coming from our Evaporative Cooler was noticeably cooler, therefore making it more efficient in its operation. I believe that is due to the ducting being in a better environment.
One concern I have is that it is difficult to know or check if the ventilator is working, as it is difficult to see or hear.
Our house cools nicely at night, as do most in Melbourne.
Eugene
Member
Something else that is coming onto the market is self-ventilating skylights. But as you said, the difference between passive ventilation, and powered ventilation is huge.
But it would be good to get an all in one skylight, solar power, ventilator.
That would be worthwhile.
KimW
Member
Hi,
Bit of recent experience with vented roof space in Perth (using Western Solar's SV12 model).
On a recent day of 40 degrees max, the temperature in the roof space got to 42 degrees max, I have a thermometer in the roof space, placed on the insulation batts. Overnight, the temperature in the roof space regularly gets down to about 20degrees.
Roof is light coloured tin, with Anitcon under the tin and R3.5 batts on the ceiling.
There should be a law against dark coloured roofs - that is my rant for tonight.
cheers Kim
lucas12
Member
The best experience we had with this was using http://www.solarwhiz.com.au as a solar ventilation solution.
This has worked very well for us so far, and it seems to be paying for itself over time. What sort of air flow are you planning to move? That will give you a good idea of what size you will need.
Have a look at 'roof ventilation blog'in google. I found that very helpful
