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Get'n er done 10-06-2010 09:56 AM

Insulation, Ventilation in Cape Cod Side Attic
I live in a cape cod and want to insulate the side attic (behind the bedroom kneewalls) to save a little money on energy. This side attic is about 30' by 6'. Right now there is about 4" of cellulose insulation between the joists in the floor of this side attic (the joists rise about 10"). There is one layer (maybe 6") of fiberglass up the kneewalls and slanted up over the bedroom ceilings toward the upper attic. There is no access to the upper attic so I don't know the situation up there.

There is no ventilation at the lower level of this side attic (no overhangs for soffits) but at the ridge of the house, in the upper attic, are gable vents and a ridge vent. Some of the insulation stuffed up along the slope above the bedroom ceilings doesn't leave much room for air flow to the upper attic--in some places there is a small gap for air, in others the insulation is stuffed tight in there between the bedroom ceiling and roof.

My question is can I add insulation without causing ventilation problems (too much moisture, too much heat in summer)? I'd especially like to blow in more to fill up between the joists in the floor. If possible I could also add more fiberglass behind the vertical part of the knee wall (not in the slanted ridge toward the upper attic-no room for more), as well as coat the back of the full-sized door (like a bedroom door) which gives access to this side attic.

And I don't want to add lower ventilation if it's going to be a big, expensive project. Right now with the inadequate level of insulation the inadequate ventilation hasn't been a problem--and our heating bills haven't been extreme. So if I can get away with adding more insulation without additional ventilation I will. If it will cause problems I'll just leave things as they are now.

Gary in WA 10-07-2010 08:37 PM

Where is this located?


Get'n er done 10-07-2010 10:23 PM

Milwaukee, WI

Gary in WA 10-08-2010 06:11 PM

If you needed to add air flow baffles, you could install them in sections from the low side. Tape them together with a 3" overlap at install as you push them up the slope over the insulation. Attic ventilation is mainly for removing heat/moisture from the attic that could condense on the sheathing. It also helps ice dams. And very little help to cooling the shingles, though you may lose the warrant unless using a special shingle for that application.(unventilated attic space) Two articles on venting, the second in more detail because it was the original: Both mention WI and Chicago, read it carefully and follow the requirements:

They concluded that the need for ventilation is related to the amount of snow to be expected in the area and the amount of insulation in the ceiling. In Philadelphia, Washington, D.C. and Chicago, roofs with at least R20 (3.5 m2kW) need not be ventilated. In Madison, Wis., Boston, and Sioux Falls, Idaho, the minimum amount of insulation increases to R-30 (5.3 m2kW), and in Minneapolis and Portland, Maine, it increases to R-40 (7.0 m2kW). From:

Use foam board or house wrap over the exterior walls to prevent wind-washing after air sealing the attic carefully:
Be sure to run it up to the baffles above and into the floor bays below the wall to prevent air movement there.

Do you have any heating supply ducts up there?

And, of course, follow your local B.D. requirements.


Get'n er done 10-08-2010 11:04 PM

Thanks, Gary. I read through much of what you shared. The gist of it seems to be ventilation is overall a good thing. Since I don't have lower attic ventilation (soffit vents) but only upper (ridge and gable vents), the ventilation is inadequate. Fortunately there aren't problems right now, probably because the insulation level is also inadequate, allowing air to flow a little more through the space. So I'm learning towards doing very little in terms of adding insulation (maybe just insulating the back of the door to the side attic). Someday some kind of continuous ventilation like the SmartVent could be easier installed when the roof is replaced. We just had our roof replaced a couple years ago and missed that opportunity. I feel that would be too expensive a project now just to boost insulation. Thus I'm pretty much stuck where I'm at.

Gary in WA 10-09-2010 12:26 PM

I hope it works for you!


FIREMEDIC007 11-16-2010 11:54 AM

It's a mess
I bought a cape 9/09. Last year the ice was so bad and the heat in the summer I decided it would be a "simple" insulation project.
1st) I removed all of the accessable old rock wool in the knee wall and crawl space floor
2nd) I cut access holes to the upper attic and unaccessable crawl space and removed the rock wool from the ceiling angles.
3rd) Because the insulation in crawlspaces are open to outside air, I used Jonhs Manville comfortTherm insulation. It is wrapped in plastic and helps protect the fiberglass from the outside air. R25 in knee walls and floor. On the floor I also put in unfaced batts R13 for sense of security.
4th) Durovents in the rafter angles of ceiling. Blocked the bottoms off with rigid foam.
5th) Cellulose blown in GreenFiber from Lowe's in the rafter angles of ceiling apprx R19 and in the upper attic to a value of R38.
6th) Rigid foam at the knee wall/floor. What happens is the cold air will suck the hot air from the joist into the crawlspace, rather than allow it go up into the finished attic. Or in my case my master bedroom and office.

If you would like a price breakdown and my project worksheet email me and I can send it to you. I can also send you a few pictures from along the way and a WAY MORE detailed explanation/problems I ran into. I just finished this project last weekend and have already noticed the difference. My furnace run once every 2 or 3 hours vs 3 or 4 times and hour.

It's a couple week project, in my case anyway. I work full time and go to school full time. Plus keep my wife off my back :)

flynavy99 12-02-2010 10:24 PM

firemedic007 could you post those pics and worksheets or mail them to me...for whatever reason can't manage to PM you. I would especially like to see how you blocked off the bottom of the slanted channels...did you fit the rigid foam with around the durovent baffles with a cutout? Thanks.

FIREMEDIC007 12-02-2010 10:43 PM

Most certainly. My email is
I no longer have all the pictures. But, I would be more than willing to describe to you what I had to do.

Around the durovent, I had a pipe dream of using spray foam. That didn't work. Not even a little, actually, all it did was make a giant spray foam mess :furious:. What i eventually did was take a bunch of scrap pieces of the batts and cut them to 6" or so. I then stuffed them into the rafters between the durovent and drywall.

The rigid foam was for the knee wall/Crawlspace floor junction.

This site is where I based a lot of my theory on. It's an older site (1995?) But fundamentally sound.

FIREMEDIC007 12-02-2010 10:44 PM

Probably help if I put the site in eh?

Gary in WA 12-02-2010 11:38 PM

"4th) Durovents in the rafter angles of ceiling. Blocked the bottoms off with rigid foam." ------- be sure to cover the foam board,


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