How to vapour barrier and insulate?
I live on the north end of Vancouver Island, British Columbia, Canada.
– My house is a two story.
– Has a carport on first floor under neath two bedrooms.
– I have closed in the car port and put in an insulated garage door.
– I am turning the, what is now a garage, into a woodwork shop.
– The shop will be heated sometimes and then sometimes the garage door will be open and no heat on.
Question #1
– How should I insulate and vapor barrier the common wall between the garage and the heated basement.
Question #2
– How should the ceiling , which is the floor between the two stories, be insulated.
– We do not have contractors who do spray foam readily available here.
– Is it possible to use closed cell rigid foam between the studs and then use spray foam in a can to seal the edges.
– Does a vapor barrier need to be applied if I use spray foam or rigid foam.
Thanks for any and all info, W.S.
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Replies
W.S.,
You can use almost any type of insulation you want, as long as you install enough of it to (at least) meet minimum code requirements for R-value, and as long as the insulation is installed properly.
There isn't much of a reason to worry about vapor diffusion problems, so I wouldn't spend any time worrying about vapor barriers. Don't put any polyethylene in your walls or ceilings. Instead of worrying about vapor diffusion, pay attention to air sealing.
More information here:
Do I Need a Vapor Retarder?
Vapor Retarders and Vapor Barriers
Forget Vapor Diffusion — Stop the Air Leaks!
"There isn't much of a reason to worry about vapor diffusion problems, so I wouldn't spend any time worrying about vapor barriers. Don't put any polyethylene in your walls or ceilings. Instead of worrying about vapor diffusion, pay attention to air sealing."
Sure, as long as he doesn't care about meeting BC's code requirements...
Poly sheeting has created as many moisture problems in temperate maritime climates as it has solved- if the inspector needs to see some sheet goods, use a "smart" vapor retarder such as Certanteed MemBrain.
Since the garage is only conditioned space some of the time, any vapor retarder is best put on the basement side of the insulation in the common wall assembly. You can use a vapor barrier latex if it's already sheathed on the basement side.
But it's true that air leaks are by far the bigger issue here (by orders of magnitude), and unless it's air-tight vapor barriers will only slow the drying rates.
Cut'n'cobble foam between studs is usually a waste of good foam, but it can be done. Either use reclaimed roofing foam(any type) from commercial building demolitions, or virgin-stock EPS. At 3" Type-II or Type-IX EPS is a minimal class-II vapor retarder, and would meet Canadian code specifications as a vapour barrier, but not so vapor retardent as to create a severe moisture trap issue. XPS (pink, blue, green whatever) would work too, but uses blowing agents that have extreme global warming potential. Foil or vinyl faced polyiso has all of the moisture trapping issues of polyethylene sheeting but might be used depending on the rest of the stackup. All standard types of roofing foam (including fiber-faced polyiso) would meet code on vapor retardency too, and using reclaimed goods is always greener than virgin stock, especially at the scrap rates you'll have in a cut'n'cobble stud-cavity application.
Air tight drywall with vapour retarder paint or alternatively the Certainteed Vapour Retarder Membrain are both code approved in BC - I just used both and passed inspection on Vancouver Island in a different jurisdiction (Qualicum Beach)
Malcolm: I believe the Canadian code reads, paraphrasing, "poly unless you can show that the wall assembly is air tight". I don't think poly is mandatory.
Patrick and John: Yes you are right, that isn't what I was getting at. Whether an emphasis on air sealing rather than "worrying about vapour diffusion" is the way to go, the BC code mandates that you provide a vapour barrier and distinguishes it from air barriers. It doesn't have to be poly, the code provides for a wide range of strategies and materials, but you can't just ignore it because you think that it isn'y very important.
Malcolm,
"You can't just ignore it because you think that it isn't very important." That's unfortunate -- because vapor diffusion isn't very important.
Fine. The BC building code has a sensible, considered approach to both air and vapour barriers - which it's quite clear neither you or the other posters have read, so it's hard to understand how you could disagree with it. If you think it is a good idea to advise posters to disregard the codes governing their projects fly at it.
Malcolm,
I don't have a copy of the B.C. code. If any GBA readers have a spare copy lying around, I'd love to receive a copy. (Send me an e-mail if you want my mailing address: martin [at] greenbuildingadvisor [dot] com.)
GBA readers in the past have quoted sections of the B.C. code -- whether accurately or inaccurately, I have no way of knowing -- and these quoted sections included language that certainly appeared to muddy the important distinction between vapor barriers and air barriers.
Air barriers are always good. Vapor barriers are often counterproductive. In some cases, however -- under a concrete slab, or on a crawl space floor -- vapor barriers are essential.
Certainly many U.S. codes have stupid sections, and in the past they had even more stupid sections than they do now. I have no problem pointing out stupidities in the building codes of any country. When a building code recommends building practices that are risky from a hygrothermal perspective, it certainly behooves every builder to do whatever they can to avoid following the code. If your building inspector is smart, he or she usually has the authority to approve of unconventional approaches, if the builder can explain why a certain code provision is stupid.
A one day online access to the code is available at this link for $20. The relevant section is 9.25. Heat transfer, Air Leakage and Condensation Control. It is also worth looking at the appendix for material that explains the thinking behind the code provisions.
http://www.bccodes.ca/building-code.aspx?vid=QPLEGALEZE:bccodes_2012_view
Just curious, but is there anything in the code specifying vapor retarders between intermittently conditioned garage spaces and a basement (which is question at hand, after all)?
Since the garage space is pretty much vented to the exterior (no matter how tight the garage door seals, they all leak like sieves) the garage space might be modeled as a 5000mm+ rainscreen gap, and the garage door the siding, from the point of view of the adjoining partition wall to the basement, which certainly affects the bulk-moisture wetting affects of that partition assembly during the rainy season, and the average temperature inside the garage will almost certainly be above the dew point of the interior air for the preponderance of winter hours, given that it's partially inside the thermal boundary.
All of which renders the vapor retardancy of that partition wall moot- it doesn't have to be very air tight at all from a moisture control point of view, but given the presumed air leakage of even the tightest garage spaces, it's important to make it air tight from a thermal performance point of view.
Dana, the BC code doesn't recognize spaces as being intermittently conditioned. It is an either/or situation. The exterior walls of the garage don't have to insulated or have an interior finish but if you do it must meet the same requirements as all other exterior walls. So garages are either heated and insulated or unheated and uninsulated.
I don't know of any provision in the code that allows you to change the requirements of the wall between the garage and the house depending on which route you took - as far as I know it is still seen as an exterior wall with all its attendant requirements.
Thanks Malcolm- it doesn't surprise me!
In that case there is no specification or requirement for vapor retardency of the partition wall between the garage & basement, only for the exterior walls, since the partition (though insulated) is inside the code-defined conditioned space.
Similarly the garage ceiling below conditioned space has no vapor retardency requirement.
Sorry Dana, I've probably been unclear. The code starts from a different point. It says exterior walls must follow the requirements for air sealing and vapour control in section 9.25. and it considers the wall between the garage and the living spaces an exterior wall. It leaves insulating the other walls of the garage as optional but if you do they also must be constructed to meet the same requirements.