Advertisement
If you have a new account but are having problems posting or verifying your account, please email us on hello@boards.ie for help. Thanks :)
Hello all! Please ensure that you are posting a new thread or question in the appropriate forum. The Feedback forum is overwhelmed with questions that are having to be moved elsewhere. If you need help to verify your account contact hello@boards.ie
Hi there,
There is an issue with role permissions that is being worked on at the moment.
If you are having trouble with access or permissions on regional forums please post here to get access: https://www.boards.ie/discussion/2058365403/you-do-not-have-permission-for-that#latest

Dormer windows - insulation and air tightness

  • 31-03-2014 2:17pm
    #1
    Maxium
    Registered Users, Registered Users 2 Posts: 22 ✭


    We in the middle of a new build at the moment and I have some questions about the dormer windows (there's 5 in total). The house is one and half story so the upstairs windows come out of the wall rather than the rafters but the cheeks of the widows sit on the rafters. The house is block build with a 200mm cavity.

    My question is we're aiming to get 0.16 u value (200mm pumped bead) in our walls and 0.12 in our roof (400mm fiberglass) and sloped ceilings (220mm metac with 37.5mm insulated plasterboard) - How should we follow this insulation through to the dormer window areas - should it be the same as the sloped roof (i.e. 200mm metac with 37.5mm insulated plasterboard) or should we be looking at something different? Should we be aiming to get a u-value of 0.12 (to match the roof) or 0.16 in this area (to match the walls) - I'm thinking 0.12.

    Also any special considerations for air tightness around this area - I'm thinking the air tight membrane from the ceiling should run down the cheeks of the dormer window and be taped to the block work and also taped to the air tightness tape surrounding the window - is this the right idea?

    Finally is there a way to achieve 0.12 in the sloped areas without having to use a 9inch rafter - could we achieve the same using a 7inch rafter (i.e. less metac but more insulated plasterboard)?

    We will be meeting with the engineer soon to discuss these items but I wanted to do a bit of research first so that I can contribute to the discussion and not appear to be completely clueless!!! Any help much appreciated.


Comments

  • #2
    Curious Geroge
    Registered Users, Registered Users 2 Posts: 1,017 ✭✭✭Curious Geroge


    Maxium wrote: »
    Finally is there a way to achieve 0.12 in the sloped areas without having to use a 9inch rafter - could we achieve the same using a 7inch rafter (i.e. less metac but more insulated plasterboard)?

    7" rafters would be standard roofing and I also considered 9". In the end, &" with 150mm metac and a 62.5mm insulated plasterboard is what I went with. Its giving 0.16 on the data I have but can stand corrected as I'm no expert in this area. I was very tempted to go with 180mm Metac but was advised strongly to allow breath space although I've used high quality breathable felt.

    I expect to put the airtight membrane on but would advise you if possible to ask the roofer not to batten out the roof to allow for the membrane to be attached easily now without having to work around rafters which is going to be painful and time consuming.


Advertisement