Hi Scott, Some of the modern spray-in foams are probably a little better than glass fiber but they can be pretty expensive to have installed particularly a small area like you describe. The main advantage is that they will fill the space snugly and completely and in the process probably help seal some air infiltration points. This will require an installer though so a small area becomes relatively expensive.
Sealing all air infiltration leaks is the first an most helpful. This might include where the wall meets the floor if you can get some access to that. The main advantage to using fiberglass bats is that you can more easily fit them into the spaces if you cut and fluff them with care. Snug but not tight or compressed. If the joist bays run over the basement wall then my advice is to extend the bats through the bay space a foot or more over the basement wall so you are insulating some part of the floor over the edge of the basement. Don't be shy to fill the entire bay, if a 2 by 10 bay then fill it with fully 10 inches of fiber glass. Depending on the design and how much exterior wall extends below the floor you can install what ever thickness of extruded foam board under the fiber glass to hold it up under the floor and to form another contiguous layer of insulation over the under side of the joists. This will hold the fiber glass up into the bay space and provide some insulation to reduce the thermal bridging through the wooden joists. I have a similar situation to yours, where I extended the master bedroom out over the first story wall by 2 feet. I filled the space with fiber glass and had room for 3 and a half inches of the blue foam under then strapping and soffit. We have had it as cold as minus 40 with no perceptible additional cold on the hardwood floor along the edge of that room. Mind you the poly air barrier up the wall also wraps under the fiber glass and on up the second story wall as well then on up over the ceiling. Hope this is helpful. If I was Han Solo I'd probably pet my wookie ----- Original Message ----- From: Scott Howell To: [email protected] Sent: Tuesday, December 29, 2009 8:35 AM Subject: [BlindHandyMan] insulation question Hi all, I have a question about a type of insulation that would best fit this specific application. I don't know if this particular application is so much unique, but any thoughts are appreciated. The way my house sits on the foundation, results in one section hanging over the basement walls by about 14 inches or so. When I first moved here there was no insulation and you can bet the floors in the bedrooms over that short area got quite cold. So, I stuffed some insulation in there and can't recall what R rating etc. is in there, but it is faced. So, I'm thinking that perhaps I'll add or replace that insulation with something, which may be even more effective. So, I'm looking for some ideas for what might be a proper insulation such as maybe attic insulation? Would the fiberglass be best in this application or some of that foam material or maybe a combo of the two? I think what is there does help, but I've noticed some cold air getting in, so I wanted to remove what is there, inspect, plug any holes or gaps and then re-insulate. THanks, [Non-text portions of this message have been removed]
