Existing dormer: Getting it air tight as possible

I want to improve the air tightness in an existing dormer which is on an exposed site. Basically the wind is blowing into the soffits and getting into the attic area which in turn is getting into the upstairs dormer area, under the skirting. The place gets really draughty when its windy.

A few questions. Can a membrane eg: (Siga) be fitted to the cold side of the studding upstairs and sealed back to the wall plate so that the draughts wont get into the building envelope? Is the any condensation risks with this? I will more than lightly fit rigid kingspan in between studs behind this membrane?

Any help appreciated.

Hi chimpster,

Afraid I don't have answers, in fact I have a question for you

> Can a membrane eg: (Siga) be fitted to the cold side of the studding
> upstairs and sealed back to the wall plate so that the draughts wont
> get into the building envelope?
Do you have open access to the "cold side of the studding upstairs"? Or are you thinking of knocking through some slabs and resealing again afterwards?

Just asking as I have a similar issue. Wishing you well, hope the job works out...

chimpster said:

I want to improve the air tightness in an existing dormer which is on an exposed site. Basically the wind is blowing into the soffits and getting into the attic area which in turn is getting into the upstairs dormer area, under the skirting. The place gets really draughty when its windy.

A few questions. Can a membrane eg: (Siga) be fitted to the cold side of the studding upstairs and sealed back to the wall plate so that the draughts wont get into the building envelope? Is the any condensation risks with this? I will more than lightly fit rigid kingspan in between studs behind this membrane?

Any help appreciated.

Click to expand...

This appears to be a common problem with dormers. In my own case I luckily had access to the area behind the studding through small trapdoors. I a) fixed Extratherm polyiso boards across the studs over the existing fibre glass to improve insulation, and b) put one layer of fibre glass between the floor joists pushing it as far as I could under the floorboards in the bedrooms; and covered this with another two layers across the joists above the level of the floor. The idea was to make a fibre glass seal to prevent wind blowing under the floor and into the bedroom through the skirting and floor boards (the room is not carpeted). On the bedroom side I sealed the gap between the floor and skirting with mastic. It's not a complete solution but it has made a difference. The room is warmer because of the extra insulation. There are still some draughts through the floor on windy days but at least they are not the gales that swirled around before. I would still be interested in the "seal" solution you propose if anyone has views on it, although I don't relish the prospect of getting in behind the studding again, particularly under the dormer windows.