Now considering that solar insolation is around 1000W/m2, imagine beaming 200 bar radiators onto the roof.

How much moving air or water do you think would be required each minute to keep the roof from heating up?

I too would also like comments on whether it's more expensive to get it done after building... we've just gotten a dark grey tiled roof installed, and no sarking.
By the sounds of it we need it!

Has anyone heard of these; Solatube or Skydome or Maestro??

http://www.solatube.com.au/homeowner/so ... awod-GY4ow


http://www.skydome.com.au/images/pdf/ve ... ochure.pdf

http://www.gaileshardware.com.au/html/h ... 300mm.html

dymonite69 wrote:

There are two studies I am aware of from the Florida Solar Energy Centre and from Queensland Uni that show roof ventilation might only cool the house by about 2-6%. Cooling the roof space does not translate to an equal amount of cooling of the house interior.

The Queensland University paper can be found here.
http://www.solarlogic.com.au/profile/Th ... tivity.pdf
It appears to have been published in 1997.

The pertinent remark regarding roof space ventilation was that providing the ceiling has R2.5 insulation then ventilation of roof space has very little effect on thermal performance. (paraphrasing is mine)

Quite frankly, I can't see how this is in fact a correct conclusion. The laws of physics - in simple terms - state that thermal energy can be transferred by convection, conduction and radiation. Inside the roof space we can discount the effect of radiation since the volume of trapped air inside is subject to heating solely due to conduction from the heated roofing material and natural convection of air withing the roof space. We know that the purpose of ceiling insulation works due to the trapped layer of air inside the insulating medium, and that providing that air does not become heated itself, then heat transfer into the rooms below due to conduction and convection (ideally) will not occur. Now by simple reasoning, if the trapped air in the ceiling space is heated over several days of 35C+ temperatures (as often happens in Perth for example) then there comes a point where the air blanket trapped in the insulation medium must become heated itself (ie. it succumbs to thermal overload), thus allowing heat to escape via the ceiling material and into the rooms below.

Carrying the simple logic to the next phase says that if the lower levels of trapped air inside the roof space do not become significantly hotter than the air inside the insulation, then heat will not be transferred into the rooms below via the insulation medium stressed by thermal overload. This effectively says that if we remove the opportunity for heat transfer via the insulation barrier then the rooms should not become heated due to the excessively hot air inside the roof space. By providing a means whereby the hot air inside the roof space can be replaced with cooler air incoming at a lower level then the opportunity for the air inside the insulation to become heated is therefore minimised and the insulation will be better able to do its job. That is the purpose of roof ventilation with regard to thermal energy transfer.

We see that many non airconditioned industrial buildings have large ventilation vents to evacuate air trapped at the upper layers of the roof space - and this isn't simply to reduce moisture and condensation effects - so it must be done to minimise heat transfer down to the lower levels where humans are working. I am fairly certain that building codes actually mandate such measures. In a home it seems to me to make sense to get rid of trapped air in a ceiling space for both condensation and unwanted room heating so to my mind installing a well designed method of exhausting heated air will reduce heat transfer to the rooms below - especially in locations where several consecutive days of high temperatures are common.

RossH, your arguments seem sufficiently logical.

The problem appears to be that there are no valid or recent studies to draw any such conclusions based on collected data.
There is even no evidence/data that roof ventilation techniques are effectively doing what they claim (e.g. whirlybirds).


The FSEC study was a step in the right direction to help clarify some of the issues, but is not entirely applicable to our local climate and building materials and techniques.

It amazes me that our government can throw vast amounts of our money at things like "insulation for everyone" - but cannot spare a few dollars to commission research in the areas of passive heating/cooling of residential premises.

There are university engineering and science students completing PhDs in all manner of topics, there are resources within the CSIRO etc that would certainly be willing to take this on. In particular, in light of the heavy recent attention on reducing our environmental impact.

RossH,

You have an almost complete grasp of the physics.

Radiation is a significant mode of heat transfer through the building envelope . Solar radiation will heat the roof which in turn re-radiates the heat onto the ceiling and then on to the occupants below.

This is by far the greatest contributor the heat load on a cloudless day (apart from radiation that is transmitted directly through windows)

The best way to reduce this is with radiant barriers through either light coloured or metallic surfaces.

In the case of bulk insulation, radiant transfer heats the top surface of the batt and then is transmitted via conduction/convection to the surface below.

With no radiant protection the surface of the batt could easily reach 70 degrees Celcius. It is also been stated that the R-value becomes temperature dependent at these levels. This is probably due to the increased convective movement of the air pockets.

It is tempting to think that reducing the roof space air temperature will alter the ceiling temperature. However, the impact is not significant.

There are several reasons for this:

• The volumetric heat capacity of the air is comparatively low compared to plasterboard i.e. 1kJ/m3/K versus 300 kJ/m3/K.
• Heat stratifies in the roof space due to natural convection. The boundary layer next to the ceiling or batts is cooler than the mean roof space temperature. If the ceiling heats up, the heated air above it will convect upwards and continues to dampen this energy flux.
• Air has poor conductivity

Therefore, the heat transfer from the roof space air to the ceiling is trivial - the heat keeps wanting to go upwards. In contrast, the radiant heat is significant and is the cause of a hot ceiling. Block this radiation and the problem is largely mitigated.

There is an indirect advantages of having a cooler roof space. It can reduce the heat load on air conditioning ductwork.

But a layer of RFL at $5/m2 has been demonstrated to reduce air con load up to 20% just by reducing the heat flux into the conditioned space.

Finally, I was not referring to the study that you mentioned. This one refers to a simulation. The conclusions drawn about roof space ventilation comes from an actual building called Research House in Rockhampton.

A lot of people live south of Rocky

To my knowledge there are no studies looking at roof ventilation in a temperate climate but the issue remains the same. Foil does a much better job at reducing the heat load than trying to remove it with ventilation once the ceiling has gotten hot. Imagine sitting on every square metre of roof is a portable bar radiator pointing down at the ceiling. Next imagine what kind of breeze you would need to whip up to prevent the ceiling heating up.

Ventilating in a mixed climate brings its own issues. Bulk insulation works best in keeping the warmth in when the air is still. An open roof space is only going to make this worse.

Today is a reasonably warm day in Perth. At 1.30pm at my location the outside temp is 34.2C. Inside the roof space it is 40C. I can't see any difference between the lower level right on top of the insulation or 2 metres higher up but I could only stay up there for 20 minutes so measurements were taken for 10 minutes at each level. The temp under the insulation is 27.1C while below the room temp is 24.4C. Given that tomorrow will be almost identical and the overnight minimum is going to be around 20C (and not for very long either) the trapped air inside the roof space will have a head start on climbing above 40C during the day.

I am just waiting till we get another hot spell with 4 or more consecutive days over 38C and overnight mins of around 28C.... I don't hold up much hope that the 14cm of trapped air inside the insulation will be able to hold out for too long under those conditions. When it finally succumbs (as it must do) there is only the 13mm of plasterboard to limit the temperature due to the extremely hot air inside the roof space coming through that final barrier. I am going to buy me a Powervent 300 as soon as I can afford the $660 and see if it does help reduce the temperature up there.

It will I did and it does

Or $660 buys you 130m2 of reflective foil which will block out >99% of the radiant heat load.

I will look forward to your report before and after measurement of under-surface ceiling temperature with your vent.

Here are the links:

Research House - Temperature Study

Performance Assessment of Photovoltaic Attic Ventilator Fans

I think you can do both. They are both cheap. Just lay concertina foil batts across the top of your bulk insulation, getting where you want to from d69's perspective ... this will cost less than $1000 and does make a difference (I have done it in the last two house I have been in). Then, given the cheapness of the power vent (or csr edmonds), why not do that as well. d69, surely this give you an incremental benefit each time. Also, you should put an eave vent on the south side of the house. Also, you must put a thermostat on the fan ... otherwise it will work when you don't want it to.

If the ambient temp remains a constant like in the tropics the drawn air will be warm air, not cool like in the temperate
areas of Aussie.
Tropics make up about 40% of the country but a LOT less people live there...
... so the main population could have some more benefit from Wbirds IMHO only.

Does insulpaint coating work on the roof? I have also heard of e-vent by Combined Metal Industries and would be interested in them if I needed it living in Perth, thank goodness I requested a surfmist roof.

I thought I would never had to say this but I am starting to reconsider the whirly debate.

A member of another forum who lives in Melbourne has been data logging roof space temperatures during summer.

Interestingly he has shown that roof space temperatures don't fall to ambient levels at night time. It makes we wonder if this is inhibiting passive cooling of the building fabric after sunset. This might also be exacerbated by the presence of insulation and radiant foil

Although I will maintain that whirlys are insufficient at countering the immense heat load during the day, they may help accelerate roof space cooling at night so that the house can shed its accumulated heat prior to the next day.

Of course, this is with the onc_artisan's proviso that night-time temperature do significantly fall.

In the past roof space ventilation was advocated for hot/humid climates to improve comfort but paradoxically its benefit may be actually be more appreciated in more moderate climes.

This is all speculation of course and a proper study for houses south of Brisbane may be in order.

The only concern is that uncontrolled roof space ventilation will not be helpful for retaining heat during winter so some means of blocking it is required during the winter months.

Good to here some more localised testing is happening
Dymonite69... I have to agree. it is my finding also they make a diff' albeit here in the south west.
We here are after all on the same latitude a Sydney,
so I figure without expiriencial data we are quite blind as to the benefits.
And hoping as you know is not good enough.


perthhomelover - I imagine you will get a negative response...
The thing that concerns me with roof painting, is that paint uses surfactants to help ingress the substrate.
Surfactants are what are in your soap
...if you intend to harvest your roof runoff (rainwater) I would advise against it.
kind regards,
Onc

I have enquired about a solar-powered roof vent which was advertised in a local paper during the week. The product is called Solar Star by Solatube. They claim 1 of them is equivalent to 14 whirlybirds...

http://www.sydneydaylight.com.au

No pricing on their website, but there is pricing (US$?) on this one: http://www.ecohaus.com/C-22/solarstar+attic+fans

Hi,
Can anyone help here please. What is the best height/location to install whirly birds on a house?
For example, when looking at the house from the street, should the birds be installed very high on the roof (close to the peak), or further down, maybe half way up the roof height?

Other info: Concrete roof tiles, 25 degrees roof pitch, sarking, 2 whirly birds.

Seeing as heat rises, wouldn't it be common sense to put them as high as possible?

BTW, No response from the company to my email requesting info on the Solar Star a week ago. Slack! (And now 3/3 companies to ignore emails/contact requests through their website in the last few weeks!)