Breathable Membrane

[^woody^]

You will have to consider what you are going to do with the roof void, or else it will end in tears.

If you are not (or can't) ventilate it, then you must make sure that it is full of insulation and the insulation is tight up against the deck too. You should have no air voids in between the roof deck and the ceiling.

Then you either put a [vapour check] membrane to stop any moisture getting into the roof structure, or use foil-backed plasterboard which does the same job

Read up on warm and cold deck flat roofs

The idea is that you don't want any warm(er) air from room hitting the cold roof deck, or else it will condense. Ventilation of the roof will evaporate condensation, but if it is not ventilated, then in must be fully insulated to remove any cold surfaces. Just putting a vapour check membrane on your roof will just trap air within it and on cold days it will condense and rot your timbers.

 

[Alastairreid]

but if i staple it to the joists (im assuming the bottom side is the warm side of the roof) how do i seal the holes the staples create?

what holes do the staples make

 

[TonyLikeDIY]

You will have to consider what you are going to do with the roof void, or else it will end in tears.

If you are not (or can't) ventilate it, then you must make sure that it is full of insulation and the insulation is tight up against the deck too. You should have no air voids in between the roof deck and the ceiling.

Then you either put a [vapour check] membrane to stop any moisture getting into the roof structure, or use foil-backed plasterboard which does the same job

Read up on warm and cold deck flat roofs

The idea is that you don't want any warm(er) air from room hitting the cold roof deck, or else it will condense. Ventilation of the roof will evaporate condensation, but if it is not ventilated, then in must be fully insulated to remove any cold surfaces. Just putting a vapour check membrane on your roof will just trap air within it and on cold days it will condense and rot your timbers.

Thanks, ^woody^

I appreciate your constructive advice. It looks as though you know what you are talking about.

What options do I have for ventilation? Do you mean to the outside?

I only allowed for 200mm insulation but will go buy some more tomorrow. So the more insulation the better?

Is this insulation ok.... http://www.wickes.co.uk/ROLL-INSULATION/Loft-Roll-Insulation/invt/161210

 

[^woody^]

It looks like you have little chance of ventilation so I would not bother trying to make some - its not necessary in this case

That insulation is fine. Have you checked B&Q - they had it for £1 a roll a week or so ago?

Cut it nicely to fit between the joists without squashing or bending it. And push it up and then tuft it out so that it expands to its correct thickness and lies flat. If you cut it too wide, it will form an arch shape when you try and shove it between the joists, and you don't want it like that.

Its important to push it up flat against the roof deck so that there are no air spaces under the deck. Then fill any gaps up the sides with some bits and pieces of the insulation.

The idea is to fill all the space with insulation

Don't forget the bits above the noggins and inside the steel beam

 

[TonyLikeDIY]

Thanks, I will do this.

Do I still need the vapour check membrane after I do this. I read somewhere that it traps moisture (when used in the attic!)

Cheers

 

[^woody^]

Yes you need it - or foil backed plasterboards

 

[TonyLikeDIY]

I already have the plasterboards so i expect I will have to buy this vapour membrane. Any idea where I can get it? Is it the same is the DPM which I put in the foundations?

 

[TonyLikeDIY]

ALSO...

(AS PER MY OTHER POST....)


Oh NO!!!!

I forgot to mention that I will be putting a 2000 x 100 rectangle hole in the plasterboard and putting an automatic projector screen into the roof space.

Is this going to pose a problem with,

a) with the heat getting into the roof space,
b) moisture getting the electronics of the projector screen wet?

Cheers,

Tony

 

[^woody^]

In that slot, fit a minimum of 50mm of Celotex or Kingspan tight up against the underside of the deck. Its foil coated which forms a vapour check. Then plasterboard or whatever inside the slot.

It's a room, you are not going to be getting excess moisture on the screen. Presumably there is ventilation elsewhere