Dormers are small additions to a home but can be a large opportunity for energy loss.
Keep in mind that where you once had only one flat roof surface that required insulation, you will now be adding four surfaces (three walls and one roof) plus a window.I will assume your home is a wood-framed home.
- I recommend that you frame the new dormer walls using 2x6 lumber. Frame the roof portion with 2x8 lumber.
- The advantage here is that you will have more wall cavity and roof rafter cavity to stuff with more insulation.
- This is especially important if you plan to use fiberglass batt insulation, which tends to be the most economical choice.
- If you can splurge and use spray foam insulation then you will be all the better for it.
And don't skimp on the window. At minimum, use a double-insulated glass, fiberglass-frame window. Your best choice would be a triple-glazed window. I recommend fiberglass because it does not transfer heat or cold like other windows as the fiberglass serves as a thermal break.
Make sure to seal every little crack and crevice with a 25-year acrylic caulk before insulating. The worst enemy of effective insulation is air infiltration. If you use spray foam insulation, this becomes less of an issue. For such a small project, you can purchase a do-it-yourself spray foam from such manufacturers as Tiger Foam. Search online for a vendor near you.
If you do all these things, you should see little impact on your overall heating and cooling system, which means less money spent on utilities and more comfort. Best of luck.
For more information:
Read "I'm converting my attic into a non-vented conditioned space. Should I use open cell or closed cell foam to insulate?" a Q&A answered by Steve Saunders.