Insulating ceiling under roof deck
Hi,
I’m building an addition which will have a balcony directly over living space. The balcony will be covered with a Duradek type covering over 3/4 inch plywood and is 8 feet deep (from the outside wall of the house). The joist are pitched, on the top side, 1/4 inch per foot as per Duradek’s instructions and level on the bottom side in order for the ceiling to remain flat with the remainder of the house. Due to the size of the existing floor joists and the distance needed under the door opening onto the balcony (3 inches min. as per Duradek again), I start out with a 8 inch ceiling / balcony floor joist and finish up with 6 inches at the outer wall. Joists are 16 O.C. As I live in Maryland (moderate climate?), any suggestions as to the best way to insulate the joists spaces? I would like to avoid spray-on foam if possible. Thanks in advance for your input.
Replies
Foam would be the best, other than cost, why would you want to avoid it? Icynene is about $.40 per board foot in my area.
6"-8"isn't much room for insulation. If we were building the space, sprayed in insulation would be a requirement because of the space limitations.
How would the space be vented if fiberglass insulation were to be used? Venting isn't an optional exercise or you'll get condensation and the related problems where it isn't needed. Yet another reason sprayed in foam makes sense.
If you don't want to have anyone spray it, you can also cut rigid foam sheets to fit and use can foam around the edges.
Good insulating
I had priced out spray in insulation for a new construction house and it was insanely expensive. Plus, there is no one local so I’m not even sure if they would travel to do a job of only 160 square feet or so.I like the idea of cutting sheets of rigid foam and placing them in the stud cavities. Looking at my newest copy of Fine Homebuilding, I wonder if the Polyisocyanurate (ISO) would work since it has foil between layers. I looked at it at my local lumber yard and it seemed to have the highest R rating of all the foam panels.Thanks “Trout” for bringing up the venting issue using fiberglass. That’s something that I hadn’t thought about.
Edited 7/16/2006 8:43 pm ET by mikemjm
Venting is a great reason in some older homes that have undersized rafters.
You can do a search here and google for a DIYer foam. I think its about $250 for a kit that always you to spray the foam yourself. I think the foam is closed cell which is the higher R value, I think about R7 per inch. For such a small area, the kit would be a good option, if you have a large space, the kit would take too long and be too expensive. You also do not have to fill the entire rafter bay with insulation, 3 inches would give you an R21. See what a box will do and go from there. I know the foam kits have been mentioned here, but I don't recall the name, just that it was a little expensive if your doing a large area.
I know abc supply has a kit in their catalog, http://www.abccatalog.com/store/viewitem.asp?idproduct=7498500435.
They have locations everywhere, they only sell to the trades at the branches but if you pay cash, they should sell to you. Or you could buy online. I don't know what the sf the box will cover, check that out. They have at least one other box, a larger one.
It seems you'd be in better shape with any of the rigid foams than fiberglass, although the better ones are, well, better. It's a good idea to use the spray foam to seal the edges so air can't pass around the sides of the foam boards.
Cheers