ventilating roof question

[Gyn_Paul]

While I'm waiting for the washing machine to work (possibly) I might as well ask another question..

I'm lining the roof of the loft. it comprises dimond-shaped black

pseudo-ardoise, on lathes, on rafters, on purlins: standard sort of

roof, really.The two sides face due East and West, and consiquently it

gets VERY hot.

I intend to cover it with panels of high-density t & g foam mounted

on the rafters, and then with plasterboard. this will leave a number of

cavities the depth of the rafters, and the width of the space between

them These will need to be ventilated, and to that end I shall

construct a cavity at the apex into which the rising hot air will flow,

and thence exit at either end through air bricks.

Obviously there will need to be vents at the lower end to allow air into the cavities.

The question is... can these supply air vents open into the room or must the air come from the exterior ?

p

[bejay]

[quote user="Gyn_Paul"]While I'm waiting for the washing machine to work (possibly) I might as well ask another question..

I'm lining the roof of the loft. it comprises dimond-shaped black pseudo-ardoise, on lathes, on rafters, on purlins: standard sort of roof, really.The two sides face due East and West, and consiquently it gets VERY hot.
I intend to cover it with panels of high-density t & g foam mounted on the rafters, and then with plasterboard. this will leave a number of cavities the depth of the rafters, and the width of the space between them These will need to be ventilated, and to that end I shall construct a cavity at the apex into which the rising hot air will flow, and thence exit at either end through air bricks.
Obviously there will need to be vents at the lower end to allow air into the cavities.

The question is... can these supply air vents open into the room or must the air come from the exterior ?


p

[/quote]

If  I have understood your question........

The lower air vents should be open to the outside.  To do otherwise would completely negate the principal of air tightness. and I think you would have an uncontrollable flow of cold air into the house when your roof isn't hot..It might help to fit an aluminised vapour control membrane to your rafters before the fittingi nsulation boards if these boards don't already have one built in.

bj

 

[Nick Trollope]

Surely, if the (essential) gap is on the "cold" side of the insulation, then both the in and out must be on the outside. Vents in the soffits and gaps along the edge of the ridge tiles works for me...

You don't want an air-tight "cold" side, but you do want an air-tight warm side.

Or have I misunderstood, too?