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Is a partial unvented roof retrofit a bad idea?

We are in Minneapolis with a 1923 Tudor house. Last year was our first winter in the house. We had bad ice damns last year and there is plenty of evidence it has had problems in the past. The second floor is a full story and consists of a shed dormer addition with no venting that I can see. The rest/original of the roof has minimal venting due to lack of (none) eves. There are only pots, but no soffit venting. Half of the story is unconditioned attic is accessible and is on the same floor as the living space. Access through a full size door. There is a tall knee wall separating the two spaces. There is a strip of inaccessible attic space along the ridge of the house that has a couple of vents as well. The bath and one bedroom have a 45 degree wall/ceiling (against the roof) between the accessible attic and inaccessible attic below the ridge. Before we purchased the home an AC unit (air handler and condenser) was placed/retrofit in the unconditioned space. There is a mess of fiber glass in the attic against the knee wall and blown in cellulose on the floor. I believe this is the cause of most of my issues. My question: can I condition the attic and convert it to an unvented roof with closed cell foam, while leaving the adjacent living space original? Is mixing vented and unvented a bad idea? The only way to access the whole roof would be to remove some of the sheathing, but I would like to avoid that if possible. Thanks in advance. Joe.

Asked by Joe Sweeney
Posted Mon, 08/04/2014 - 21:04

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2 Answers

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1.
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Joe,
The type of house you describe would benefit from blower-door directed air sealing by an experienced home performance contractor. It sounds like there is no clear delineation of your home's air barrier.

That's step one: figure out where the air barrier is, and plug the air leaks, with an attempt to make the air barrier continuous.

Once that's done -- and it will be a huge job -- you can improve the insulation.

There is no problem combining unvented roof assemblies with vented roof assemblies, as long as your house has a continuous air barrier.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 08/05/2014 - 06:31

2.
Helpful? 0

Thanks Martin,

I am working with someone now to do a blower door test. I am thinking of doing my attic first and doing the rest in stages if need be. If I sealed the accessible attic with closed cell spray foam and the rest of the roof/attic remained original I do not think I would be creating any additional problems that were not there prior to sealing the attic. I am hoping I can attack the biggest problems first; HVAC duct work in an unconditioned space, major ice dams over attic and inability to install proper venting in attic. Joe

Answered by Joe Sweeney
Posted Tue, 08/05/2014 - 08:56

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