Ceiling void advice 100mm or 150mm insulation. Void or avoid?

Bruceslades · 2 Feb 2017

Another question regarding my garage conversion. I am struggling with insulation for the roof.

The garage is detached from the house.

I have recently had the flat roof replaced with a EPDM roof. It sits on 150mm rafters. It is a cold deck roof. I was planning on using 100mm celotex in between and then 52.5 insulated plasterboard. But as it is a garage my height to ceiling is already low. 2290mm. I still have 80mm floor insulation plus 18mm flooring, plus finished floor, so I was thinking that maybe I would fill the ceiling void right the way up to the deck with 150mm celotex and then put a vapour barrier underneath the rafters before attaching uninsulated 12.5mm PB.

Re the 150mm celotex, I know that there should be 50mm ventelation but I can't see how I can have a vented cavity as how would the air flow past the rafters that would be at parallel with the vents - there are no vents at the moment. Or I would have to put a lot of vents along the side of the garage between each rafter. Anyhow I can't see how I can have a vented void. So I was thinking 150mm so that I don't need insulated PB underneath or thinner insulated PB if I do need it. Does this seem reasonable?

I am assuming that the insulated PB is to stop thermal bridging, but I may be completely wrong but I can't see how it's a major issue with 150mm wood. I am happy to be told otherwise. I am an amateur at all this and so any help and guidance much appreciated. I really want to order some insulation so that I can get cracking.

Also, I rang Building Control before I started doing any of this and they told me that I didn't need to notify them about any of the garage conversion. With this in mind, although I am trying to stick to target U values and building regs I am also not tied by them (as far as I am aware).

Sorry for rambling post, fell like my head has been insulated after spending all day trying to work out what to do.

^woody^ · 2 Feb 2017

Fill the roof void completely with insulation, ensuring no voids at all, and insulate continuously across the bottom of the joists, then plasterboard. You'll need a vapour barrier, which could be a distinct membrane, the insulation with taped joints, or vapour-check plasterboard.

You'll need to work out amount and type to meet your required u-value.

motorbiking · 2 Feb 2017

celotex specify a 50mm air gap. i'm assuming you have some sort of uninsulated garage door? in which case over specifying the roof is pointless. A roof U value of <.5. in hkthis situation in more than plenty.

Bruceslades · 3 Feb 2017

motorbiking said:
celotex specify a 50mm air gap. i'm assuming you have some sort of uninsulated garage door? in which case over specifying the roof is pointless. A roof U value of <.5. in hkthis situation in more than plenty.

I will be creating a stud wall on the inside of the garage door and insulating with 150mm celotex and insulated plasterboard.

^woody^ said:
Fill the roof void completely with insulation, ensuring no voids at all, and insulate continuously across the bottom of the joists, then plasterboard. You'll need a vapour barrier, which could be a distinct membrane, the insulation with taped joints, or vapour-check plasterboard.

You'll need to work out amount and type to meet your required u-value.

Thanks woody. I am hoping that if the 150 is packed tightbti the deck then cances of condensation forming will be minimum.

Is it essential to insulate continuously across he joists? I think if so then I will go for thinnest available to maximise ceiling height.

Cheers for the help.

endecotp · 3 Feb 2017

Bruceslades said:
Is it essential to insulate continuously across he joists?

Not doing so obviously increases the heat loss, by an amount that's not difficult to calculate.

But you also have to consider the possibility that the colder areas on the surface of the plasterboard where the joists are will form condensation, or some other cold-related problem.

Using the thinnest-available insulated board seems like a good choice to me. Beware, sometimes the quoted thickness of these boards is the total thickness, sometimes its the thickness of the insulation!

^woody^ · 3 Feb 2017

Bruceslades said:
it essential to insulate continuously across he joists

If the insulation needs to conform to Part L b/regs, then yes. And if you don't want any pattern staining from the joists, yes.

Bruceslades · 4 Feb 2017

endecotp said:
Not doing so obviously increases the heat loss, by an amount that's not difficult to calculate.

But you also have to consider the possibility that the colder areas on the surface of the plasterboard where the joists are will form condensation, or some other cold-related problem.

Using the thinnest-available insulated board seems like a good choice to me. Beware, sometimes the quoted thickness of these boards is the total thickness, sometimes its the thickness of the insulation!

ok thanks. going to go with the PL4015 - 15mm insulation and 12.5 plasterboard.

^woody^ said:
If the insulation needs to conform to Part L b/regs, then yes. And if you don't want any pattern staining from the joists, yes.

Fair enough. definitely going with the thin insulation with vapour barrier and plasterboard included.

TBH not sure if it needs to conform or not? When I rang building control before starting all this they told me that i could just get on with it as it is a garage room and won't be used for sleeping in. I told them that I was putting a shower and toilet in so that I don't have to walk wearing dirty gear through the house and they said this was this OK too. I ran electrics out there and got an electrician to do the connections and notify. Everything else I will do myself. However I want to try to make sure that it does conform to any regs especially as far as insulation is concerned as I don't want the finished room to be cold and unusable.