More Q&A Spotlight
Andrew K has a project that should ring a bell with lots of homeowners: What’s the best way of insulating a room over the garage so it becomes useful, conditioned space?
Andrew lives in Massachusetts, in Climate Zone 5A, and would like to turn the space, what real estate agents used to call a “bonus room,” into an office. The roof is framed with 2×8 rafters and already has both soffit and ridge vents, but no insulation whatsoever.
“I have looked at closed-cell [spray foam] insulation, but quotes have come in at about $5,000-$6,000 to insulate the entire roof deck,” Andrew writes in a Q&A post at GreenBuildingAdvisor. “So I’m looking for cheaper alternatives.”
What he’s got in mind is adding fiberglass batt insulation to the rafter bays, leaving a 1-in. space for ventilation, plus 2 in. of rigid foam on the interior, then drywall.
Should Andrew install a vapor barrier? Should he be using faced batts? And which kind of rigid foam insulation would be the best choice, extruded polystyrene (XPS) or polyisocyanurate?
Those questions are the start of this Q&A Spotlight.
More ventilation, better insulation
Both GBA Senior Editor Martin Holladay and Dana Dorsett would opt for a ventilation gap greater than 1 inch gap that Andrew is planning, but they recommend slightly different approaches to insulating the rafter bays.
First, says Holladay, a 2-inch gap will be better than 1 inch. As far as insulating the rafter bays, he writes, dense-packed cellulose insulation will perform better than fiberglass batts, and polyiso insulation is more environmentally friendly than XPS.
“If you install 1×3 or 1×4 strapping (furring strips) on the underside of the polyiso,” Holladay adds, “you’ll have an easier time installing the drywall. And if you choose foil-faced polyiso, the air space will provide…
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11 Comments
What about the floor?
I was a little surprised that there was no discussion of insulating or sealing the floor. I would assume the garage is unheated, and there is the issue of exhaust. Any thoughts?
Response to Eric Peterson
Eric,
The original question that led to this Q&A Spotlight article concerned roof insulation.
For a discussion of methods to insulate the floor of a bonus room, see:
How to Insulate a Cold Floor.
Thanks Martin!
good article, I hadn't thought about holding the insulation tight to the subfloor..
SIPs
Another great alternative when dealing with vaulted ceiling are SIPs. They work perfectly with vaulted ceilings and I personally prefer steel SIPs over OSB SIPs but either one would work. Depending on panel thickness, one can see R-48 with a 12" EPS panel. I know many people dislike vaulted ceilings because of the issues brought up here in this article but utilizing SIPs in such an application is a good match.
Walls and ceilings between a garage and living space
In addition to neglecting the issue of insulating the floor between the garage and the new conditioned space, this article neglects to address either the issue of fire or of fumes from the garage below.
I am in the process of a deep-energy-retrofit-like renovation of a split-level ranch-ish house and am wondering how to address the party wall and ceiling separating the garage from living space...
Response to David Bettridge
David,
This article is one in a series of articles called "Q&A Spotlight." The article summarizes comments from a single thread on our Q&A page. The original question concerned insulating a sloped roof assembly, so it's only reasonable that the posted answers focused on that question.
Your question concerns air sealing. You are correct, of course, that it's very important to air seal the floor assembly that separates a garage from a bonus room, just at it's important to air seal the common wall between an attached garage and the attached home.
The two best tools to help with your air sealing efforts are a blower door and a theatrical fog machine. For more information on these tools, see:
Blower Door Basics
Pinpointing Leaks With a Fog Machine
Thanks Martin, the link to
Thanks Martin, the link to the cold floor article you shared in your response above has some great tips, and nice clear drawings so thanks for that.
The name of this article though is "How to Insulate a Room Over a Garage" which implies insulating all the walls, floor, ceiling, as well as dealing with issues specific to garages versus other kinds of cold spaces below finished rooms.
It should either deal with all those issues or have its name changed.
Sorry to pick on you when I find so much useful information on this web-site without ever thanking you for it.
David
OK -- here's a new title
To GBA readers,
Eric Peterson didn't like the original title to this Q&A Spotlight.
David Bettridge didn't like the original title to this Q&A Spotlight.
So the article now has a new title. I'm sorry if the original title was confusing.
What I did...
Our code requires 2.5" vent space. On my new house I used 2x10 for rafters then nailed 1x3 on the sides adjacent the roof sheathing and 1/2" plywood to those. The remaining rafter bay is filled with rock wool batt. Continuous 3" EPS covers the bottom of the rafters and 1x4 (with very long screws) supports the drywall. The vapour barrier is between the 1x4 and the drywall. This seems to work very well. We had a lot of scrap plywood from cutting out all the windows and a few additional sheets were inexpensive. The system is a bit labour intensive and you need to be careful nailing so close to the shingles. However it won’t collapse like cardboard chutes and is somewhat vapour permeable (top surface). The way I look at it there is zero opportunity to repair or correct this assembly so for me it’s got to be robust. (Oh, and no pot lights in the cathedral ceilings!)
Oh good lord
You guys can yap all day and never agree or make a conclusion. This sounds like a government discussion, rather than "experts". The fact that you can't agree on anything says you are either both wrong, both uneducated, or this whole discussion is a waste of everyone's time. I know it wasted mine!
Response to Igor Mastovitch
Igor,
This article is one of a series of articles called "Q&A Spotlight." The intention is to summarize the type of dialog that occurs on GBA's web forum. As is usual on such forums, a diversity of views are expressed.
Fortunately, GBA Technical Director Peter Yost agreed with my advice this time around -- and he even recommended my article on the topic. So, at least on this issue, there is consensus.
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