Page 2 of 3
1. The manufacturers state the sarking needs to be draped (ie hang down between the purlins) not pulled tight.
2. The reflective surface is only on one side of the sarking.
BTW, I'm fascinated by the explanation of why ventilating the roofspace doesn't help to cool the house.
This was discussed on envirotalk as well.
What is not intuitive is that a heat reflective surface also doesn't radiate heat well either (look up the terms reflectivity and emissivity). Therefore the backing of the foil gets heated up but it can't actually send that heat out the other side. The heated backing however will begin re-radiating the heat back towards the source. Therefore, the foil seems to be 'reflecting' heat back out again. Foil up or foil down doesn't affect its function (as long as the shiny side faces an air gap). If the shiny side touches something than heat no longer travels by radiation but by condution. Since foil is highly conductive, it WILL transfer the heat readily.
The convective flow rates need to be extremely high to remove the radiant heat from the roof. Remember that every 1m2 of the roof surface heated by solar radiation is equivalent to one bar radiator. Can you imagine how much air you need to blow on this to remove the heat.
The second point is that heated air doesn't contain actually much heat energy (look up the term specific heat capacity). It won't warm up the ceiling (which has a higher heat capacity) much at all.
Dynamite - Theory from a couple of sources is all well and good - but be careful of the web not being all true and correct :d
That's why I am careful with my sources.
Research house is a project by the University of Queensland and the methodology is described very carefully.
The Florida Solar Energy Centre has had multiple scientific publications on energy-efficient design and again describes its methods in detail.
You can check out their website for other research that they have done.
As you are aware the scientific method ensures that somebody could read the paper and potentially reproduce the experiment themselves. It is completely transparent to the reader.
I won't base my position purely on commercial sites, advertisements or non-expert conjectures.
We have one on our roof, its light color, so don't think its got anything to do with the roof color. Isnt it to do with the 'five star' energy rating? in our sales contract its added into that section as an additional cost..
I was in the old Perth gaol last weekend and the 1800's gaol has an excellent whirlybird that is a 3 foot diameter fan that vents the entire room through the roof. On a 27 degree day it was naturally spinning with the convected air that was moving through it.
The trick with roof ventilation, is to ensure that you are using it to draw air into the house, and venting out through the roof space ensures you create good ventilation, as well as remove the heat from the roofspace.
As you indicate the example is a different application. If I lived in the roof space, a whirlybird will help by assisting cooler breezes enter and encouraging evaporative cooling of our perspiration. However, it makes little difference to the living space below.
The trick with roof ventilation, is to ensure that you are using it to draw air into the house, and venting out through the roof space ensures you create good ventilation, as well as remove the heat from the roofspace.
One option for existing houses or non ideal airflow designs is to use a system like the cardiffair.
A correct application of whirlybirds which would work. The problem is that most people don't do it this way and get minimal benefit from them.
Dymonite69 wrote:
Quote:
That's why I am careful with my sources.
Research house is a project by the University of Queensland and the methodology is described very carefully.
The only problem is the Research House didn't use whirlybirds for roof ventilation but vents in the gables.
In section 2.0 Introduction
Quote:
wide eaves and a colorbond high pitched (25 degrees) metal roof which has all three (3) gable ends open to natural ventilation.
Also the day that the testing on roof ventilation open was a Dec 2002 day which is possibly the one mentioned as Max temp of 35C with an average of 29C. However the day measured for roof ventilation closed was Nov 2003 and no statistics are given for that day. How do we know that both days were similar?
Yet this observation from the report:
Quote:
The effect of no cross ventilation (Figure 7- November 2003) is that from approximately mid afternoon (2.00pm) the roof space temperature of the closed roof space does not decrease as rapidly by comparison to the open roof space. Figure 7 above suggests the effect of air movement is a slight cooling down of the roof space from about mid afternoon (2.00pm) to later in the evening (6.00pm).
So it does appear that roof ventilation does play an important part in keeping a house cooler.
Do you have any studies that say whirlybirds don't work?
Dymonite69 wrote:
Quote:
The effect of no cross ventilation (Figure 7- November 2003) is that from approximately mid afternoon (2.00pm) the roof space temperature of the closed roof space does not decrease as rapidly by comparison to the open roof space. Figure 7 above suggests the effect of air movement is a slight cooling down of the roof space from about mid afternoon (2.00pm) to later in the evening (6.00pm).
Ventilating the
roof space will help cool it. All the studies I have seen show this. The problem is that I don't live in the roof space. I live in the
space below and the study showed that this had minimal effect on those temperatures.
Quote:
As you indicate the example is a different application. If I lived in the roof space, a whirlybird will help by assisting cooler breezes enter and encouraging evaporative cooling of our perspiration. However, it makes little difference to the living space below.
In the case above at the gaol it is a cathederal ceiling, some 10 or so metres high - so a fan does vent the rooms living/occupied space
Big Kev - As for Queensland - lets say it is in Brisbane - Nov 2003.
On average Nov is 3 or so degrees cooler than dec - was it that year ? jmm.
Wind at 2pm is that less - marginal.
http://www.bom.gov.au/jsp/ncc/cdio/cvg/av?p_stn_num=040214&p_prim_element_index=0&p_display_type=statGraph&period_of_avg=ALL&normals_years=allYearOfData&staticPage=
Steve
BOY, I started to read the link above but I don't have a spare 3 hours to go through the whole thing, because they are relatively cheap so if they make a very small difference I think it may be worth it when you consider the whole cost of the project.
If you put foil in you will make a real difference. Actually about 20% reduction in cooling requirements. And it will be good for your air con system and as a condensation drainage plane.
Whirly's in a humid environment may actually be detrimental to moisture control by bringing in unwanted moist air into the roof space and rotting your timbers or rusting your fixings.
In Vic the foil is required by law (building code) for houses with a roof pitch of 25degrees and below. This will mean that the a WB is not needed for the condensation will run down the foil.
The condensation can happen more in colder climates than hot, ie more condensation in houses in Melbourne than Brisbane. How do you ask? Well the cold air on the outside of the house, cools the moist hot air in the roof from your exhaust fans quicker. Thats what the rep at colorbond told me anyway. But for better advice speak to the plumbing commission.
http://www.pic.vic.gov.au/www/html/7-home-page.asp
PS. If I'm wrong correct me.
So as a test of your theories -
Grab a section of silver sisalation
Do the same with another box at the same time with sheet of tin and some R3 or 4 insulation and tell which one works best.
See figure 3 for a variation on your experiment. Batts vs batts and foil.
http://www.fsec.ucf.edu/en/publications ... N-6-86.pdf
The condensation can happen more in colder climates than hot, ie more condensation in houses in Melbourne than Brisbane.
You have sort of the right idea but the same thing can happen anywhere. What is important is the dew point.
The air contains a known amount of moisture given any temperature and humidity. At a certain temperature the moisture will condense. This is called the dew point. It will occur after the sun goes down and the earth cools.
The higher the humidity, the closer will the dewpoint to the existing temperature. Furthermore, the greater the difference between min/max temps, the more likely you will get dew forming. It may not need to be a big difference if it already quite humid.
When the roof cools below the dew point, condensation will form on the surface.
The answer is to keep moist air from constantly circulating into a cooler roof spaces and cavities in the first place (hence the probelm with whirlybirds, roof vents and soffit vents). The second point is to prevent cold surfaces forming by placing insulation next to them.
Quote:
The answer is to keep moist air from constantly circulating into a cooler roof spaces and cavities in the first place (hence the probelm with whirlybirds, roof vents and soffit vents). The second point is to prevent cold surfaces forming by placing insulation next to them.
I believe this is not quite correct.
Lets use an example of the southern states of Australia. Often the day can become quite warm and cool rapidly in the evening. If the roof space is warmer than the outside temperature, it is more likely that condensation could form on the inside of the roof.
The use of vents or whirlybirds, etc allows the air temperature inside the roof cavity to equalise with the outside temperature much quicker. This means that the dew point is less likely to be reached. Obviously at 100% relative humidity the dewpoint is equal to the current temperature but this situation hardly ever occurs in the southen states. Whereas in the northern states this situation is more likely to occur.
To dismiss something out of hand in a climate diverse country such as Australia is simply short sighted.
Lets use an example of the southern states of Australia. Often the day can become quite warm and cool rapidly in the evening. If the roof space is warmer than the outside temperature, it is more likely that condensation could form on the inside of the roof.
The use of vents or whirlybirds, etc allows the air temperature inside the roof cavity to equalise with the outside temperature much quicker. This means that the dew point is less likely to be reached. Obviously at 100% relative humidity the dewpoint is equal to the current temperature but this situation hardly ever occurs in the southen states. Whereas in the northern states this situation is more likely to occur.
To dismiss something out of hand in a climate diverse country such as Australia is simply short sighted.
BigKev,
I don't quite follow your logic here. The roof space is going to eventually cool down overnight in any case. So the question is if the moisture content in the roof space air high enough to result in condensation.
If the roof space is open to atmosphere than its relative humidity is going to be the same. If it is sealed then it will be two separate systems (and ideally lower).
If the air trapped in the roof space is drier than the outside, the chances of it condensing is going to be less whether or not it cools quickly or cools slowly.
My argument is not to ventilate the roof space and keep the lower moisture content inside it at a stable level (hot or cold). Ventilation may equilibrate temperatures quickly but it also equilibrates humidity as well. If you keep bringing in fresh moist air inside as the roof temperature falls, more and more condensation accumulates.
All the stuff I am saying is pretty much repeated in this document on building science. It covers the principles and then the implementation in different climate settings.
http://www.buildingscienceconsulting.co ... rriers.pdf
Dyno - how many celing to roof spaces have you actually been in?
My house in the hills with vented gables, no sisalation and an iron roof showed no signs of condensation under the iron - and that was the 1880 place with original hardwood trusses.
One of my houses in the tropics - slotted eaves vented roof space, insulated ceiling, with no sisalation - showed no condensation on the trusses in a 10 year roof.
Venting this area in OZ is not a major issue IMO - excepting exhausting bathroom steam into that space - or using it to vent hot air from the house and draw cool air in from below.
And in your article it states this.
Quote:
Vapor barriers are also a cold climate artifact that have diffused into other climates more from ignorance than need.
The history of cold climate vapor barriers itself is a story based more on
personalities than physics.
Rose (1997) regales readers of this history.
It is frightening indeed that construction practices can be so dramatically influenced by so little research and reassuring indeed that the inherent robustness of most building assemblies has been able to tolerate such foolishness.
IMO - Australain climate of the majority of the polulation - excepting some collder areas - the issues of major venting and air movement is not a major issue. But I'll leave that bit to the real designers to work out as required.
So far all my houses have not had an issue, including my current 30 year old house with sislation and ceiling insulation and no major venting features. And I've been in all my roof spaces in winter and summer / wet & dry season - no condensation issues.
Steve
One of my houses in the tropics - slotted eaves vented roof space, insulated ceiling, with no sisalation - showed no condensation on the trusses in a 10 year roof.
the issues of major venting and air movement is not a major issue.
You are probably right to say that ventilating the roof space even in a humid environment in Australia can be safely done in the majority of circumstances without risk of condensation.
My point about moisture control was peripheral to the thread. The real question is there any evidence that demonstrates that ventilating the roof space is a good thing. In short, do we need to bother? Currently, I can't find research that shows that it is necessary or helpful. It doesn't really help much in reducing living area temperatures and the only possible benefit is to reduce the cooling load on air-con ducts in the roof space. A radiant barrier would achieve more than ventilation. Ventilation as an intervention hasn't been show to assist in moisture control.
I believe in the "less is more" principle. If you can get away with doing nothing, then do nothing.
And in your article it states this.
Quote:
Vapor barriers are also a cold climate artifact that have diffused into other climates more from ignorance than need.
The history of cold climate vapor barriers itself is a story based more on
personalities than physics.
Rose (1997) regales readers of this history.
It is frightening indeed that construction practices can be so dramatically influenced by so little research and reassuring indeed that the inherent robustness of most building assemblies has been able to tolerate such foolishness.
This thread has slightly deviated from 'What use are there for whirlybirds' to 'Moisture control and vapour barriers'. Perhaps start another thread about this?
Does anyone know when the whirly birds are installed on a new colourbond roof, our roof went on a few weeks ago but there is no sign of the "Birds" yet!
Q:Well if you have a bathroom or 2 with an exhaust fan in each and love a shower,
with no roof vent where will the vapor go?
A:On the insulation
The fix: attach a fume duct to the roof vent and discharge the vapor outside.
Most of the water entrapment in a roof space comes from the bathroom
Without removing at the point of creation you will be feeding the damp/dry cycle,
your ceiling space will go moldy but you won't see it, though you might smell it.
...until you get pinhead size spores that grow into mold.
Sarking loose and ducted ventilation
Onc_Artisan
PS I am wearing my hat as a waterproofing contractor of 20+ years
hey, vd
if you are building with PD, it should be installed not long after the roon in
nh
We have a colorbond roof with 2 whirley birds located near the shower exhaust fans. Take a drive around and look at light colored roofs and the black along the ridge capping - all those hot showers venting into the roof cavity with no venting. Have never had a problem with mould in the bathrooms - guess it all depends on how hot & steamy you have your shower and whether you put the fan on during the shower or after (which is useless IMO).