Insulating sloping ceiling/DIY spray foam insulation

[Kaymo]

Hi,
As you can see from the pic below the upstairs of my 110 year old house has a sloping ceiling and although I have the flat part of the loft well insulated I have not yet been able to find a solution for the sloping section and I am convinced I am losing a lot of heat through here (in the largest bedroom the slope accounts for nearly 1/3rd of the ceiling area).

I understand that there needs to be a gap left and previously I was considering sliding insulation boards down from the loft but its going to be difficult to get a close fit as the distances between the battens is not uniform as it might be in a modern house. Then I saw this http://www.spray-insulation.co.uk/product range.htm#1 DIY foam and I wondered if this would be a better and quicker solution. Does anyone have any experience of this stuff, both to install and its performance in comparison to solid boards?

Finally, both of these solutions involve insulating between the wood. I hear a lot of talk on these forums about wood, or metal nails etc being a thermal bridge - would it be better therefore to pull the plaster board off from inside the room and insulate from inside - this would be tricky also due to the angle and particularly the original cornice.

Thanks in advance.

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[freddiemercurystwin]

The problem with spray foam is that it blocks up all the ventilation and your timbers will eventually rot. Its much better to try and remove enough of the plasterboard to enable the insertion of some decent insulation.

 

[Kaymo]

Thanks Freddie, I had been thinking that I would be able to spray just to a depth that would allow ventilation over the top but I gather from what you are saying that its not that easy to control and will just expand to full the void?

 

[freddiemercurystwin]

Correct.

 

[Kaymo]

Thanks again Freddie.

Another question, pulling down the plasterboard etc is going to be major work and if I am going to do that it would make sense to insulate the walls as well and with other priorities in this old house that is just not an option in the near future. So, it looks like my best bet will be to get insulating board and slide down the gap for now - my question is, if I do this now, can I then add more insulation later from below - i.e. is it possible to sandwich two pieces of insulation together or does the join/slight air gap likely to cause any problems?

 

[freddiemercurystwin]

Any insulation is better than none so if you have some gaps then so be it not much you can do about it. If you decide to insulate the lot later on consider just how much it's going to cost you overall and whether you'll really get a pay back, cranking the heating up a few degrees may well end up cheaper especially if you're only there for 5 or 10 years.