Insulation question...

[Jim O'Dell]

Some of you will remember the box bay window I added to the west side of my shop during the exterior rehab, for the miter saw. ( see: http://www.sawmillcreek.org/showthread.php?t=14427 post # 27) I started with finishing the interior of it this weekend between dog shows. The roof for this slopes up, but stays under the eave of the building's roof. In insulating it, do I need to keep air space above the insulation? There is no way for air to circulate up to the roof vents. It is a self contained roofing space approximately 4' X 4'. I could easily vent the bottom edge but not sure that would help anything. Or can I just put the insulation up under the roof deck and put my OSB against the vapor barrier? TIA!! Jim.

ps. I might have enough of the pink rigid foam insulation to get 1 1/2" of it against the bottom of it's roof if that is better than fiberglass.

[Matt Ocel]

Jim,
If it were me, I would batt it with an R-30 then a vapor barrior.
I wouldn't worry about ventilation as it is a very small area.

[Jim O'Dell]

Not near enough room for R30...R13 might be tight. Thanks Matt! Jim.

[Don Bullock]

... I started with finishing the interior of it this weekend between dog shows...

Sometimes our dog shows just get in the way, don't they? I hope yours went well.

[Chase Gregory]

If you use the foil faced bubble material you can get R45 in a 3 1/2" rafter by using three consecutive layers!

I wouldn't worry about air space in that instance and with fiberglass batting.

If you were using a reflective material (like the foil faced bubble, foil faced ridgid, etc... you need at least 1" air space to make a conductive break. It is much harder for heat to 'jump' than conduct and the thin reflective products really need that break to perform.

Chaser

[Jim O'Dell]

Unfortunately Don, we weren't even in the hunt.
Thanks for the info Chase. That is good to know. Jim.

[Jason Beam]

Generally, as I understand the rule, if it is 100% sealed tight you don't have to vent it because if air can't get in neither can vapor. It might actually be worse to open the bottom without opening the top, since there'd be no cross-vent. I'm far far far far far from an expert, but that's my understanding of it.

[Ken Garlock]

Jim, do a google on "insulating spray foam", and you will get nearly a half million hits. I would think that you could find a kit whereby you could fill the space completely with foam. It seems to me that if the space is a sealed area, the foam would be the way to go. The foams that I have heard of give around an R-7 to R-8 per inch, but I haven't looked at the market since 2001 when we were building.

Good luck.

[Don Bullock]

Unfortunately Don, we weren't even in the hunt.
...Jim.

That can make for a looooong weekend.

Fortunately ours was somewhat better. While our class female showed better than she ever has, the best she could do was win the BBEx class on Friday and Saturday and she was Reserve on Friday. On a much better note, our two year old male beat the number one basset hound two of the three days.

Due to problems with my shop plan the shop is being delayed while some of the plans are redrawn to "please" the county officials. Due to all the regulations and codes here, I'd never be able to build my own shop like you are doing. Jumping through all the building and safety hoops is very frustrating and costly. I almost gave up on the shop portion last week, but the numbers my contractor came back with were in the right range. It looks like I still am within my budget if the county ever OKs the plans and lets us start the building.

I'm glad to see how well your shop is progressing. The progress you and others are having is encouraging me to keep trying to get mine done.