Insulation and condensation
I am curious about the consequences of not insulating a block or concrete wall in a conditioned space, particularly, but not exclusive of a basement. I know that a typical uninsulated wood framed wall will not have condensation/mold/rot problems because the cavity will stay warm and dry itself out, but is that also true of a masonry type situation, if I were to frame against it and drywall it. My gut tells me that the warm air will condense against the cold wall and cause problems, but then it doesn’t seem to in a wood wall. Am I missing something?
Also, I have used 1″ rigid foam (not foil faced) in my walls against the sheathing and spray foamed the edges to seal it up as a cheaper alternative to the “flash and batt”; is there any reason I shouldn’t be doing this? I am assuming the non foil rigid is semi permeable so I am under the impression I am not causing a problem with drying to both sides.
When insulating a basement with fiberglass batts, I have been told the batt has to contact the surface (i.e. sheathing, etc) in order to work properly. when you are insulating walls that are separating finished from unfinished space, there is no surface to contact. Is this an issue?
Lastly, I want to air seal my basement (the home is 100 years old) but I have a gas furnace, water heater and dryer down there. How do I know if I have made it too tight, or how do I avoid it preferably?
thanks, I know I threw a lot out there but its my first time on here, I had some questions building up!
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Replies
Patrick,
Concrete is a good conductor of heat. The soil on the exterior side of a basement wall is usually colder than the interior air, so a basement wall is usually cold. That can encourage condensation.
Wood is not as conductive as concrete. Wood studs adjacent to a concrete basement wall will not be as cold as the concrete. Moreover, wood is hygroscopic, so condensation won't occur on the studs. Instead, under the right (=wrong) circumstances, the moisture content of the studs will rise.
Yes, you can use sealed rigid foam as a substitute for spray foam in the flash-and-batt method. This is called "cut-and-cobble."
Your rigid foam doesn't have to be vapor-permeable; it just has to be thick enough to keep the stud bays above the dew point. More information here: Calculating the Minimum Thickness of Rigid Foam Sheathing.
For batts to perform at their rated R-value they need air barriers on both sides. In a basement wall situation the exterior air barrier is rightly the requisite amount of air-impermeable insulation to keep the average winter temperature of foam/batt inferface on the above grade section above the wintertime dew point of the interior air (typically 38-40F), otherwise you'll have periods of persistent condensation leading to mold inside the cavity.
It's pretty difficult to make 100 year old home so tight that it's a problem without doing a full-gut rehab or very extensive air sealing. But to tell when you're "there", conduct a backdrafting test on the atmospheric drafted water heater under two conditions:
A: Close all exterior windows & doors, open up the kicthen & bathroom doors as well as the door to the basement. Then turn on all bath & kitchen exhaust fans, as well as the drier, and use a spider-web or smoke stick to see if air is coming down the flue at the spill hood at the water heater.
B: Same conditions as A, but turn on the air handler of the furnace, and re-verify the draft at the spill hood on the water heater.
On the above grade walls a cut'n'cobble approach works just as well as a flash foam. It doesn't need the same R-value as an exterior insulating sheathing approach- it's the foam/fiber ratio that needs to be maintained. eg:
Say you live in climate zone 5 and have full dimension 2x4 walls (4" cavities, not 3.5"). IRC code spells out a minimum R5 for exterior sheathing on 3.5" of fiber cavity fill, or R1.43 per inch of fiber insulation depth (see: http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm ). With an inch of foam against the sheathing you would then have 3 inches of fiber, thus the foam needs to have an R value of at least ( R1.43 x 3" = )R4.3. An inch of Type-II EPS is only R4.2 @ 75F average temp, but is R4.5 @ 40F average temp, and it's guaranteed to be colder than that when the interior face is at the ~38-40F dew point of interior air, so in fact, you're in pretty good shape, no need to bring it up as high as 5". The fact that the foam itself doesn't adsorb moisture the way wood sheathing does (and foam is tolerant of intermittent wetting/fogging) means there will be no moisture stored during condensation events at the foam/fiber interface. Since the average temp will be above the average dew point you won't get liquid water puddling at the bottom of the cavity over time either, just intermittent fog/dry/fog/dry events on the surface of the foam as outdoor temps go through their daily cycles. During extreme cold events you may even get a frost event, but it will be short-lived, and still dry rapidly.
In climate zone 6 only an inch of EPS could be an issue though, since there you need R7.5 on the exterior of 3.5" stud cavities or R2.14/inch. With 3" of fiber-fill you would then need R2.14 x 3" = R6.42, which is only doable in one inch with 1.5lb polyiso or some grades of 2lb spray polyurethane. With only 1" EPS you may even get real frost accumulation that ends up dribbling a bit of liquid water when it thaws.
So, it kinda matters where you are, and what type of foam, and how deep the cavities really are.