Anyone have experience / advice re garage roof insulation?

[skotl]

Following on from my Rust! thread, I was going to take the advice to insulate the roof of the garage.

My plan was to fit this Celotex stuff and run it down the lengths of the rafters, leaving 50mm airgap as the manufacturer recommends.

Only problem is that the manufacturer also recommends that there is ventilation in the void above panels. This is what my roof looks like;

So that would suggest that I'd need to drill a hole in the board and mount a grill for every run? There are only ten of them, but it does seem a tad OTT to have ten grills on the outside facing.

What do y'all think?

Ventilation required?
Every run?
Or maybe every fifth run with vent holes through the rafters?

Advice appreciated!

 

[Kent]

I suggest you bond the boards on and fill between joists with exp foam. Forget venting a space if you can make it not exist to my mind. BTW were are your 3x2 's between?

 

[Shedendman]

It's a garage,is it worth all what they say on the tin?
If you know its water tight,few coats of oil based paint stick up some rockwool etc and board it up with plasterboard or ply,why spend money with celotex?

 

[Tigman]

Kingspan is what the builders use , and vent the soffit's with a few holes and stick the soffit vents in !

 

[Tigman]

or is it facias ? one or the other !

 

[skotl]

I suggest you bond the boards on and fill between joists with exp foam. Forget venting a space if you can make it not exist to my mind. BTW were are your 3x2 's between?

Not sure about ignoring both bits of the manufacturer's advice! Are the 3x2s short battens between the rafters? If so then nope - none of them! However I could put them in after I mount the boards, both to reinforce and hold the boards up.

 

[skotl]

It's a garage,is it worth all what they say on the tin?
If you know its water tight,few coats of oil based paint stick up some rockwool etc and board it up with plasterboard or ply,why spend money with celotex?

as above rockwool and plasterboard. keep it simple

TBH, that sounds a lot more work. I 'kin hate painting and I particularly hate painting ceilings

The cost of the celotex from Wickes is going to be around £100 so I might save ~£30-£50 (?) but cost a lot more in hassle?

 

[Shedendman]

They're called noggings,they stop the joists from twisting,i wouldn't really worry about them tbh,again why spend more money on extra timber?

 

[skotl]

They're called noggings,they stop the joists from twisting,i wouldn't really worry about them tbh,again why spend more money on extra timber?

Cheers. And it's just a hardboard roof with felt, so no weight up there.

 

[Shedendman]

The reason why i said paint it is to give the contact glue a better hold to stick the wool up,you dont have to,from what i see of the underside it looks pretty clean board,god i've smacked in a few 4'' nails every 18'' or so either side of the joints just to keep the stuff in place once its up just plaster/ply board it over

 

[J.B. Welding]

Hey There Bud. The air gap above is to prevent condensation forming on the cold underside of the roof board thus stopping mushrooms etc growing up there, but are you intending to plasterboard/sterling board under the joists? If so then 'stick' or staple some rockwool up there its much simpler and cheaper-but if you are going for a closed cell, foil-backed insulation board finish (generic branding ) then you need to get it airtight all around or you will lose the benefit. Even with the insulation tight up to the roof you will still get condensation in there which will turn to stinky mould without the air circulating to keep it dry.

Noggins are really called dwangs you know-in Scotland anyway

 

[skotl]

Hey There Bud. The air gap above is to prevent condensation forming on the cold underside of the roof board thus stopping mushrooms etc growing up there, but are you intending to plasterboard/sterling board under the joists? If so then 'stick' or staple some rockwool up there its much simpler and cheaper-but if you are going for a closed cell, foil-backed insulation board finish (generic branding ) then you need to get it airtight all around or you will lose the benefit. Even with the insulation tight up to the roof you will still get condensation in there which will turn to stinky mould without the air circulating to keep it dry.

Noggins are really called dwangs you know-in Scotland anyway

Cheers for that. So if I use celotex a generic foil-backed board with an air gap, what's your thoughts on the ventilation? I'd have to vent each run?

 

[DecK]

Is there a cavity in the wall or is it single course block? If theres a cavity and it hasn't been closed up it should provide enough circulation between the joists once you can vent the cavity.

 

[J.B. Welding]

You could try without for a year and then pull a board down and have a sniff-then decide? But ideally, as ever, if your gonna do a job.........
Is there an overhang on the roof outside where you could possibly create an air gap, or is drilling holes in the facia the only way?

 

[Shedendman]

Come on,own up,the wifes got the hump with you spending so much time in the garage she's told you to go b****y live in there,so your getting it all snug ready for winter,lol
I see a wood burning thread coming along,lol

 

[rikrobson]

I'd go with the rockwool & plaster board.

 

[skotl]

You could try without for a year and then pull a board down and have a sniff-then decide? But ideally, as ever, if your gonna do a job.........
Is there an overhang on the roof outside where you could possibly create an air gap, or is drilling holes in the facia the only way?

Nah - no overhang at the sides. There is at the back, and we've been plagued by wasps in the last couple of years so I have insect netting over that end.
So if I was to drill holes in the side cladding, I'd probably mount as small a grill as possible over it and put insect netting behind.

Who'd have thought a garage would be so much sodding hassle?

 

[Tigman]

you can buy vents for the facias they are about 50mm round drill with hole cutter and push them in !

 

[Tigman]

33p from wickes

 

[J.B. Welding]

It's nae bother pal, juist a frame o' mind no?-git in there, git it done and git oot-building like, piece o' pish!