Attic Truss Insulatioin and Air tightness

DeeJunFan

So looking at the options below what do you think would be the best way to insulate attic trusses?

Is it better to have the insulation and airtightness layers together?

I have a sketch of the options but dont seem to be able to attach a pic

The Air tightness detail is something i'm concerned about as if i make the rafters my air-tightness line there will be multiple timber struts etc that will have to be taped around. A lot of work and a lot of potential mistakes.

It would be much simpler to make the wall of the attic Room the air tightness line as its pretty straight and uninterrupted.

The issue for me is i dont really understand the concepts enough to know if i can have the insulation and air-tightness separate.

For example

Slate
Slate Battens
Counter Battens
Breather Membrane (eg Tyvek)
Rafters (Full Fill or no insulation)
Attic Room (Walls full fill or no insulation)

If i go rafters full fill would the rafter line need to be my air tightness layer or could the attic room be my air tightness layer?

From what i have read if i use a Breather felt and counter battens then there is no need for ventilation of the attic? Would this be acceptable to BCO??

Thanks
Damian

DeeJunFan

Anyone got any ideas?

DeeJunFan

Hi,

This is the sketch i was referring to.

basically i need to know if i can have the air tightness layer and the insulation layer separate or do they need to be together (as in contact with one another)



https://www.flickr.com/photos/126303063@N08/14935135736/in/set-72157646577574641

Thanks
Damian

MicktheMan

DeeJunFan wrote: »

.

basically i need to know if i can have the air tightness layer and the insulation layer separate or do they need to be together (as in contact with one another)

You can of course, but your insulation will be as effective as if it were on the garden wall!:)

DeeJunFan

MicktheMan wrote: »

You can of course, but your insulation will be as effective as if it were on the garden wall!:)

ok, thanks Mick

So are you saying the air tightness layer is going to impact that much on the insulation?

There have been some people advising me that if i was going to insulate the attic at rafter level the best place to put my air tightness layer would be the first floor ceiling. Leaving the attic within the house thermal envelope but not in the air tightness envelop but thats the reason i'm asking the question.

There is a lot of conflicting advice

MicktheMan

DeeJunFan wrote: »

There have been some people advising me

Who are these "some people"?
What professionals have you on board?

MicktheMan

DeeJunFan wrote: »

ok, thanks Mick

So are you saying the air tightness layer is going to impact that much on the insulation?

There have been some people advising me that if i was going to insulate the attic at rafter level the best place to put my air tightness layer would be the first floor ceiling. Leaving the attic within the house thermal envelope but not in the air tightness envelop but thats the reason i'm asking the question.

There is a lot of conflicting advice

So, in reality, what you are saying is to rely on heated air outside your airtightness barrier in a ventilated space to be kept warm by your insulation envelope. See the problem?

DeeJunFan

MicktheMan wrote: »

Who are these "some people"?
What professionals have you on board?

have an architect. haven't spoken to him much recently. Don't know how much experience he has with low energy builds as that was not the reason for choosing him. Difficult planning and he was the best for that.

No other professionals as yet. have been approached by Renewables consultant but have seen some previous work and am not at all impressed.

Trying to educate myself as much as possible.

The some people would be an adviser from an air-tightness company (tape and membrane producer)

If i go for rafter level insulation i wont be ventilating the attic. Would go for breather membrane and counter battens.

So if i go rafter level insulation i should have air tight layer at the rafter level and if i go insulation in the room in roof level that should also be my air barrier.

Thanks, that is what i wanted to confirm.

DeeJunFan

Mick if you don't mind, could you give me your opinion on spray foam for this job?

If i got the rafters spray foamed there would be no need for a separate air tightness layer. so i should be getting insulation and air tightness all in one go?

BryanF

DeeJunFan wrote: »

Mick if you don't mind, could you give me your opinion on spray foam for this job?

If i got the rafters spray foamed there would be no need for a separate air tightness layer. so i should be getting insulation and air tightness all in one go?

Spray Foam is not air-tight, show me an EN standard data sheet that states different.

DeeJunFan

BryanF wrote: »

Spray Foam is not air-tight, show me an EN standard data sheet that states different.

Suppliers claim it is. Thats why i was asking the question. The price was the thing that was really putting me off.

BryanF

BryanF wrote: »

Spray Foam is not air-tight, show me an EN standard data sheet that states different.

Suppliers claim it's is, but when you question the manufacturer they will not make this claim. As I said above -with all product read the manufacturers data sheet ensuring it's to relevant EN standard.