Breathable membrane or vapour control layer for roof

[roosie]

I have a flat warm room on my detatched newbuild garage.

from bottom to top it has: NOTE the ??????? needs a a control barrier but does it need to be the breathable type or weatherproof type.


Plasterboard
Roof joists
18mm Ply
??????????
Celotex Insulation
18mm ply
Sarnofil Membrane

 

[Xenon]

Heavy gauge visquene would be usual VB, 1/2 ply will do.

 

[hardmetalking]

This is one expensive garage roof!

So you can't fit sarnafil directly onto insulation?

The vapour control is the most important part IMO.

Is your garage heated? If not I don't understand why you would want a warm roof?

If you are going ahead with the two different layers of ply then you should have insulation above and below the first layer. The idea is to stop warm air condensing on a cold surface. You need an air tight vapour seal as well as plenty of insulation to separate the cold and the warm parts.

 

[roosie]

So what would be your reccomendation for roof layers? Its not a heated garage but just want it not freezing in winter months as ill get a space heater in. Will insulation make a big difference in roof? Unless i put insulation in rafters and just a ply layer on top with sarnofil.

 

[freddiemercurystwin]

Are you going to insulate the walls? Otherwise there's not gonna be big gain.

 

[hardmetalking]

I think I would stick with your original build but add insulation between the rafters and a good vapour control where the ??????? is.

Not sure if this will be enough to keep the cold out. Need a decent seal on the garage doors and constant low heat. This is a lot of effort and expense for a garage......

 

[roosie]

Does the vapour control need to be breathable?

 

[noseall]

Does the vapour control need to be breathable?

PARADOX ALERT!

No, vapour control would be pointless if it were breathable.

It needs to be impervious like polythene or foil etc.

 

[roosie]

no problem cheers


i take it 1200 will be too thick really or overkill

 

[noseall]

I think I would stick with your original build but add insulation between the rafters and a good vapour control where the ??????? is.

You could also fix foil backed plasterboards for belt 'n' braces protection.

 

[Deluks]

I would agree with the idea of insulating the garage roof but not in the pricey manner you suggest.
12mm plasterboard, vapour barrier sheet (you can buy rolls of this from wickes, screwfix etc.) Thick enough to do the job without being too heavy to handle.
Use a cheaper insulation such as polystyrene sheet or loft roll, fully filling the void. 12mm ply or 18mm osb3. Final roof covering.
Wouldn't epdm work out cheaper than sarna?

 

[roosie]

Being you would insulate between rafters, wouldnt that be a cold roof system so need airflow though joists to outside?

 

[Deluks]

Yes, but it shouldn't pose a problem if the vapour barrier is continuous, taped at all overlaps and round the edges. It's just a garage, no one will be living in it. A thin layer of insulation, 20mm would do, under the roof deck or behind the plasterboard would do if it bothers you.

 

[roosie]

ok ive had a change of mind.


can someone tell me if this diagram will work ie not moisturise the rafters?

again thanks everyone for the previous comments

 

[Deluks]

Vapour barrier goes at the bottom, under the plasterboard.