Roof insulation - how much is enough?

[Downramp]

Afternoon all

I'm currently installing a colorbond roof on a little weekender shed project that's I've been working on for a while. The rafters are all up now and I'm about to put down the battens to mount the colorbond on. However I'm not sure if I should put the sarking down after the battens, or before.

Following a poke around the forums the accepted wisdom seems to be that keeping a small void between the colorbond and the sarking helps keep the place cooler in summer - so I'm thinking of putting the battens on top of the sarking. After chatting further to the blokes at Bunnies - who admittedly didn't sound confident - I'm looking at using insulation with an additional layer of sarking on the underside, which has an R-rating of 1.5. They reckon it'll keep me warm/cool enough, but I'm sceptical.

So the setup would look like:

colorbond
----------
battens
----------
sarking
----------
insulation
----------
more sarking
----------
gyprock

Ideally I'd go for thicker insulation, but the ceiling's cathedral style and I've built the rafters out of old 4x2 - so I don't have a lot of depth to work with between the gyprock and the colorbond.

What do you reckon? Is a r-rating of 1.5 plus an additional layer of sarking going to be good enough, or am I kidding myself?

[Barry_White]

Downramp

The normal procedure is if you are using insulation you only need one layer of foil with Colorbond.
It goes;

Colorbond
Insulation
Foil.

The best product to use is Blanket with the Foil adhered to it. Bradfords call theirs Anticon which stand for no condensation.

The easiest way to do it is to lay your battens first then lay the Anticon down from the ridge to the gutter one row at a time and then fix your sheeting over the top and just fix your laps first. By doing it this way it will stop the Anticon from blowing away before you get the roofing fixed.

The whole idea is to have the insulation hard up underneath the roofing iron so that it stops the condensation so no air gaps.

After fixing all the roofing go back and fix the roof in between the laps.

Added Comment:
You will find the minimum thickness Anticon is 50mm.

[pharmaboy2]

BTW, the anticon blanket cuts down rain noise on the roof a great deal, so another good reason to use it. FWIW, when i had the opportunity, I used 2 layers of insulation, one was the blanket, and the other was on top of the ceiling joists - particulraly useful and effective for holding in the heat in winter, though you're looking at a cathedral ceiling so i dont know how much extra effect you would get compared to a flat ceiling with roof void.

But putting roofing down on blanket sure as hell made me nervous - its much nicer to be able to see exactly where the battens and the joists are - I suppose thats why the roofers wear dunlop volleys - thin enough to easily feel exactly ewhats under your feet through the blanket!

any tips on that bazza?

[DJ’s Timber]

What Barry says, Anticon is the best for your situation. My shed is lined with it whilst my house has the insulation in the roof cavity on top of the ceiling and the rain is louder in the house but can barely hear it in shed

[malb]

Remember also, that while isulation is great for keeping heat out, once the building heats up it will also trap the heat in with similar efficiency, giving a long cool down cycle.

[Mickj]

I just built a patio using a similar method.
1st rafter
2nd Shade Cloth
3rd Battens
4th Anticon
5th Colorbond

The reason for anticon on a patio was purely for noise control. My bedroon window is just above the patio and I didn't want to be woken by all the noise....it works quite well.

I also put down a layer shade cloth...this was purely to cover up the foil, so that when you look up, all you see is a cream colored ceiling(and rafters of course)

[addo]

In response to the OP's plan, I've used R1.5 blanket, then once roofed strapped batts up between the cathedral ceiling's rafters with fine wire and staples, then sheeted it in. Fitting batts overhead means a decent hat is a must, or you get fibres right down the back and front of your shirt!

Cheers, Adam.

[Barry_White]

BTW, the anticon blanket cuts down rain noise on the roof a great deal, so another good reason to use it. FWIW, when i had the opportunity, I used 2 layers of insulation, one was the blanket, and the other was on top of the ceiling joists - particulraly useful and effective for holding in the heat in winter, though you're looking at a cathedral ceiling so i dont know how much extra effect you would get compared to a flat ceiling with roof void.

But putting roofing down on blanket sure as hell made me nervous - its much nicer to be able to see exactly where the battens and the joists are - I suppose thats why the roofers wear dunlop volleys - thin enough to easily feel exactly ewhats under your feet through the blanket!

any tips on that bazza?

If you want to feel safe because not being able to see the battens and the rafters you could lay down safety mesh before putting the blanket down.

http://buybuildingsupplies.com.au/30...re-p-3705.html

[Downramp]

Thanks all - your advice is much appreciated

[MrFixIt]

Hi

Isn't it interesting how people's opinions vary

So the setup would look like:

colorbond
----------
battens
----------
sarking
----------
insulation
----------
more sarking
----------
gyprock

This is quite a good method and will work well.

Ideally I'd go for thicker insulation, but the ceiling's cathedral style and I've built the rafters out of old 4x2 - so I don't have a lot of depth to work with between the gyprock and the colorbond.

You didn't say if the rafters are exposed or not, so if your rafters are NOT exposed, you have 100mm depth with which to work and this is very useful.

What do you reckon? Is a r-rating of 1.5 plus an additional layer of sarking going to be good enough, or am I kidding myself?

I think it will be better than just the sarking.

When using sarking YOU DO NEED an air gap between the roofing material and the sarking itself for BEST PERFORMANCE.

Aluminium foil, the material used in sarking, IS NOT AN INSULATOR aluminium foil conducts heat VERY EFFICIENTLY. Placing sarking in direct contact with coulourbond roofing will cause the sarking to DIRECTLY conduct heat from the colourbond. Air is a reasonably good insulator. This is WHY insulation batts work, they trap the air within the fibres and help prevent the circulation of the air which would then become hotter in a convection cycle. (That's why convections ovens - fan "powered" - do work, they circulate the heated air.)

Mounting the sarking or Anticon in direct contact with the colourbond will NOT prevent condensation (if it is likely to occur). IMHO any condensation that occurs will be TRAPPED between the sarking and the colourbond material.

Condensation forming under the colourbond or on top of the sarking will "run off" more readily if the two materials are NOT in direct contact.

Many years ago when I built my second storey extension I installed a COMPLETE Jarrah ceiling (oooh it was nice ) dressed exposed 8 x 2 rafters and laminated beams covered by Jarrah flooring.

As this ceiling was effectively directly below the tiled roof the only space to work with was the depth of the battens (25mm). Because of this very tiny space, I laid TWO sets of battens. One set of battens ran vertically from the ridge to the gutter. This set of battens was designed to create an air gap (25mm) between the jarrah "flooring" mounted on top of the rafter and the sarking. Sarking was then installed across these "vertical" battens before installing the tile battens proper.

The nett result was a 50mm air gap between the tiles and the Jarrah ceiling in the middle of which was the sarking. This provided an effective insulation barrier within the limits I had to work with.

As you probably know, hot air rises and will gather at the highest point. It would be beneficial to include some means by which this trapped air could escape.

hth

[Barry_White]

Mounting the sarking or Anticon in direct contact with the colourbond will NOT prevent condensation (if it is likely to occur). IMHO any condensation that occurs will be TRAPPED between the sarking and the colourbond material.


hth

Peter

With my 11 years with Lysaght selling roofing and Insulation 4 years installing insulation and 35 years in the building trade and the information available from the Bradford Insulation site the above statement doesn't quite agree with the facts. The name Anticon describes what it does and that is prevent condensation. By keeping the Blanket hard against the steel roofing will prevent condensation. That is how every commercial roof that the architect calls up insulation for is installed.

The only time I would recommend foil on top as well as the bottom of the insulation would be with a tile roof.

http://www.bradfordinsulation.com.au...CRBL/ACRBL.pdf

http://www.bradfordinsulation.com.au...ding%20P25.pdf

[zongatron]

hey guys,

am i right here in saying that the anticon is an insulating blanket and sarking combined?
Similar to the aircell product?

Could i insulate with only the anticon relatively effectively?

[Barry_White]

hey guys,

am i right here in saying that the anticon is an insulating blanket and sarking combined?
Similar to the aircell product?

Could i insulate with only the anticon relatively effectively?

Zongatron

Anticon is a minimum of 55mm up to 100mm thick fibreglass insulation blanket with aluminium foil adhered to it with a 150mm flap of foil to overlap onto the next row of blanket & foil when laying it.

Have a look at the links I posted in the above post.

And yes you can insulate only with Anticon.

Several Companies make Blanket and Foil. Anticon is Bradford Insulations name for Blanket and Foil.

It is nothing like aircell.

From looking at the details of aircell it looks as if you wouldn't be able to lay it without flattening some of the air cells and rendering it useless.

Anticon would just spring back if you trod on it.

[MrFixIt]

Bradford Insulation site the above statement doesn't quite agree with the facts. The name Anticon describes what it does and that is prevent condensation. By keeping the Blanket hard against the steel roofing will prevent condensation.

Oops, I should have left out the word Anticon in my statement

[Barry_White]

Oops, I should have left out the word Anticon in my statement

Peter

I now understand what you meant now was that the foil on its own wont prevent condensation and that certainly is true. Like you say its not the foil the insulates but the air trapped between roofing and the foil that does it.