Attic ventilation
sdnapier
| Posted in General Questions on
I just read your great article on attic ventilation. My Maine home was built in 1867. I have a metal roof, no ridge vent, and no soffit venting. There is an exhaust fan in the one window. I read this comment near the end of the article: “Generous ventilation channels can reduce the risk of ice damming if you live in a location with deep winter snows”. Are you referring to the debate about 1″ or 2″ spacing between the insulation and the roof? Thank you
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Replies
S. D. Napier,
Q. "I read this comment near the end of the article: 'Generous ventilation channels can reduce the risk of ice damming if you live in a location with deep winter snows.' Are you referring to the debate about 1 inch or 2 inch spacing between the insulation and the roof?"
A. Yes, but the comment also refers to a performance comparison between roofs with no vent channel at all and roofs with a vent channel.
If your roof has ice damming problems, I can make recommendations. If your roof has no ice damming problems, and no visible mold or moisture problems on your roof sheathing, I wouldn't worry too much about attic venting.
Hello Martin,
Thanks for the reply. I do have a small area of roof where I get ice damming. An addition was added years ago that is perpendicular to the main house. Where the old roof and addition roof come together at 90 degrees I have some ice damming. Gutted the bottom room of the addition this summer, sealed all air gaps, installed foam board insulation. At this point there is still a little ice on the addition roof but the majority of it is on the main house section. The attic over the addition is not yet insulated except in the floor. There is also an interior knee wall at the 90 degree junction that is not yet insulated. I was going to use spray foam insulation in the cracks and then install foam board. This should stop the icing. I just haven't started due to the huge debates re ventilation.
So I guess I have 2 questions: Without soffit ventilation I don't really need baffles, correct? and the second, do I need to do anything about ventiltion? I have no mold or moisture problems currently. Any advise? Thank you. Sheryl
Sheryl,
Q. "Without soffit ventilation I don't really need baffles, correct?"
A. I would need to know more to give you advice. Among the details I would need to know are the following: (1) What type of insulation do you have between your ceiling and the great outdoors? (2) Is the insulation resting on top of a horizontal ceiling, or does the insulation follow the sloped roofline? (3) Is the insulation touching your roof sheathing at any locations?
Q. "Do I need to do anything about ventilation?"
A. I'm not sure. You'll have to answer my questions for me to understand your current situation. In the meantime, you should probably read this article: "All About Attic Venting."
Ah...so there is no insulation currently in the attic except what's under the floor boards which appears to be blown in. As there are floor boards my plan was to insulate the sloped roofline. Does that make more sense?
Sheryl,
If you want to insulate along the sloped roofline, you have several decisions to make, including these:
1. Do you want to create an unvented insulated assembly or a vented insulated assembly? Either way will work.
2. What type of insulation do you want to use? If you choose a vented assembly, you can choose from a wide variety of insulation types. If you choose an unvented assembly, you have just two choices: either rigid foam above the roof sheathing, or closed-cell spray foam on the underside of the roof sheathing.
For a full explanation of your options, see this article: "How to Build an Insulated Cathedral Ceiling."
I looked at the picture of the air flow up to the vent ( how to build an insulated cathedral ceiling). There is no ridge vent in my metal roof. I want to use foam board. Please forgive me as I don't know all the terms but are you saying I would have to take the roof off, put on the foam board, and then reinstall the roof? I had planned to just put it in the rafter bays from inside. Thanks for hanging there with me.
Sheryl,
Q. "There is no ridge vent in my metal roof."
A. If you want to create a vented roof assembly, it's a fairly easy job to install a ridge vent. You can hire a roofer to do the work.
Q. "I want to use foam board. ...Are you saying I would have to take the roof off, put on the foam board, and then reinstall the roof?"
A. If you intend to use rigid foam to create an unvented roof assembly, that is exactly what you need to do. But there are other options, including (a) a vented roof assembly that includes mineral wool batts or fiberglass batts, or (b) an unvented roof assembly that includes closed-cell spray foam installed on the underside of the roof sheathing.
" I want to use foam board. Please forgive me as I don't know all the terms but are you saying I would have to take the roof off, put on the foam board, and then reinstall the roof? I had planned to just put it in the rafter bays from inside."
Yes, that's what he is saying. Foam boards between ceiling rafters is almost impossible to correctly detail in regards to air tightness.