Quality professional advices at: http://www.casver.com.au/

It depends on how much glass and how long you run the air-conditioning.

Assuming 20m2 of glass, with single glazed aluminium frame with U=6, temperature inside 25, outside 30 (temperature difference = 5). The rate of heat gain through the windows is area * U value * temperature difference = 20 * 6 * 5 = 600W.
If your air-conditioner has an energy efficiency ratio of 3, then that is 200W of electrical power. If you run the air-conditioner for 5 hours a day, that's 1kWh per day.
If you convert to double glazed windows with U=3, then you halve the energy requirements.

Insert your own numbers in the above.

I expect you to get more return for your money with external blinds on the windows.

how much could you actually save in air-con cost if you double glazed a two storey house ?

An interesting benefit of sliding security screens is the reduction in heat through the covered half of the door. I've noticed our west facing glass doors where we recently put aluminium security screens are signficantly cooler than the other panes of glass. I don't have a thermometer to measure but the covered door is 'cool' perhaps 25deg to the exposed pane's 'hot' 45 deg perhaps. Interestingly the aluminium security mesh though black and in full exposure to afternoon sun remains cool as well - perhaps re-radiating and diffusing as quickly as it absorbs heat.

thanks for kicking off discussion thread regarding double glazing - we're effectively working through the list: "draughtproofing, ceiling insulation, slab edge insulation, wall insulation and lastly double glazing" as per Dymonite69 posting some time ago. Just did a posting in another forum regarding underfloor insulation, as have been putting in foilboard under the floor. Was wondering if any can relate experiences with insulation and/or double glazing suppliers. We're in Orange (no evaporative cooling in our house!) - looking at DG Window options. If it can make it work, will order recycled polyester insulation from outside the CW region, and then overlay over current insufficiently thick (about 4 cm) mineral fibre batts.

cheers

When sealing up an exposed rafter ceiling, I would like to know what others think about the idea of sandwiching an insulating material (perhaps XPS) between the new ceiling lining and the undersides of the rafters. The rafters (on 800mm spacings), running the full length of the ceiling, are in contact with the roof and I imagine will be a significant thermal bridge.

Is it worth insulating this bridge and, if so, what is the best material to use?

In a typical brick veneer wall the outer surface will increase the cavity R-value from around R0.16 to R0.6, obviously depending on the emissivity of the anti-glare coating used.

I'm trying to figure out what kind of value the inner surface would be providing if it has no air gap and is in contact with the bulk insulation.

I've seen the single sided RFLs being used and I've always wondered whether that inner surface alone is actually having any insulative effect at all.

Ben1, there is foil on each side of foilboard. You don't put anything against either surface of the foilboard (except at fixing points), so there is an airgap on each side.

ghostgum, if the air gap must ideally be on both sides of the foil, why are foilboard manufacturers bonding foil to their bulk insulation?

"The first (termites) is not a problem where I live"
Lucky you.
Yes, I suppose I was mainly worried about termites not being seen, and possibly other insect nests and vermin.
It is sort of reassuring now, to be able to see all the timber exposed (or at least one side of it)

It is a possible problem, I suppose, but no one has mentioned it until you did. <img src="http://www.ata.org.au/forums/my-plugins/bb-smilies/default/icon_smile.gif" title=":-)" class="bb_smilies" />

The two problems I can imagine are termites and moisture ingress. The first is not a problem where I live and the second will surely become apparent when it reaches the second lower ceiling?

Ben1, I am also thinking of enclosing my exposed rafters, for insulation.
But am a bit worried this will leave cavities (between the rafters where the insulatin is), that can never be accessed or inspected.
Does anyone think this could be a problem ?

You want an air gap on each side.

Given a heat source, the near side of the RFL will reflect most of the heat back. Some of the heat will be emitted from the far side of the RFL. If you touch the far side against something (e.g. wall insulation batts) then it will conduct even more heat than if it has an air gap on that side.

If there is an air gap on one side only, then it I expect it should be on the side with the heat source.

Thanks, ghostgum, that helps me a great deal. The only thing I am wondering about now is which side the air gap must be on. Am I correct in understanding that the air gap must be on the opposite side of the foil to the radiant heat source?

If double sided RFL is used and an air gap is placed on both sides of the foil, will the insulation work in both directions? (With approximately similar performance, as you've indicated.)

Ben1,

From an infrared (heat) perspective, both sides are shiny, reflect heat, and have low emissivity. The anti-glare side is usually outwards to avoid blinding the person installing it or the roof. The anti-glare side is usually blue or green in the visible spectrum, but transparent in the infrared spectrum.

Actually this is not entirely true, the anti-glare side has slightly worse performance than the shiny side, but the difference isn't enough to worry about.

ghostgum,

I am a bit confused about the orientation of RFL. If one side (the shiny side) is reflective and the other side (anti-glare) has low emissivity, then why would you place RFL shiny side down in a roof when you want to reflect incoming radiation in summer? Isn't that the wrong way around? I understand the motivation to keep the shiny side free of accumulated dust, but it just seems that it is facing the wrong way.

Thanks.

Thermilate is an insulation paint additive which could be used to paint the underside of your ceiling. Google Thermilate for some help.
We also have a polystyrene cube which we pump-in to cavities (usually minimum cavity width of 50mm. The product INSULBLOC has a rating of R4 for 100mm. Also good for wall cavity brick veneer, double brick and clad homes. Hydrophobic so it will not track moisture.

I'm intrigued that you think that R5 batts are 150mm deep! What material are you using? <img src="http://www.ata.org.au/forums/my-plugins/bb-smilies/default/icon_smile.gif" title=":-)" class="bb_smilies" /> I understood that fibreglass R5 batts are 210mm deep.

Ben1
Can we assume form your description that the "existing ceiling lining boards" are on top of the rafters?

If you do not intend to remove the roof, then you will have to put RFL underneath the ceiling lining boards, so a film which can be glued to that surface would probably be easier to cut and fit.

With 245mm depth to the rafters there is ample space for 150mm R5 batts. Plus a 30mm air gap above them = 180mm.

You will need to install a false ceiling to support the batts, and fibro or ply or similar board is the easiest and cheapest way.

Simply attach 25 X 25mm battens 55mm up from and parallel to the lower edge of the rafters. Cut board ceiling to width between rafters and slip it up and in to rest atop these battens.

Slide pre-cut to size insulation batts down into the space thus created.

The 'last' one on each row run some fabric strips above the batt to hold the batt down onto the ceiling panel as it is being installed, then once ceiling dropped into place, pull down gently on strip to ensure batt sits down on ceiling no up agains old cailing, then carefully remove fabric strips for re-use on next row.

Sounds complicated but easy once you've done it once. <img src="http://www.ata.org.au/forums/my-plugins/bb-smilies/default/icon_smile.gif" title=":)" class="bb_smilies" />

I am also investigating insulation for closing in an exposed rafter cathedral ceiling. The rafters are 245mm deep, so I am thinking of using R5.0 (210mm) fibreglass batts with RFL of some kind sitting on top of the batts (giving a 35mm air gap between the top of the insulation and the existing ceiling lining boards). My question is: is this the best insulation combination? What sort of RFL should I use?

Regarding potential moisture buildup with foil barriers and celing vented exhaust fans I note that Aircell do make a 4mm floor use foil laminate (R0.12)that is air permeable (lots of tiny holes) rather than the thermal break 6.5mm product(R0.2)which may reduce worry. They are about twice as efficient at keeping heat out as holding heat in. Anyone who has pulled a low sloped roof will know how dirty the sisalation gets with dust dirt & nests etc. The steeper the roof and the less open the roof to the air the cleaner it will keep. If you are relying on foil only then pehars a cheap sisalation above with the appropriate 25-30mm air gap will help keep it clean and give you two good air gaps

I think point that JB is making is not what temperature is reached with the evap cooler (which is usually about 2/3rd the way between dry bulb temp and dew point) but the potential to bring it into the the thermal comfort range of < 18C dew point.

You can pretty much look at the historical BOM data and if your climate is normal > 16 dew point then an evap probably won't be adequate to stay comfortable.

A better estimate of the evaporative cooler temperature is the wet bulb temperature, which is the dry bulb temperature minus Delta-T. The following are the weather observations about 10km from me
http://www.bom.gov.au/products/IDV60901/IDV60901.95867.shtml
The wet bulb temperature is warmer than the dew point.
For explanation of the BoM observations, see the "About weather observations" link on the web page.

It's going to be warm in Melbourne tomorrow, so if you want to see how an evaporative cooler works in Melbourne on a hot but dry 40C day, have a look at the BoM observations tomorrow night.

Now I see what you mean. Dew point is a convenient parameter that encompasses both terms. < 13C and evap will work well. > 18 and it will struggle.

http://www.swampy.net/humevap.html

D69
I don't think that introducing a dew point temperate is confusing and unnecessary, the "real feel" temperates given by weather presenters are based on this. In fact I always use this dew point calculation from the int/ext to decide if I should turn on the AC or just a fan. It's not hard at all, and both humidity and temperate need to be realised to control comfort. Even air pressure plays a role....
I think that if households would use commercial size AC with HRV and de-humidification, with dew point oriented temp control, then household energy consumption would decrease significantly in comparison, and comfort level would increase.

BTW I did say: "Evaporative cooling works by "absorbing" heat from the surrounding air", this is otherwise known as "cooling". I try to refrain from using the "cooling" terminology, simply because most people think that you can "make cold", which you cannot, you can only make "less heat", by making something else even warmer.

Also Thx for the link, adds some other factors to human comfort perception.

Invoking dew point as a means of measuring human thermal comfort is awkward, confusing and unnecessary. You also have omitted the important point that evap AC lowers the air temperature (sensible heat).

Dew point is essentially the temperature at which the moisture in the air will condense i.e. reach 100% relative humidity.

It depends on the initial dry bulb temperature and relative humidity of the air.

Cold dry air has a low dew point. Hot humid air has a high dew point.

Evaporative AC works really well in dry conditions. With very low humidity e.g. desert air the AC can drop temps up to 20 degree.

Human comfort levels are best described with psychometric charts. Empiric studies demonstrate most people can tolerate a certain range of dry bulb temperatures and relative humidity. By superimposing before and after conditions with different cooling systems it is then possible to determine whether the evap or refrigerative AC is required.

See Givoni bioclimatic chart:

http://nceub.commoncense.info/uploads//Saberi.pdf

Fig 6.1.4.

Thanks for the replies guys,

I am at Aldinga Beach about 40km south of Adelaide about 1km inland.

Jeff, we have placed a door in the corridor between the bed/bathroom area and the living/cooking area to enable 'zoning'

With any luck we have designed intelligently and won't need too much heating/cooling, but coming from Northern England 15 months ago we are still not used to 40c and like you say it must be easier to get it right before we start building.

The windows will be openable (is that a word)

We were thinking evap to reduce running costs but I'm new to all this aircon malarchy.

Brentus
If your going for DG make sure you get not-metallic (ie aluminium) and non-conducting frames. The frames themselves can conduct more heat than a single pane glass itself, with DG the difference would be even worse.

On evaporative AC:
Human temperature comfort levels are actually linked to the dew point of water in the atmosphere. By adding moisture to the atmosphere, especially when it is already saturated (humid), will only make it feel even more uncomfortable. Evaporative cooling works by "absorbing" heat from the surrounding air, by warming the evaporated water up to produce more humidity, and by depositing "cooler" humidity on your body. Likewise if you continuously increase the dew point with evaporative AC in a closed house without sufficient ventilation, or reintroduction of dry air to be humidified, it will become uncomfortable. http://en.wikipedia.org/wiki/Dew_point

If you look at the graph and table down that page, you will see that you are just as comfortable with the dew point temperature at 12'C: at 37'C @ 20% humidity (Hot by dry, with possible respiration/skin drying problems though!) also at 11'C @ 100% humidity (Cold but muggy), or the more ideal 25'C @ 40% humidity. You could say that the perceived comfort level is the the point at which the body can regulate perspiration/evaporation against ambient temperature by itself, for it to maintain the body core at 37'C. This is what your artificial AC climate control should allow.

A refrigerated AC actually dries the air and reduces the dew point. If you recycle that air (which is standard in most split and ducted AC systems) the dehumidifying is actually increasing its own cooling efficiency, and at the same time increasing your bodies sweat evaporation rate, that cools you even further. The water in the air is also the best conductor for heat from the humid air to your body, dry air is much better, and allows natural body perspiration/evaporation to function effectively.

If you are concerned about cost and energy consumption, the best way is to limit the areas you climatise to areas of high traffic, insulate and seal these areas as best as possible, (even 2-3x better than the rest of the house, that is if you have doors to separate the zones!) and regulate according to the weather conditions effectively. This way you can get away with installing just a few split system reverse-cycle ACs, and still be comfortable when switching off those not in use. BTW seeing that you said "new house", I can only recommend to get the design spot on before you build. The more effort you put into planning (including getting professional help!) the more cost effective it will be. To design in our minds and on paper costs less than in brick, mortar and tears. <img src="http://www.ata.org.au/forums/my-plugins/bb-smilies/default/icon_wink.gif" title=";)" class="bb_smilies" />

BTW welcome to the ATA forum.

Catopsilia
My plans to get up your way (on my way to see my brother in Grafton and also check out permaculture farms up on the coast) were thwarted as you could imagine by the flooding and road closures earlier this month. Maybe next time.
Regarding effective cooling this time of year. What about the use of water?
I used to have a fernery with a misty spray system just outside the lounge room in a house in Bendigo (yes it could get hot there too)and the cooling effect was fantastic.
The same effect can be achieved (theoretically) by having a strategically placed pond between the room you want to cool and the prevailing wind.
Every bit helps.

Brentus. The answer depends critically on your local climate. If you are in the tropics evaporative won't work even with open windows. If you need significant winter heating (and most houses in Australia use more energy for heating than cooling) then DG is appropriate to keep that heat in and reduce energy loss. If you live in a hot dry climate, like Perth, then evaporative cooling will work and uses less energy than reverse cycle, BUT as you say, it is essential to open the windows and let the humid cooled air flow through. The closed DG windows will still help to keep out the heat until its time to turn on the evap AC. In fact even without evap AC I'd always want opening windows to take advantage of free sea breezes and cool night time air.
I don't really agree with Catsopilia's comments that because you don't use something often it can be cheap and inefficient. I think we should always strive to use devices that are as efficient as possible and that we can afford.
It all really comes back to your priorities - saving money, reducing energy use/GHG ? Paying whatever is necessary to minimise use ? The most "effective" system would probably be fully automated, computerised, reverse cycle AC with individual room controls, but then I was just in an office where this was being used and because adjacent rooms had different set temps, one AC kept going on to the heating cycle because the cold air from the adjacent room kept leaking in and dropping the temp below its set point.