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Critique my planned workshop roof

M

Miffer79

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Hi all,

Only just registered, but long time lurker. I've got a quick question about the pitched roof I'm planning for my new workshop build that I would be grateful if more knowledgeable people could cast an eye over.

From the inside of the building out it will be:

Plasterboard ceiling
VCL (not foil backed plasterboard so required?)
6x2 joists with 150mm of batt insulation
18mm OSB sheathing outside
Waterproof roofing membrane
Felt shingles

I'm not planning to have a vented roof so no ridge vent or soffit vents etc, is this an ok design for a warm roof, any obvious problems?

Cheers,

Brian
 
If it were me I'd be having a 50mm gap between the cold side of the insulation and the underside of the decking to allow some ventilation to the roof structure.

So 150mm batts and 200 joists or stick with 150mm joists and higher performance insulation like celotex 100mm thick.

Bob
 
Hi Bob, thanks for the reply. Can I ask why you think the air gap is better? Just curious as that was my first plan to do it that way but researching I came to the conclusion (rightly or wrongly) that I didn't really need to and it would make it easier to build, and less expensive getting a vented ridge and I can cram now insulation into the same space.

I would like rigid insulation but it's a big building (7mx7.3m) and it's a big cost, certainly when I can get batts for a song, maybe an upgrade down the line if I feel it needs it

Brian
 
I believe that with no air flow on the cold side of the insulation, condensation will collect in the roof and could in time rot the timber.

I'm not an expert so hopefully Mike will be along in a bit to give an experts view.

Bob
 
I went for the air gap, an I think the reason is for ventilation to dissipate any condensation build up between the cold and the warm.

Insulation can be sourced cheaper at the outlets that do seconds, and cheaper still if you can be flexible on availability, thickness to within about 20 mm or so, and quality/varying thickness. Then buy a pallet and sell on what you don't use if you have much over.

E.g. Secondsandco, but their website seems not to have anything on it but they may be selling on eBay.
 
Thanks Malcolm, I've looked at rigid insulation seconds, I still might go for them for the extra performance but I'm getting rockwool batts virtually free so hard to pass up.

I'll have an air gap if required but I'm just trying to figure out if it's actually needed or just preference or legacy practice as I thought the modern breathable membranes don't require it, not like the old roofing felt. I think I need to do more research.

Brian
 
Hi Brian,
I think the issue is that your OSB decking is pretty well sealing the roof from being able to breathe so putting breathable membrane on top won't help that.
As you are using felt shingles you will need the support of the osb decking.
But moisture laden air can get in and out with variations in weather conditions and this can lead to condensation and possible rot over time.

hth

Bob
 
Good point Bob I never really thought about the OSB being so vapour proof, looks like I'm researching ridge vent for shingle roofs.

Brian
 
How about cross battening onto your rafters with say cheap tiling battens and then fitting the OSB. Your vents can then be incorporated in the barge boards in some way.
Never seen it done but just trying a bit of lateral thinking?

Bob
 
You absolutely must leave an airgap into the void above your insulation in these circumstances, and that isn't just because it says so somewhere. The actuality is that lots of rooves built in the 60s and 70s failed because of the lack of such air ciruclation, with condensation in the roofspace leading to rot, and mould problems, and even the collapse of some rooves only 10 or 15 years old.

There are lots of good technical reasons I can give you if you are interested in the next few days, but for now, follow the advice you've been given and ensure good air circulation above the insulation.
 
Thanks Mike, I've ordered soffit mesh to make my own soffit vents and think I've found a suitable ridge vent to work with felt shingles so I'm going vented. I think I just couldn't get my head around where the moist air is coming from with a VLC inside and breather membrane outside but I suppose it isn't going to be sealed perfectly so moist air will get in and cause problems.

Cheers, Brian
 
It doesn't need moisture to get in for moisture to be a problem. All air contains moisture. If you cool that air its ability to hold water is diminished, and the excess will condense out on adjacent cold surfaces, such as the underside of your roof (OSB and rafters).
 
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