Thread: Insulate or not?

I have EXACTLY this problem in my studio. It is a converted garage with a tin roof and sarking.

Im not keen to take the roof off, too hard, too expensive.... so I thought about how to insulate effectively.

After all my research I've 98% decided on these polystyrene foam panels:

https://www.justritestore.com.au/hom...ed-polysyrene/
https://www.bunnings.com.au/foilboar...ation_p0810087
https://www.justrite.com.au/products...lation-boards/

They can be stuck up onto the underside of the roof and the manufacturer says they are almost miraculous. I keep thinking to myself of an old fashioned esky. If it can keep a bag of ice frozen for two days, then it must be OK. I've seen all the new houses going up around here use them (as 15mm). Since it hits 40+ in sumer and -8 in winter, they must be sort of OK.

What I like is they are huge, easy to trim and fit, can be stuck up and cheap.

Despite the terrible "insulation" I have now, I'm using a reverse cycle aircon to keep it to 20 deg and 50% humidity and it costs me < $2 a day to run it (It has a kwh usage/output monitor built into it).

Kingspan make some good insulation options as well

My experience is that just doing between the purlins tends to concentrate the condensation issue and create rain in winter, inside the shed. Not really what you want. I think you either do, properly, or don’t do.

the question I suppose is what is the mechanics of the forming of the condensation, and will adequate ventilation prevent the issue occurring?

cheers

Enfield guy is spot on - if you're going to put up roof insulation or sarking, it needs to be in a continuous run so water/condensation is carried away (if done only between purlins, it'll leak out at that point some time).

As Beardy mentions, Kingspan have a product called Insulshed 50, and I've put this immediately underneath the roof sheeting (in between it and the purlins), and then the same on the side walls.

It makes a big difference, and I have zero condensation issues inside the shed, irrespective of outside conditions.

If this is the case, which I don't doubt, why would they sell the polystyrene foam specifically to be adhered to a roof in between joists/purlins? I see it everywhere now (I saw a new building in Fyshwick, an industrial suburb, being fitted with 50mm poly just yesterday. They adhered it to the roof with double sided tape and little clips on the sides.)

I have sarking on my roof now. What would occur if I should use the poly sheets, cut carefully, to fit tightly up against the purlins, underneath the sarking, and use the recommended tape to join them so there are no gaps?

How would this be any different to using the Kingspan Insulshed? I ask for Kingspan refuse to release their R-ratings and the fitting instructions page is broken.

TIA

Edit: here is a sheet where they mention doing just as I wish: https://www.justritestore.com.au/ass...atasheetlr.pdf

Edit 2: a product from Bradford: https://www.bradfordinsulation.com.a...r-your-project

.... Im a bit concerned now. Obviously I dont want some Evil Black Mould of Death or a rusty roof!!!

Better check and see what the fire ratings are for the foam and whether it need to be covered. Here you can't have exposed styrofoam because of the toxic gasses when it burns.

Pete

Foilboard is an excellent product, a well designed system with a choice of those plastic fixing clips with screws to suit the various panel thickness and steel or timber battens. I like that they give you proper R value calculations for the whole roof system not just the panels, makes it much easier to work out what grade you need. While sarking is a benefit (gives you an insulating air gap between the sarking and the foil backing on the eps sheet) it's not essential. All joins are sealed with foil tape, which seals good even with thermal expansion in the building. As its a rigid board any condensation will run off if there's sufficient pitch, but otherwise it'll evaporate away. Never seen it cause a problem.

I have no experience with the other products mentioned. But I can say when you're 4.5m up in a self propelled Tele lift, any kind of rigid insulation is much easier to work with! Oh, and I'm not fond of heights, anything quicker and easier is better!

Originally Posted by woodPixel

If this is the case, which I don't doubt, why would they sell the polystyrene foam specifically to be adhered to a roof in between joists/purlins? I see it everywhere now (I saw a new building in Fyshwick, an industrial suburb, being fitted with 50mm poly just yesterday. They adhered it to the roof with double sided tape and little clips on the sides.)

I have sarking on my roof now. What would occur if I should use the poly sheets, cut carefully, to fit tightly up against the purlins, underneath the sarking, and use the recommended tape to join them so there are no gaps?

How would this be any different to using the Kingspan Insulshed? I ask for Kingspan refuse to release their R-ratings and the fitting instructions page is broken.

TIA

Edit: here is a sheet where they mention doing just as I wish: https://www.justritestore.com.au/ass...atasheetlr.pdf

Edit 2: a product from Bradford: https://www.bradfordinsulation.com.a...r-your-project

.... Im a bit concerned now. Obviously I dont want some Evil Black Mould of Death or a rusty roof!!!

I couldn’t see on your climafoam link where it said it could be used under a metal roof? It does refer to slab with ballast over them which I have done before for and under plywood membraned roofs but I did not see a reference to metal roofing

Foilboard gets my vote. I'll be lining my build in the next couple of months. We did a mate's about 6-7 years ago, easy. Why place between purlins? Fix against the underside of the purlins. Simple strip battens run down the rafters for edge support, and clips/whatever and tape for the butt joints between Foilboard panels. Run them lengthways ridge to wall , rather than parallel to purlins. They require a decent air gap (see the spec sheet) between them and the wall/roof surface to achieve their R value. For the record, the green face is as thermally reflective as the silver face; the green normally faces outward to stop reflection glare during construction - to prevent safety issues onsite/ on adjacent sites/ passing traffic / annoying the neighbours...

Originally Posted by Beardy

I couldn’t see on your climafoam link where it said it could be used under a metal roof? It does refer to slab with ballast over them which I have done before for and under plywood membraned roofs but I did not see a reference to metal roofing

As Beardy has said (I was scrolling down to reply), I couldn't see this either. Then again, I"m no expert, so might be missing something?

All of that aside, given your situation, where you have sarking in place, I see no issue fitting these in between your purlins - in fact (and I probably wasn't clear in my original post), that is how I see this type of product being best used.

If you install as you suggest in your post, I see no risk of condensation in between this product and your sarking (which is what you want) - the area between your sarking and the metal roof will be where your condensation sits, and natural ventilation should dissipate that fairly quickly.

At least, this is the way I figure it, but I"m happy for someone to tell me I'm wrong!