insulation...foil up or foil down??

[shads]

hi all...

putting on a new tin roof next week over a deck and extension. im using an insulation and foil blanket... problem is ive been getting alot of conflicting answers as to whether the foil side or the insulation side is up against the tin and whether its under the battens or above?

interested to hear all opinions...

im using corrogated zincalume with steel tophat battens over a hardwood skillion roof frame

cheers

[woodbe]

Foil down usually, mostly to help prevent excess radiation of the installers.

For clean foil, it doesn't really matter which way it goes. Shiny aluminium foil is highly reflective and it has very low radiance, so the effect is very similar whichever way it is installed.

If it gets dusty on top, the reflective nature is reduced somewhat, so on balance, facing down is best.

woodbe.

[PDub]

Hi

I used timber battens over hardwood skillion and blanket went on top - the battens help stop it sagging.

Aside from reasons already mentioned the insulation usually goes against the tin to make a pocket where warm moist air trapped under your roof **doesn't** meet cold colourbond and hence form condensation. Otherwise the roof blanket might as well be sarking with batts underneath.

Not a pro of any kind so feel free to shoot me down...

Paul

[Bloss]

The answer as I've said before is - depends! It all depends on the major direction of heat flow and what its intended to do - foil facing under a tin roof should be on the under side for cold and temperate climates, or on the upper side for tropical climates. :2tsup:

It is important that when rolling out over purlins sufficient sag is allowed in between to let the insulation to expand out fully.

See: http://www.bradfordinsulation.com.au/Bradford/UploadedFiles/0c/0c1d9cbe-55f7-46d7-b4f7-4f4a27bb99b1.pdf

This is a useful document, but I note that the R-value tables are incorrect for the ceilings - my guess is it's an old table that someone has forgotten to update. They suggest R2.5 for Canberra for example and it is R4.0!

A bit of a harder choice in Brisbane where you are as it is sub-tropical neither temperate or tropical! Which probably explains the diversion of views you received. The simple question is for your house which way are you mostly trying to have the heat reflected - and my guess is you want the the summer heat reflected back - so foil should be facing out.

[woodbe]

The simple question is for your house which way are you mostly trying to have the heat reflected - and my guess is you want the the summer heat reflected back - so foil should be facing out.

Bloss, it doesn't matter. The foil will be as effective facing up as down. When it faces down, it doesn't reflect, but its Emissivity (http://en.wikipedia.org/wiki/Emissivity) is very close to the same as its reflectance. It's a quality of polished, shiny metal. Bottom line is the heat doesn't pass.

woodbe.

[Bloss]

Bloss, it doesn't matter. The foil will be as effective facing up as down. When it faces down, it doesn't reflect, but its Emissivity (http://en.wikipedia.org/wiki/Emissivity) is very close to the same as its reflectance. It's a quality of polished, shiny metal. Bottom line is the heat doesn't pass.

woodbe.

Sorry but it does - not much mind (as you say 'very close'), but there is difference and that is why the manufacturers offer different instructions for different climates. It also has an impact on the vapour/ condensation control too for the tropics. But not something to die in the ditch over certainly. In the ACT it is recommended foil down and I reckon if I did work in Brisbane (which I never will) I'd still do it that way for the neater look and lower likelihood of any fibres ever coming off the blanket side. I know that's an oddity to suggest following the 'destructions', but I do read them and find that few manufacturers are ignorant of their own products. (But don't get me started about batt manufacturers and there playing around on R-value ratings and the building standards committees!)

[Pulse]

Some bad physics happening here guys,

Wikipedia (http://en.wikipedia.org/wiki/Radiant_barrier) explains that reflectance plus emissivity equals 1 for a particular wavelength and angle of incidence.

Problem is that everywhere you look websites say that foil insulation "reflects heat" that's because its easy for people to understand because they see that its shiny.

Air is a poor conductor of heat (conducted heat) and foil is a poor emitter of heat (radiant heat). Therefore covering a hot object in foil and surrounding it with air will prevent that object from heating its surrounds.

Bloss explained it well here (http://www.renovateforum.com/f193/anyone-have-ceiling-batts-foil-insulation-under-metal-roof-88636/):

Foil as sarking or even retrofitted is a great addition for summer impact, won't hurt, but doesn't make much to winter where batts are the go. Batts slow the transmission of heat, but do heat up and if the airspace above is hot relative to the ceiling the bats are sitting on then the heat in the batts will be transmitted to the room below.

The aluminium foil has extremely low emissivity so heat hitting the top surface (from the tiles or colourbond) is not radiated to the roof space below. If the roof space temperature can be reduced by even 5 degrees that will improve the performance of the batts in summer and often the temp reduction can be 10-12 degrees or more in the roof and can lower the temp in the house by 3-5 degrees.

The attached is an old document, but explains the issues quite well - the physics haven't changed!


Since most foils are double sided (http://www.icanz.org.au/handbook/typesofinsulation/) with both having a pretty low emissivity, they do work both ways, just better when the antiglare side is facing the heat you are trying to stop, which is the opposite of what you initially think.

So in most places, shiny side down to the roof space prevents heat gain in the roof space.

As Bloss pointed out the foil blanket instructions for Anticon suggest "foil facing under a tin roof should be on the under side for cold and temperate climates, or on the upper side for tropical climates." but this is related to condensation, not to correct orientation of the foil. That why its called "anticon".


This Bluescope article (http://www.bluescopesteel.com.au/files/CTB-111.pdf) explains it. In summary, we all know that moist warm air will condense on a cold surface. So in a temperate place the house is full of warm moist air which is kept away from the roof by the vapour barrier. The bulk insulation also prevents the warm foil from losing heat to the cold roofing.

In the tropics if you had the same arrangement then the warm moist air would condense on the inside of the vapour barrier, wetting the insulation and rusting the roof, so the vapour barrier is kept close to the tin and insulated from the cold room, so that water doesn't condense on it.

Ok all done

Cheers
Pulse

[autogenous]

Aircell - Building Insulation, House Insulation, Energy Efficiency, Australia (http://www.air-cell.com.au/)

[Bloss]

Pulse got in before me . . . . :- Aircell and similar work well enough, but are just a little better than regular foil and are not an equivalent alternative to the composite foil blankets like Anticon.