Insulating roof query.

PG.Tips

Hi there, I've done a lot of searching on this forum and others, and am still confused by conflicting advice.

I'm insulating upstairs in our house, it's a refurbishment project.
I'm insulating with the roof pitch.

So far I've put 100mm EPS between the rafters.
Next I'm going to batten and fix insulated plasterboard.

Should I put a vapour barrier up?

If so should it go up before the battens, so the battens hold it in place?
Or should it go over the battens, up against the back of the plasterboard's insulation?

Also, is the 1200g 'Builders Film', basically thick polythene I have suitable to use a vapour barrier?

Thanks.

Calahonda52

what felt is on the roof under the slates/roof tiles?
How is the roof space currently ventilated?
see page 69 and following here:
https://www.nsai.ie/S-R-54-2014-Code-of-Practice.aspx
Its 12Mb so not for the phone:D

whats the purpose of the air gap between the eps and the insulated PB?
If this is not 1000%, yes 1000% airtight then it will negate the eps because you need it to be airtight

PG.Tips

Thank-you.

The roof is actually metal sheet (Non drip cladding), common in old cottages.
It's got a breathable felt, and we also counter-battened the roof to make sure there'd be no condensation issues.
The soffit has a plenty of vents, and there's a good gap behind the insulation I've put between the rafters.
Due to the nature of the roofing and ridge, there's also good ventilation at the ridge.

The air gap between the EPS and PB is just a result of the battens, these are needed to create enough fixing places for the PB.

PG.Tips

Looking at it further, it's the battens that are causing me the issue, as they're creating an airspace, preventing me pushing the PB/vapour layer up tight against the rafters.

This is due to the roof type, as the rafters are a metre between centres, too far for fixing the plasterboard to them, without running battens too.

Mmmm?

Calahonda52

Well then make the battens maybe 50mm and full fill with insulation and finish with plain PB.!

You might even put the the battens so as you can use the small pb boards, a little more expensive but easy to work with in an attic

If its all foil covered eps and all taped up with Alu foil tape then no need for VCL as the Alu is impermeable!

What depth are rafters ?

PG.Tips

Yes, i think you're onto the solution.

Basically I add the battens and then insulate between them, leaving a flush insulated surface to attach the plasterboard onto, after taping.

Sounds like the way to go.

The rafters are 9 inches deep, if I remember rightly.

PG.Tips

I'll have to find out about the smaller boards, I didn't know you could get smaller sizes. That'd be handy for upstairs.

Thanks, again.

Calahonda52

eg
https://www.goodwins.ie/products/id-56.html?filter_set%5B0%5D=4%2C44%2C502&
You will need the 12.5mm one for fire regs and, depending on how you do it, a batten all along the bottom and top

Is it foil covered EPS and are u taping with foil tape??

PG.Tips

Ok, thanks, I'd know it now, will enquire locally.

It's foil covered EPS I used between the rafters.

The battens I have are only 20mm deep, looking at the EPS board it seems to start at 25mm thick, but I could just get some thicker battens I guess.

What's your thoughts/opinion on a product like this ? -

https://buildwithme.ie/2l2-multifoil-30mtr-sq-roll

I wonder could I use something like this between the battens and taped up, before the plasterboard?

Calahonda52

20mm not enough for a 1meter span
As for the link not at all suitable for your project
Is the EPS all in snug and taped up with no gaps etc or have just cut it and have it in place.
Is it supported at the back?

PG.Tips

Ok, so what thickness would you recommend for that span?

I'll go with EPS then, between the battens too.

The EPS between the rafters is good and snug, not taped yet, but I will before fixing battens.
It's not supported at the back, but isn't going anywhere.

Calahonda52

Okay, lets be bold here:
what if I told you with the breathable barrier on the outside you could look at full filling the 9"

Solar gain may be an issue for you as well as what is called decrement delay
http://www.greenspec.co.uk/building-design/decrement-delay/
will the room be occupied, have you windows

PG.Tips

I'm aware I could have filled the entire depth, what with the counterbattening and membrane.
It's down to budget that I haven't.

The two rooms upstairs both have a good large velux on the north face, for ventilation.

PG.Tips

Interesting link, thanks.

Calahonda52

No problem its just that be prepared for solar gain, maybe not that dramatic in your case as there is a 4" gap behind the insulation, and if the ridge ventilation is good then even better.

The denser the insulation the better for solar gain/ decrement delay.

I know a little about this because I live in one that suffers badly from these problems, the big south facing velux only make it worse

I am considering stripping the roof and going at it from outside.

Re the battened, I would think 2" square would be fine, you don't have to fill it with EPS, a service cavity behind the pb would be fine, its just no air gaps in the insulation layers as discussed earlier

PG.Tips

Well Calahonda52, you've helped a lot, so thank-you.

Great to get some info now and get it right, rather than regret something I did in the future.

ATB.

Calahonda52

PG.Tips wrote: »

Well Calahonda52, you've helped a lot, so thank-you.

Great to get some info now and get it right, rather than regret something I did in the future.

ATB.

Atb ditto! Over and out