Browse Forums Eco Living Re: Roof space heat - eave vents and whirlybirds ... 2Oct 07, 2009 11:57 am 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 house used eave vents. In contrast foil sarking can reduce air con requirements by 20%. The more significant cause for overheating is direct radiant heat from the sun that heats the roof and then radiates onto the ceiling. Once the house gets heated it is much more difficult to remove it again. You need fairly high ventilation rates to achieve this. Therefore the best way to prevent overheating is to stop the heat getting in the first place i.e. shading the windows, paint roof white, sarking under roof and around walls, trees/plants to shade the walls. The second line is to improve ventilation. The best methods is to direct cooling breezes through the house. Letting hot air upwards by natural convection is the last step if you can't achieve the above but the flow rates significantly less. Re: Roof space heat - eave vents and whirlybirds ... 3Oct 07, 2009 1:56 pm Thanks dynomite69 I was hoping for your input as your posts repeatedly came up in my forum searches on this topic. To summarise, what you are saying in direct relation to my situation ... I could only expect to achieve something much much less than a 2-6% improvement if I was even able to successfully ventilate my roof space. What I'm hearing is that it is not worth the effort - other approaches will have a greater effect. Re: Roof space heat - eave vents and whirlybirds ... 4Oct 07, 2009 2:05 pm For the price of a whirlybird or two though it's worth a go. From my personal experience, installing a pair of them onto the roof of our Perth home immediately and dramatically made a difference. The house was easily a few degrees cooler inside. If the heated air can be expelled from your roof space before the energy has a change to soak into the rest of the house then that's what will make a difference. It won't prevent a heated roof structure radiating heat onto the rest of your house, but I can tell you that for me anyway it was way more than a 6% difference. Re: Roof space heat - eave vents and whirlybirds ... 5Oct 07, 2009 2:13 pm Another method is to paint your roof WHITE. I don't know what you think of the aesthetic aspect of this but I note that you mentioned that your roof is grey. Linky: http://www.abc.net.au/science/articles/ ... 465812.htm Re: Roof space heat - eave vents and whirlybirds ... 6Oct 07, 2009 3:20 pm JilaMint What I'm hearing is that it is not worth the effort - other approaches will have a greater effect. Yes. We have no roof ventilators. Our is just a passive solar design in the Adelaide Hills. During the last summer heatwave with temps in the mid-high 30s for a week, the house interior did not rise above 28. This was when the house was unoccupied with no window coverings and windows left closed. I attribute this to: 1) Good passive solar design - west/east window area <4%, north windows shaded with 450mm eave 2) Radiant foil under roof and on walls 3) Western wall shaded by rainwater tanks 4) Eastern face buffered by garage (which did get hot in itself) 5) Heat reflective paint (this might have been redundant) The south side of the house was also a degree or two cooler than the north side. You can always punch a hole later in your roof for a ventilator if you really it will make a difference. Re: Roof space heat - eave vents and whirlybirds ... 7Oct 07, 2009 4:34 pm Thudd For the price of a whirlybird or two though it's worth a go. From my personal experience, installing a pair of them onto the roof of our Perth home immediately and dramatically made a difference. The house was easily a few degrees cooler inside. If the heated air can be expelled from your roof space before the energy has a change to soak into the rest of the house then that's what will make a difference. It won't prevent a heated roof structure radiating heat onto the rest of your house, but I can tell you that for me anyway it was way more than a 6% difference. Did you already have adequate ceiling batts ? Re: Roof space heat - eave vents and whirlybirds ... 8Oct 07, 2009 4:38 pm bigdenis Another method is to paint your roof WHITE. I don't know what you think of the aesthetic aspect of this but I note that you mentioned that your roof is grey. Linky: http://www.abc.net.au/science/articles/ ... 465812.htm I don't think it will make that much of a difference for colourbond. The fact that it is corrugated, made of steel and somewhat reflective (shiny enamel-like finish) would override the colour I imagine ... but I'm just guessing, have no numbers to back that up. There's probably also a good reason why you don't see too many white colourbond roofs - the glare would be a problem I would think. Re: Roof space heat - eave vents and whirlybirds ... 9Oct 07, 2009 5:25 pm JilaMint don't think it will make that much of a difference for colourbond. The fact that it is corrugated, made of steel and somewhat reflective (shiny enamel-like finish) would override the colour I imagine ... but I'm just guessing, have no numbers to back that up. There's probably also a good reason why you don't see too many white colourbond roofs - the glare would be a problem I would think. Actually FSEC also looked into this. White roofs work. It is one of the most cost-effective things you can do. True that metals are reflective. But reflectivity is a surface property so there is no contribution of the underlying material if the coating is non-reflective. Re: Roof space heat - eave vents and whirlybirds ... 10Oct 08, 2009 11:09 am dymonite69 Actually FSEC also looked into this. White roofs work. It is one of the most cost-effective things you can do. True that metals are reflective. But reflectivity is a surface property so there is no contribution of the underlying material if the coating is non-reflective. Thanks dynomite69 I tend to agree with you that "white roofs work" - but there is no specific data comparing white metal against darker metal roofs. A Bluescope technical report says that most colours in the Colorbond product range will have a similar gloss level and might be expected to give similar amounts of reflection. Which was the point I was trying to make about Colorbond when I said - "I don't think it will make much of a difference for Colorbond". The coating is sufficiently reflective that a lighter coloured variety would make little difference. I found the FSEC study that you were referring to. For anyone interested - the light version of the report is here: http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-1220-00-es/index.htm Interestingly, the control house has a dark shingle roof and there is no house in the study with a dark metal roof (only a white one). There's a CSIRO thermal modelling study (which I can't find anywhere) that apparently shows zinaclume (which is basically un-painted colorbond as far as i can tell) steel roof reduces the cooling energy usage to less than half compared to a red tile roof with no other insulation present. With insulation present the cooling bill is 15% less for the zincalume roof. That kind of implies that the "insulation" has a greater overall effect than the roof type - even though in this case the roof type is still a significant factor. Lots of bits and pieces of data, but the question still remains, if I reduce the roof/attic space temperature via some ventillation method, will that make any difference to the internal house temperature or to the A/C efficiency considering that the ceilings are already sufficiently insulated. I'm leaning towards thinking that it might make only a slight difference. Thudd tells us it was a noticeable difference for his house. Re: Roof space heat - eave vents and whirlybirds ... 11Oct 08, 2009 1:29 pm re Colorbond and Zincalume There is no good data on emissivity values for colorbond colours. Bluescope have not published them. A coating with low emissivity has high reflectivity. Interestingly Bluescope are releasing a new range of colorbond with a new heat reflective coating. This suggests that there is room for improvement with the basic range. The only info I came up with is from a commercial site advertising heat reflective paint: http://www.skycool.com.au/john%20bell%20qut%20paper.pdf They say the absorptivity is 50% of Colorbond. (colour not stated) In comparison foil has a reflectivity of > 99% (i.e. emissivity of < 1%) re emissivity, reflectivity Reflectivity of non-metallic surfaces is not always predictable. Generally light colours are better. The visible light reflection is usually an indicator but that is not always the case. 'Glossiness' is not a good indicator. re roof vents I can't comment on Thudd's anecdote. All studies that have looked at this as a proper before and after or side-by-side comparsion have not shown roof ventilators, ridge vents or soffit vents makes much of a difference. If it worked reasonably well, I would have expected at least one study that showed it made a difference. The Your Home technical manual now also says that whirlys don't work (after saying for years that it did) Re: Roof space heat - eave vents and whirlybirds ... 13Oct 14, 2009 10:36 am dymonite69 All studies that have looked at this as a proper before and after or side-by-side comparsion have not shown roof ventilators, ridge vents or soffit vents makes much of a difference. If it worked reasonably well, I would have expected at least one study that showed it made a difference. The Your Home technical manual now also says that whirlys don't work (after saying for years that it did) Did that study included Eco-vents - the thingys that open up when the under roof temp hits 35 degs ? Chris My father rode a camel, I drive a car, my son flies in a jetliner, his son will ride a camel.Saudi saying Re: Roof space heat - eave vents and whirlybirds ... 14Oct 14, 2009 10:30 pm In the absence of a radiant barrier, ventilating a roof space with a whirly is insignificant. Here is the rationale: The roof space air temperature contributes a small heat load into the living space because: - Functionally the roof space and living space are separate systems - Natural convection in the roof space results in a much cool boundary layer at the top surface of the ceiling (lower than the mean air temperature) - The volumetric heat capacity of air is low i.e. it doesn't contain a lot of heating energy - The thermal conductance of air is low - Trivial amounts of heat energy is actually conducted to the ceiling material and hence to the living area In contrast a significant contribution to the heat load is radiant heat from the roof which is generally an order of magnitude higher. Yet the convective thermal flows removed with passive fan vents are small compared to the radiant heat. To purely negate the radiant heat load by ventilation would actually require about 1 air change per minute. as recommended by the US Dept of Energy. The largest impact would be to duct a roof ventilation system directly into the conditioned area to draw hot air away from the occupants - not the roof. Re: Roof space heat - eave vents and whirlybirds ... 15Oct 18, 2009 8:08 pm I've been thinking about the question of colour... and then it struck me. My work colleague and i have the same model car. Same year, same manufacturer, same model. Hers is black, mine is white. When parked out in the sun, my interior gets pretty hot, but hers is like an oven!! I can't comment on other colours, but ours are at opposite ends of the spectrum and the difference in temperature is subjectively substantial (we've never put thermometers in our cars to quantify this though). Re: Roof space heat - eave vents and whirlybirds ... 16Oct 20, 2009 10:15 am bigdenis Hers is black, mine is white. When parked out in the sun, my interior gets pretty hot, but hers is like an oven!! That's the classic difference between heat reflection and heat radiation. Re: Roof space heat - eave vents and whirlybirds ... 17Oct 26, 2009 8:47 pm Jillamint - roof vents will make a bit of a difference - but IMO Adelaide heat is not as intense as WA heat and the benefits may not be as much as you require from the sounds of your posts. As for the grey roof versus white roof - certainly makes a difference, http://www.colorbond.com/index.cfm?objectid=8B47C5CE-A52F-FE7E-1932836BE38355FB but I suspect that the heat is getting in via other means. ie exposed windows, non insulated walls, walls that are heating up during the day, or more likely non insulated areas of your ceiling where the insulation it is not is installed properly. Re: Roof space heat - eave vents and whirlybirds ... 18Oct 26, 2009 10:10 pm We have chosen an Ironstone roof (dark grey) and while it looks terrific it will obviously get hot, we have opted for a couple of whirly birds and the whole house is insulated but we now wish we spent the extra (about $3500) and got the sarking, I was up in the roof last week with our Air Con friend and it was only mid 20's and it was VERY hot up there, admittedly the Whirly Birds are in yet but a high 30's + it will be an oven up there! We might go through a season and after that see if it's worth getting sarking, does anyone know if it's MORE expensive getting it done post building ? Re: Roof space heat - eave vents and whirlybirds ... 19Oct 27, 2009 8:09 am From The Your Home Technical Manual - Common Myths (click on link) "Myth: Roof ventilators will keep your house significantly cooler. Fact: Roof ventilators do not make an appreciable difference to house temperatures if the roof is insulated, particularly if reflective insulation is installed. If your ceiling is uninsulated a ventilator might make a small difference, but insulation is a better investment. There may, however, be other valid reasons for installing roof ventilators such as moisture removal." Re: Roof space heat - eave vents and whirlybirds ... 20Oct 28, 2009 7:24 pm dymonite69 In the absence of a radiant barrier, ventilating a roof space with a whirly is insignificant. - Natural convection in the roof space results in a much cool boundary layer at the top surface of the ceiling (lower than the mean air temperature) And guess what will happen to that boundary layer if you introduce enough ventilation to stir up the air space without actually allowing enough air out to take much heat away! I believe an experiment could well find that only having a couple of eave vents or a single small whirlybird could be worse than having none at all. "Now, supposing a house to have a southern aspect, sunshine during winter will steal in under the verandah, but in summer, when the sun traverses a path right over our heads, the roof will afford an agreeable shade, will it not?" -- Socrates, ca. 400 BC Shade the brick with large eves. Since going to minimum of 600mm eaves on my build, I've noticed much improved cooling off the house after hot summer days. Can see the… 1 4869 Old Home Restoration / Renovation Cheers mate! This is exactly what I was thinking but wasn't sure if it was something I'd find on the shelf. I agree with you, fixing the brackets to the board will look… 2 4986 retail its around double the price of a similar sized actron/dakin system from memory, They are excellent systems though. But with how builder gouge on AC/heating, you… 4 17032 |