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Question DetailsAsked on 10/27/2016

When insulating a garage in Wisconsin, should I use rolls or panels?

I have to do this by myself and panels look easier. My garage is attached and has the spaced studs to put the rolls. Since I'm in Wisconsin, I need to know if one or the other works better.

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Assuming of course this garage will be heated (at least at times, like if you are using it for occasional car maintenance or as a workshop). Otherwise, unless trying to limit summer heating makes no sense to insulate it unless you are trying to further limit heat loss from adjacent living space walls or retain some household heat in it so the cars don't have such an extreme cold winter start condition.

With normal stud spacing (16 or 24" on center), you can do either - fiberglass is almost certainly cheaper, and easier to put up, and you don't potentially lose interior space. Only real drawback is if you are sensitive to fiberglass - in handling it, even with gloves and long-sleeved clothing or tyvek coveralls you still get poked a lot by the glass fibers it is made of, and while some people like me can roll in it without adverse effects (made me real popular as a kid on a construction crew - guess who got all the attic and crawlspace insulation assignments - though paid better too), other people get real sensitized by it and rash up or feel real itchy for several days after handling it. However, is basically fireproof and does not outgas chemical fumes (which makes a difference to some people either due to snesitivity or due to environmental protection concerns), whereas foam insulation outgases fumes for a very long time and is almost always highly flammable, and generates profuse amounts of deadly smoke when it burns.

Since you are in a cold area, you want decent insulation, probably as much as you can reasonably fit into the walls - typically R11 to R15 in 2x4 walls (depending on brand and insulation density), R19-25 in 2x6, R23-30 roughly in 2x8 walls, though you can put thinner insulation in the thicker walls if you don't want that good an insulation - it does not have to fill the full thickness of the stud bays though if you are planning on using

When you say "panels" I am not sure it you are talking true insulated panels (commonly applied as part of a siding system, but also available separately - commonly with one face foil covered). Talking insulation values - the higher the R-value the more insulation it provides per inch of thickness - stone/brick/concrete is about R=0.1/inch, fiberglass batt insulation about R3.5-4.2/inch, expanded polystyrene (usually white or pale blue or pale green, same material as white foam coffee cups) board insulation about R3.2-4.2/inch depending on brand and density, extruded polystyrene (normal pink and blue board you see around) around R5/inch, polyisocyanurate up to R6-6.5 - though this latter product is not recommended in exterior walls because it absorbs and holds moisture.

Using panel insulation on the outside of your wall (under siding and water barrier) can be done - but generally in enough thickness to be meaningful causes issues with doors and windows and even eaves because you are talking 3-4 inches of insulation or more so it pushes your siding well out beyond the normal location. In cold climates, it can also cause moisture problems because it tends to act as a vapor barrier, which in your area should be on the inside of the wall to prevent airborne moisture getting into the wall and freezing. You do NOT want two vapor barriers in a wall - traps moisture in the interior of the wall.

So - proper way to insulate in your case is inside the wall or on inside face. Board insulation does not work well in the stud cavities because the studs are not straight or perfectly spaced - so you get a lot of board trimming needed, and you also have to use spray foam in a can along the insualtion board edges to get a full insulation seal between the boards and the wood. If doing this (normally for industrial buildings) the board is usually "edge stripped" - cut with a hot wire cutter to about 1/2" narrower than the space, to provide a wide enough gap along the edges for spray foam to be used. Also - commonly board insulation (except on special order) is only available in 2 and 4 foot widths - so does not match your stud spacing whether you have 16 or 24" on center spacing (so roughly 14-1/2 or 22-1/2" stud bay width - usually different at corners), meaning you would have to cut ALL the boards to width and also waste a lot of material - can be quite a hassle.

Board insulation can also be put over the inside face of the wall - but again a width issue unless you have 4' wide boards available in your area because otherwise 24" board over 16" studs means a lot of wastage and cutting to get the board edges over the studs for nailing. (You can glue free hanging edges, but gets real messy and the adhesive is brain-melting - VERY nasty stuff to work with because of the fumes, which will continue to outgas and permeate the house (assuming attached garage) for weeks. Also, even more problems with the thickness if applied on the inside of the studs - not only do windows and doors have to be trimmed out for the added thickness, but you also lose 2-8 inches of the garage interior dimension in each direction because of the insulation board thickness - plus the 1/2 or 5/8" drywall thickness on each wall to cover it. You also have to use much longer (so quite substantially more expensive) drywall fasteners to go through the insulation into the studs.

Board insulation is not allowed to be exposed - HAS to be covered with fire-rated drywall.

For all these reasons, board insulation is used in probably not more than 5-10% or so of your type applications - and most of those are as exterior foam board under the siding or in (cringe) EIFS - exterior insualtion and finish system - where the foam board with some spray-on coating becomes the insulation and siding both. (Please do not do that - MANY moisture problems showing up in houses that were done that way in the last 20-25 years or so).

So - that leaves fiberglass batt - which comes sized for 16" on center studs and also fairly readily available for 24" (may require special order) stud spacings, goes up VERY easy (roll out as you push into stud bay or buy as bundled wall-height precut lengths, stapling the facing at the top to the top plate of the wall (top horizontal 2x) so it does not slip down in the space over time. You would want paper-faced (also called "kraft" faced because the brown paper is called kraft paper) so you can staple the paper to the studs, and make sure it has a binding strip in it (not all do) - which is a bonded reinforced center strip designed to keep the fiberglass matt from peeling of from the paper facing and slumping. Or alternatively, get foil-faced insulation with overwidth foil (designed so adjacent rolls have the foil overlap each other on the stud) which is stapled at the stud and then taped on the seam with foil vapor barrier tape.

On the inside face of the wall, to stop airborne moisture (from any water source in garage, from your body mnoisture evaporation if using as a workshop, and especially from rain and snowmelt water evaporating and dripping on the slab from parked cars), if not using foil-faced insulation you want to staple up a vapor barrier - usually 6 mil polyethylene sheeting extending from bottom to top of wall and ideally across ceiling as well - not always done on ceiling if living space above.

Then you need fire-rated (Type X) drywall of the thickness prescribed for your building code. Commonly 1/2" but sometimes 5/8" is required between garage and living spaces (adjoining walls and ceiling if living space above). Check with your local building department on requirements for your area.

For an unfinished garage, in some areas it is legal to leave the joists exposed, fire-block any penetrations in the walls to overlying living space, and use exposed fiberglass insulation in the stud bays, though it picks up a LOT of dirt very quickly, and may tend to slide down in the stud bays over time - pulling loose from the top staples. If this is done, you need fireproof paper facing on it - available on special order usually. Or use unfaced or foil-faced insulation and directly staple the top of the insulation to prevent slumping - though this gives a poorly insulated top few inches, and is not so resistant to slumping as sstapled faced insulation. Where exposed insulation is used, sometimes a spray adhesive is sprayed on the inside face of the exterior sheathing to help keep the insulation from slumping when it is applied.

Lots of Youtube videos on how to apply insulation of either type - just be sure to check out several so you don't accidentally follow one that is not quite on the mark - there are some laughable how-to videos out there which are more how-not-to videos.

Remember - foam insulation and plastic vapor barrier cannot be left exposed - have to be covered with drywall - and in some areas even fiberglass insulation cannot be left exposed in attached garages, even though it is basically fireproof. In those areas, exposed insulation has to be rock wool - looks like and comes in batts like fiberglass, heavier, somewhat more expensive, made from mineral fibers. Generally, due to the itch factor and the speed with which it turns brown from dust accumulation, most people don't want exposed insulation anyway.

Answered 3 years ago by LCD

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