Insulation

[groeneaj]

Going back to Andy's initial post, it looks like he has Gyprock attached one side of his rafters and the roof purlins the other.

I'm not sure that any of the insulation products discussed are designed for that situation. As far as I know, ceiling insulation is designed to provide a thermal barrier between the roof cavity and the living areas. And to be fully effective the roof cavity needs to be able to exhaust the hot air generated inside the roof from solar radiation to the outside, with replacement air drawn from the under the eaves where the air is typically cooler.

I also understand that it is important to maintain an air gap between the insulation and the roof -- otherwise you get thermal coupling with the hot roof which conducts the heat into the ceiling material.

Perhaps Andy's most effective insulation will be to ventilate the roof cavity, so that the air heated through contact with the very hot roof is exhausted outside the house and replaced with cooler air drawn from the shady side of the building. A forced air flow coupled with 25 to 50 mm of rigid foam insulation might do the trick.

The other option worth investigating is installing a sprayer to wet the roof on very hot days. When I lived in Broken Hill the workshop was fitted with a lawn type sprayer system which sprayed water onto the roof on very hot days. The cooling effect of the evaporation reduced the heat radiating from the iron into the building.

I've been looking at fans to exhaust the hot air out, then grilles around the eaves to draw cooler air in. The roof space is ridiculously hot so this makes sense to me. Perhaps a powered exhaust fan or similar on the side of house pushing air out?
I'll look into the sprayer system, I think one of the neighbours has one so I'll check it out.

[Chris Parks]

I think a major problem with roof ventilation and fans is that the inlets for make up air are too small as are the fans, they need to move a huge amount of air but even if they did it is a losing battle. Adding insulation to the roof cavity creates a heat bank and when the internal temperature should be dropping because the sun has set the heat absorbed by the insulation is released during the night. The only true way to prevent the heat build up is to prevent the sun striking the roof but that while not impossible is a costly exercise. Australian house design is rubbish when it comes to climate control and it is one of the world's mysteries why rooves in this country have not evolved to prevent this heat build up.

[justonething]

Hi Andy
Please bear with me on a bit of environmental thermal science. The transmission of heat transmission is a 3 component sandwich:

• Radiation from a higher temperature object to a lower temperature environment;
• conduction through contact;
• convection through circulation of air.

To use a over simplified model, The roofing iron absorbs the radiation from the sun and can rise to a temperature of 70 deg. The outside temperature being lower will enable the outdoor air to carry some of heat away, but the remainder is transmitted to the inside surface of the roofing iron. Insider the roof cavity, heat will be radiated from the iron to all the surrounding surfaces through radiation as well as heating up the air through convection.
Heat gain from the roof in summer is different than heat loss through the wall in winter in that the surface temperature of the inside of the roof is say 70 deg, While in winter, the objects inside the house are typically in the 20s. This means that heat gain from the roof has overwhelming components of radiation as compared to mostly convection in the case of winter.

What does all that mean? To stop the heat gain in summer, the most effective way is to minimise the radiation heat tranmission, by only 2 means:

1. Shade
2. To provide reflective surfaces to reflect the heat back to the source.

The first thing I'd look at is to provide shades if possible, by planting trees. If it's acceptable, re-paint the roof with the lightest possible colour, subject to own taste and desire. (A mirror reflect most of the radiation and a black belly stove reflects almost nothing). The next thing would be to look at foil insulation at the under side of the roof, maintaining a small air gap between the foil and the roofing iron. I have seen some polystyrene backed foil somewhere that you can cut to size and put it between rafters. They should be good.

The problem with roof ventilation cowls is that it doesn't provide enough air flow to carry away sufficient heat to lower the roofing space temperature. I'd say you'll need 30 air changes an hour to be effective (changing the volume of the space 30 times an hour). The fan would have to quite big and hence noisy.

By the way, I don't think for the amount of money you spend on spraying insulation, it would be a cost effective solution for your problem in summer. You are better off spending money on a evaporative cooler.

[NCArcher]

For where you live Andy, a roof spray system in conjunction with the insulation might be a good idea. That might preclude the use of roof vents though. And while roof vents are ineffective by themselves they can assist the insulation to work properly by lowering the temperature in the roof space. If you get roof vents, buy good quality ones because a squeeky one is really annoying.

[crowie]

G'Day Andy, For what it's worth here's what we did...
We replaced out tile roof with metal adding insulating blanket directly under the new roof [we do have eave ventilation holes around the houses 600mm eaves]...
We already had R2.5 batts in the ceiling space plus 2 whirlybirds and added 2 additional whirlybirds....
The end result is the house has stayed cooler in summer and warmer in winter....
I don't know how that stacks up with the science or what others have said but it works for us who life 15mins from yourselves...
Cheers, Peter

[forrestmount]

We have always implemented the same idea as Crowie however we also add whirlybirds. The basic idea is prevent the roof space reaching a higher temperature whilst insulating the roof cavity from the rooms under the ceiling. This “design” also lets me go into to roof space in summer without the extremes. I don’t really know why I am need to go into the roof space but there always seems to be a reason to go up on the hottest summer days.


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[Fuzzie]

When we moved into our house it had a dark Decramastic style tin roof and no insulation. I could not spend any time in the roof space due to the heat. The first time I tried I nearly passed out and only recovered by jumping into the cold pool to drop my core temp. We reroofed in light coloured metal decking with sarking (not insulated blanket, no whirlybirds) and put insulation batts on the ceiling. I can testify that I can now get into the roof space at anytime of the year and it is a comfortable temperature. With insulated foil instead of the straight sarking, I expect it would be cooler again, but I think it might only be marginal.

The upstairs bedrooms still got quite hot. We have now replaced the largish upstairs windows (north facing) with tinted glass and there has been a considerable drop in the room temps. Hot air rises and not all the heat is coming in from the roof space. Tint film on the glass and opening upstairs windows to allow the rising hot air in the rooms to escape works wonders. We now seldom need AC to reduce heat, it is all about dropping the humidity, the rest of the time a ceiling fan is sufficient.

[groeneaj]

Thanks for the help so far.

Downstairs is really cool, even on ridiculously hot days it doesn't get that hot. I've always thought this was because of the double brick construction. So I've concluded that it has to be the roof/upstairs as it's poorly insulated.

In regards to installing shade/planting trees, this is not really possible. The water sprayer I've briefly looked into but I need to do more research.

Foil insulation was mentioned, but can I install this even though there's sarking attached to the roof? Could I lay the foil insulation essentially on top of the gyprock, leaving a gap between the insulation and roof. Is there any point having two reflective surfaces?

Cheers

[justonething]

Foil insulation was mentioned, but can I install this even though there's sarking attached to the roof? Could I lay the foil insulation essentially on top of the gyprock, leaving a gap between the insulation and roof. Is there any point having two reflective surfaces?
Cheers

Generally speaking, if you are building a new house, you would use reflecting sarking as a combined sarking/reflective foil insulation. The difference between the two is stated here which should answer your question with regard to having two reflective surfaces. But unless you're willing to lift out your roof, you will need a different kind of insulation that you can fit the insulation from inside the roof space. This foil boards or to be exact, the cathedral 25, allows you to cut and fit them in between the rafters or using their special anchors, placed under the rafters.

Now on top of that, you could also have additional insulation batts between the studs in walls or between ceiling joists.

[Brian45]

Hi,
I have installed a few home with insulation. Now there are a number of products you could used. The one I always used was polyester. It A non-itch and allergy free product, which will keep the leader of the opposition happy. Additionally my kids had asthma so I was very careful what I installed in my place. It usually comes in 1 m long by 430 and 580 widths which fit neatly between joists. The walls in your photo not sure what insulation they have used, looks like a mix of polyester, I used to use green for the walls as it had a 2.5 R rating, and white in the ceilings. The mix of polyester in the walls could be part of the reason the room stays hot.

Polyester is more $$$ however I feel it is worth the extra. Just bare in mind don't mess around in the ceiling areas unless the power is off, electrical wires are placed over the insulation not under, light fitting and downlights should have a collar placed around them so insulation is not up against the fitting. If my memory does not fail me I think it was 200 mm clearance around downlight fittings.

To walk around in that void area I would lay a board across the joists to make things easier. I cannot help myself, sorry, whoever shot that roof batten on needs glasses. Whirlybirds will also assist.

Brian

[groeneaj]

I'm really liking the foilboard cathedral 25 that was posted last week. It seems very easy to install and will just slide between the timbers.

https://www.foilboard.com.au/products/cathedral-252

This is a pic of the flat area of the roof where the blow in insulation is. My only concern is that I'm not sure if I'll be able to reach the last section of the roof. If you look at the pic, the bottom right noggin is where I'll be able to push the foilboard through to from the open roof space. From the noggin to the ridge of the roof I'm not entirely sure I can reach. I may be able to squeeze through if I don't eat for a week.

If I insulated everything else and not that last little bit (probably 500mm), would I be wasting my time?

IMG_2894.JPG

[ian]

Hi Andy
could you use a board (or strip of ply / MDF) as a ramp to get the insulation into that last space?

[Bushmiller]

Andy

One comment, which is of absolutely no help to you at all, is that two storey houses are very difficult to cool. To give some idea, in the realms of passive solar housing you cannot have a two storey house! All the heat ends up in the upper storey of course. I would definitely go with whirly vents (several in your case), but as I think has been already stated make sure your ceiling has vents to equal the potential of the whirly birds.

If you have to either repaint of re-roof (both look to be in good order at the moment) I would certainly consider light colours. With the bats I would seriously consider a product other than fibreglass. Our son used to work for a builder and they were more than happy to install insulation, but flatly refused to use fibreglass. Our son installed the polyester type bat for us and these you can tear to shape (in one direction only) to fit the space. Having said that I would also look at other types such a wool etc..

Regards
Paul