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Community and Q&A

Insulating behind a wood burning stove in a basement.

Mike Marino | Posted in General Questions on

I’m in the process of finishing off my basement. Generally it stays pretty dry other than the occasional gutter malfunction. I built a 2×4 stud wall with r13 batts. Behind the studs and in front of the block is a layer of plastic sheeting. For the 1/2 of the basement that has been done for years, I don’t seem to have many moisture issues. However, I’ve decided to incorporate the wood burning stove into the finished section. Same deal, 2×4 studs with unfaced batts in this section covered by concrete board and eventually will cover that with stone. Again, plastic sheeting between the two walls spaced an inch or so from the block. The problem is, since this part of the wall gets very warm as opposed to the rest of the space, I’m getting crazy condensation between the plastic and the insulation. The wall hasn’t been stoned yet so now is the time to fix it. What would you recommend, that will hold up to high heat, to use in place of the batting/plastic setup?

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    The inch of space between the studwall and the foundation is a thermal bypass, and very difficult to air seal. If instead of air, filling that space with an inch of rigid foam board with the edges sealed, trapped up against the foundation with the studwall would be a far better approach, since it blocks the convection loop through the cavity.

    The only way condensation can occur on the plastic is if the plastic is colder than the dew point of the proximate air. Heating up that body of air with the wood stove does not increase the moisture content (and thus does not change the dew point) of that air, but it might be driving moisture out of the studs into that air. If the batts have facers on the warm side of the assembly the moisture release rate is relatively low, which may be part of the issue.

    If you can, get rid of the plastic, replace it with an inch of EPS wedged behind the studs. By having R4 between the cool side of the fiberglass and the cold foundation the temperature at the cool edge of the fiberglass will be higher, hopefully above the dew point of the room air. Using unfaced batts (rock wool preferred, due to the much higher melting temperature) would also allow any moisture that gets into the studwall a drying path. As long as you've stayed with the stove manufacturer's or building code clearance requirements to walls it should be fine.

    Speaking of code, technically the source of make-up air for a wood stove has to come from a location below the firebox of the wood stove, which can be a problem in basements. Ideally you'd be able to use a ducted combustion air kit with the wood stove, which would take the room out of any backdrafting path, but if the combustion air duct's input is above the firebox, it becomes a potential mini-flue when the wood stove door is open.

    From IRC 2012 (http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_10_sec006.htm):

    " SECTION R1006 EXTERIOR AIR SUPPLY

    R1006.1 Exterior air.
    Factory-built or masonry fireplaces covered in this chapter shall be equipped with an exterior air supply to assure proper fuel combustion unless the room is mechanically ventilated and controlled so that the indoor pressure is neutral or positive.

    R1006.1.1 Factory-built fireplaces.
    Exterior combustion air ducts for factory-built fireplaces shall be a listed component of the fireplace and shall be installed according to the fireplace manufacturer’s instructions.

    R1006.1.2 Masonry fireplaces.
    Listed combustion air ducts for masonry fireplaces shall be installed according to the terms of their listing and the manufacturer’s instructions.

    R1006.2 Exterior air intake.
    The exterior air intake shall be capable of supplying all combustion air from the exterior of the dwelling or from spaces within the dwelling ventilated with outside air such as nonmechanically ventilated crawl or attic spaces. The exterior air intake shall not be located within the garage or basement of the dwelling nor shall the air intake be located at an elevation higher than the firebox. The exterior air intake shall be covered with a corrosion-resistant screen of 1/4-inch (6 mm) mesh.

    R1006.3 Clearance.
    Unlisted combustion air ducts shall be installed with a minimum 1-inch (25 mm) clearance to combustibles for all parts of the duct within 5 feet (1524 mm) of the duct outlet.

    R1006.4 Passageway.
    The combustion air passageway shall be a minimum of 6 square inches (3870 mm2) and not more than 55 square inches (0.035 m2), except that combustion air systems for listed fireplaces shall be constructed according to the fireplace manufacturer’s instructions.

    R1006.5 Outlet.
    Locating the exterior air outlet in the back or sides of the firebox chamber or within 24 inches (610 mm) of the firebox opening on or near the floor is permitted. The outlet shall be closable and designed to prevent burning material from dropping into concealed combustible spaces. "

    In practice this isn't always a problem, but it's something to be aware of.

  2. GBA Editor
    Martin Holladay | | #2

    Mike,
    You have made several errors, as Dana correctly pointed out.

    Basement wall insulation needs to be in contact with the concrete or masonry wall.

    Interior basement wall insulation can be rigid foam or spray foam -- but never fiberglass batts.

    You never want to include any polyethylene when insulating a basement wall on the interior.

    For more information on the correct way to do this work, see How to Insulate a Basement Wall.

  3. Steve Vigoren | | #3

    May or may not be appropriate, but back in my wood burning days, I put a piece of metal between the stove and the wall. Set it on a couple bricks to get it off the floor, and do something to space it out from the wall at the top. With the air able to circulate behind it the metal doesn't get that warm and it keeps the wall from getting hot. I used a piece of metal floor decking I had so it was fairly stout, a little heavier than steel roofing.

  4. Mike Marino | | #4

    I appreciate the feedback. In the sections I'm finishing now, I will put rigid foam behind the studs against the block. After doing this, would it then be problematic to use batts over the rigid foam? Or is my best bet to leave the stud cavity empty? And as far as the metal sheeting, I was going to try to avoid doing that because I wanted the look of stone around the unit but it may be wise to at least use some metal around the closest part of the stove.

  5. Expert Member
    Dana Dorsett | | #5

    Using a combination of exterior-side foam with unfaced batts is standard practice, and not problematic. The foam is vapor-retardent, and limits the rate of groundwater moisture migration into the studwall on the below grade section, and the R-value of the foam relative to that of the total R limits the amount of wintertime condensation that can form on the above grade section.

    If you want to be really conservative on the R-ratio you can follow the IRC prescriptives for exterior insulating sheathing for wood sheathed above-grade walls when only Class-III interior vapor retarders (eg, standard latex paint) are used, which varies by US climate zone:

    http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

    https://www.greenbuildingadvisor.com/sites/default/files/images/DOE%20climate%20zone%20map.preview.jpg

    Since neither batts nor the foam are damaged by wintertime condensation events (unlike OSB or plywood), using the IRC prescriptive foam for wood sheathed above-grade applications is conservative- you can safely cut that by 1/4 and be fine in a basement application, as long as there is nothing moisture-susceptible between the fiber & foam. If you cut by more than 1/4 you cut into the thermal performance of the fiber due to longer term frost/ice formation potential, and when it thaws you may have liquid water dripping down the face of the foam. As long a it isn't in copious quantities, the liquid-water that runs down the face of the foam ends up in the warmer below-grade portion of the cavity and re-evaporates. Air leaks are a bigger hazard in that regard than the vapor permeance of the finish wall.

    Metal heat shields would only be necessary if you violated the clearance-to-walls specifications of the stove manufacturer. Having a stone or brick surround is both more aesthetic, and it provides some thermal mass that will still be radiating back at the room hours after the fire went out.

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