Silver sided sarking - which way up??

[rsser]

Hi all,

This is where my limited knowledge of physics and the apparent custom in fitting sarking collide :(

The house has a pitched tiled roof with seagrass loose fill insulation in the ceiling. It's OK in winter but not much good at keeping the place cool in summer. I've fitted some rotary ventilators and they help but not enough, cos the ceiling space is like an oven even on a mild day.

I've heard of people fitting sarking to the underside of the rafters to reduce the radiat heat from the tiles warming up the ceiling space. In fact, I have some govt stuff on insulating your house showing just this.

Thing is that it shows the reflective side down, when I would have thought for this purpose you'd want it up wouldn't you??? To reflect the radiant heat back out?

What do you think?

TIA,

Ern

[glenn k]

Yes they show it blue side up, This is because when you are putting a roof on you put the foil down and then the iron or tiles and it is shiny and shines in your eyes. It is less efficient this way but blame the unions for wasting energy.
You can still buy double sided silver thats what I used.

[echnidna]

Try laying a roof over silver sarking on a sunny day, its downright dangerous.

[rsser]

Thanks Glenn.

Any advice on where to get the double sided stuff? Would clearly be ideal.

I'd asked at Robot trading and they said it was rare as hen's teeth.

TIA,

Ern

[kenmil]

Try laying a roof over silver sarking on a sunny day, its downright dangerous.


Try these...... :cool:

[echnidna]

Thanks Kenmil tried that but sunglasses dont help much.

[journeyman Mick]

Even the best sunnies won't stop your eyeballs being fried if you work with foil in the tropics. Don't know about the unions stopping the use of double sided foil - most blokes are self employed in the housing industry up here and we don't need a union to stop impossible work practices. If somebody wanted me to lay double sided silver foil I'd simply refuse. :p

Mick

[Theva]

Ern,

Insulationsolutions make one with reflective surfaces on both sides (sisalation 430 from memory). Try the following link and ask for the local supplier in your area:

www.insulatiuonsolutions.com.au

The upper surface will not stay reflective for long in the roofspace; dust in roof space will take care of that.


Regards,

Theva

[MrFixIt]

Yes they show it blue side up, This is because when you are putting a roof on you put the foil down and then the iron or tiles and it is shiny and shines in your eyes. It is less efficient this way but blame the unions for wasting energy.
You can still buy double sided silver thats what I used.
Hi

Way back in 1972 when I had my first home built, the builders put in the double sided sisalation. When I added my own second storey, I also put in the same double sided material. So of course it had a reflective surface facing up.

I have seen that they now do it with single reflective sided and wondered why they did it with the blue side facing up. My neighbour put in his own and he asked which way was up and was told blue faces up, reflective side down.

I don't know if the unions were to blame - but it would not surprise me if they were :-(

I consider it to be illogical to have the reflective side down, for all intents and purposes they may as well make sisallation with two blue sides!

If I ever have to install sisalation again I will place it reflective side up. While it is a little harder on the eyes than the blue side, selecting a suitable time of the day to install it and suitably positioning yourself on the roof will minimise the inconvenience and the danger.

When I did mine it was on a tiled roof and I did all the work including battens and tiles over the sisalation. Due to the fact that I had a full Jarrah ceiling with exposed dressed 8x2 rafters, I had to put down a set of battens to keep the sisaltion off the Jarrah ceiling timber, then the sisalation then another set of battens for the tiles. I did not find it that difficult :D

regards

Peter

[silentC]

It depends on whether you want to keep the heat in or out, doesn't it? ;)

Sisalation is double sided - both outer skins are aluminium - but one side has a blue coating on it to stop the glare. It probably does make it less efficient in reflecting radiant heat, how much difference that would make under a roof, I don't know. They say to install it with the reflective side facing the air gap, so that means if you're putting it under a roof, it would be blue side up.

I gather it is more useful in stopping air from circulating in the ceiling space and helps with insulation that way.

[MrFixIt]

It depends on whether you want to keep the heat in or out, doesn't it? ;)

Sisalation would prevent much of the heat loss that would normally escape between the tiles (not quite the same on a "tin" roof of course). It also makes a big difference on the amount of "wind" that blows through the roof - this can be a good thing or a bad thing depending on season or climate.

It probably does make it less efficient in reflecting radiant heat, how much difference that would make under a roof, I don't know.Quite a lot I would think, as the blue painted aluminium would then heat up much more than if the reflective side faced the tiles/tin.

They say to install it with the reflective side facing the air gap, so that means if you're putting it under a roof, it would be blue side up.
I think you will find that they mean the air gap between itself and the roofing material, ie the approximate 25mm air gap between the battens and the roofing material. AFAIK the sisalation is laid over the rafters and then the battens and roofing material over the sisalation. This provides the "insulating" air gap between the sisalation and the roof proper. This is how it was installed in my first home and how I did it when I built (myself) my second storey extension.

The sisalation then relfects the radiated heat that is conducted through the roofing material.

This the reason for the "air gap". This air gap allows some air movement above the sisalation and under the roofing material, which in turn helps in the prevention of heat build up.

As you may have seen in a previous post (by me), I had to install battens ABOVE my Jarrah timber ceiling (floorboards on top of the exposed ceiling rafters), then install the sisalation, THEN install more battens for the tile fixing. This left an air gap below the tiles and an air gap above the timber. Air is quite a good insulator especially if the air is moving - blowing between the roofing material and the insulation.

[outback]

I dunno if you put blue up or down. I also know that silver sarking is different to insulation batts in their function.



BUT

Moving air ain't gonna help with insulation.


This the reason for the "air gap". This air gap allows some air movement above the sisalation and under the roofing material, which in turn helps in the prevention of heat build up

[MrFixIt]

Hi Outback

I dunno if you put blue up or down. I also know that silver sarking is different to insulation batts in their function.
It sure is :D The sisalation (sarking) attempts to prevent the heat from getting into the roof (btw it also makes for a good "waterproofing", leaks through cracked tiles are, generally speaking, diverted down the roof space instead of leaking through the gyprock ceiling etc)

Whereas batts are designed to prevent heat transfer though the ceiling.


Moving air ain't gonna help with insulation.Moving air SURE DOES help with insulation. That is the potential heat transfer from the tiles (roofing material) into the roof space. If the air is moving it is more difficult to maintain a thermal transfer.

A prime example of this is the "tropical roof" on some 4WD vehicles. The sun beating down on the "upper" roof cover dos not affect the "lower" roof ie the vehicle roof to anywhere near the same extent as the same vehicle without the tropical roof. This is enhanced when the vehicle is in motiion as the air stream flowing between the two surfaces (upper and lower) helps extract the warm air radiated from the underneath of the "upper" roof.

Obvioiusly there is a finite limit to this. When ALL components get to the same temperature the cooling effect is diminished.

regards

Peter

[glenn k]

Quote
"Moving air SURE DOES help with insulation. That is the potential heat transfer from the tiles (roofing material) into the roof space. If the air is moving it is more difficult to maintain a thermal transfer."

Insulation like bats polystyrene fluff foil bats etc all rely on stagnant air pockets. Moving air is good to cool surfaces but it is a real problem when heat needs to be retained.

[MrFixIt]

Quote
"Moving air SURE DOES help with insulation. That is the potential heat transfer from the tiles (roofing material) into the roof space. If the air is moving it is more difficult to maintain a thermal transfer."

Insulation like bats polystyrene fluff foil bats etc all rely on stagnant air pockets. Moving air is good to cool surfaces but it is a real problem when heat needs to be retained.
Hi

Precisely :D

What we are talking about here, is not the retention of heat but the prevention of the transfer of heat from outside the building via the roof surface into the roof space - subsequently into the rooms below. The fact that
Moving air is good to cool surfaces is not a problem with sarking/sisalation as the idea is to NOT retain heat but prevent heat transfer in essence what we want to achieve is to prevent the sarking/sisalation from getting hot in the first place :D

Kind regards

Peter