Hey i'm in west michigan and have had a furnace installed in my 36x22x10' garage, it has two insulated doors and nothing in the attic/sidewalls yet. I've finished electrical and ready to close it up. I'm having the attic blown in and a 20x9' section of drywall blown in so I don't have to remove the drywall. That leaves me with do I inset staple kraft batts, face staple kraft batts, insert unfaced batts and cover with 6 mil poly and caulk the top and bottom to the wall plates?

Got a ZIP code? (climate matters)

What type of siding? (construction matters too)

Are you heating the place all the time, or only when you want to work in the garage when it's cold out?

In most cases you won't need to bother with interior side vapor retarders in a garage, and true vapor BARRIERS like polyethylene sheeting can actually raise the risk of mold/rot. The kraft facer on a batt is a Type-II vapor retarder, but it has variable vapor permeance, becoming more vapor-open if the moisture levels in the cavity reach mold-growth levels. Unlike a house, most garages don't have indoor sources of moisture like breathing/bathing/cooking humans driving the dew point of the garage air up, and even the best weather seals on garage doors are pretty leaky, allowing quite a bit of air exchange (even though you'd like less), which purges any moisture at a faster rate than the much tighter house.

The most important thing about installing batts is to tuck them in at the edges and corners to make sure there are no long skinny gaps where the framing meets the outdoor sheathing, then tugging them gently back out so that they're just proud of the interior stud edges for a compression fit when the wallboard goes up. Face stapling the batts ensures that there are no long skinny channels where the framing meets the wallboard.

Using a batt knife to carefully sculpt the fit around electrical boxes, and splitting batts to accomodate wiring rather than just mashing it all in there improves performance too. The key is to get a complete cavity fill, no compressions, channels, or voids.

Air tightness of the exterior side matters (for performance) too. The crackage at the plywood/framing corners is bigger than it looks, as are the seams in any plywood/OSB sheathing. Caulk the framing to the sheathing around the full perimeter of EACH cavity with a purpose made sealant or polyurethane caulk (not painters caulk), and tape any seams in the sheathing. Any wiring or plumbing penetrations of the framing or sheathing should be sealed with can-foam. Any doubled-up top plates, headers, jack studs etc should have the seams caulked, and any micro-width cavities between studs that are too tight to stuff fiber insulation should be can-foamed (primarily for air tightness) as well.

Posted By Dana1 on 07 May 2018 10:15 PM
Got a ZIP code? (climate matters)

What type of siding? (construction matters too)

Are you heating the place all the time, or only when you want to work in the garage when it's cold out?

In most cases you won't need to bother with interior side vapor retarders in a garage, and true vapor BARRIERS like polyethylene sheeting can actually raise the risk of mold/rot. The kraft facer on a batt is a Type-II vapor retarder, but it has variable vapor permeance, becoming more vapor-open if the moisture levels in the cavity reach mold-growth levels. Unlike a house, most garages don't have indoor sources of moisture like breathing/bathing/cooking humans driving the dew point of the garage air up, and even the best weather seals on garage doors are pretty leaky, allowing quite a bit of air exchange (even though you'd like less), which purges any moisture at a faster rate than the much tighter house.

The most important thing about installing batts is to tuck them in at the edges and corners to make sure there are no long skinny gaps where the framing meets the outdoor sheathing, then tugging them gently back out so that they're just proud of the interior stud edges for a compression fit when the wallboard goes up. Face stapling the batts ensures that there are no long skinny channels where the framing meets the wallboard.

Using a batt knife to carefully sculpt the fit around electrical boxes, and splitting batts to accomodate wiring rather than just mashing it all in there improves performance too. The key is to get a complete cavity fill, no compressions, channels, or voids.

Air tightness of the exterior side matters (for performance) too. The crackage at the plywood/framing corners is bigger than it looks, as are the seams in any plywood/OSB sheathing. Caulk the framing to the sheathing around the full perimeter of EACH cavity with a purpose made sealant or polyurethane caulk (not painters caulk), and tape any seams in the sheathing. Any wiring or plumbing penetrations of the framing or sheathing should be sealed with can-foam. Any doubled-up top plates, headers, jack studs etc should have the seams caulked, and any micro-width cavities between studs that are too tight to stuff fiber insulation should be can-foamed (primarily for air tightness) as well.

49534 zip, vinyl siding. Will be kept 50f most of the time to aid ice melt, 65-70 when using periodically. I did foam all plate penetrations and caulked where I saw light between sheathing and also between joints in the double studs up top, my thought on poly was it'd aid in air sealing more than vapor concerns. It sounds like caulking each cavity then carefully insert stapling them to the cavities is the trick here. These are 14.5-15" x 9ft cavities, it appears faced batts of this size are special order everywhere unless I want to cut for each cavity and tape at the seam.

49534 is in Kent County MI, which is US climate zone 5A.

With the inherent back-ventilation of vinyl siding it would not need (or want) an interior side vapor barrier even if it were fully conditioned occupied living space (which has higher moisture drives than a garage.) Even the kraft facers aren't necessary- standard latex paint on wallboard would do.

See the IRC prescriptives for Class-III vapor retarders by climate zone here:

https://up.codes/viewer/wyoming/irc-2015/chapter/7/wall-covering#R702.7.1

Vinyl siding qualifies as the "Vented cladding over wood structural panels." exception.

So unfaced batts, drywall, but seal all sheathing joints top bottom and studs to concrete with construction adhesive for air infiltration?

You can use unfaced batts if you like, but if it's easier or cheaper to use kraft faced, they're fine. Kraft facers have variable permeance, and are more vapor-open than latex paint when the humidity level in the cavity is high enough to support mold.

For sealing concrete to wood use polyurethane caulks formulated for concrete (just not the "self leveling" type.) Construction adhesive won't stay flexible enough over the long haul for concrete-to-wood. It will probably be OK with wood-to-wood air sealing, and will make the assembly much more rigid. But polyurethane caulks tend to do better long term, and won't ever be brittle enough to crack if the building flexes a bit in a high wind.

Posted By Dana1 on 08 May 2018 07:29 PM
You can use unfaced batts if you like, but if it's easier or cheaper to use kraft faced, they're fine. Kraft facers have variable permeance, and are more vapor-open than latex paint when the humidity level in the cavity is high enough to support mold.

For sealing concrete to wood use polyurethane caulks formulated for concrete (just not the "self leveling" type.) Construction adhesive won't stay flexible enough over the long haul for concrete-to-wood. It will probably be OK with wood-to-wood air sealing, and will make the assembly much more rigid. But polyurethane caulks tend to do better long term, and won't ever be brittle enough to crack if the building flexes a bit in a high wind.

Thank you, to clarify how would Kraft faced be easier? I suspect also using kraft with face staples would annoy the drywaller? Also i'm a little confused, osb plus viynl siding is "greater than 1 perm" so don't i need a class iii vapor retarder like a kraft face anyway?

Vinyl siding is back ventilated and not air tight- the humidity of the air behind the siding tracks that of the outdoor air, and it doesn't matter how high or low (it's actually extremely low, well under 0.1 perms) the vapor permeance of the vinyl itself happens to be.

The most susceptible part of the assembly is the OSB wall sheathing, since that's where the bulk of the wintertime moisture accumulates, since it doesn't pass moisture freely from one side to the other. Dry OSB has a vapor permeance of LESS than 1 perm (a Class-II vapor retarder), but as it reaches moisture content levels high enough to support mold it runs about 5 perms- the middle of the Class-III vapor retardency range. As long as moisture isn't being injected into it from the interior side faster than it can dry into the air behind the vinyl siding it's fine. During the winter when the OSB is cold it is much colder than dew point of the indoor air in a house, so there is a vapor pressure gradient pulling moisture via vapor diffusion though the drywall, and that moisture collects in the OSB. In the spring when it starts to warm up, moisture is released from the OSB in both directions- the entrained air in the fiberglass rises in humidity, and the direction of vapor pressure oscillates with outdoor temperature. As long as the maximum moisture content of the OSB doesn't get too high, it will dry to both the interior and exterior before it's temperature rises to levels where mold can grow quickly.

Wallboard is VERY high permeance- north of 25 perms, and without paint on it the peak moisture content of the OSB can rise to the danger zone. But with standard latex paint applied the vapor retardency of the interior drops to 3-5 perms, the low to mid-range of Class-III vapor retardency, and about the vapor retardency of OSB at high moisture content. That is a low enough vapor permeance to limit the peak moisture content in the OSB to a safe level without blocking the ability of the OSB to dry toward the interior, the way 4-6 mil polyethylene would.

With the ability of the assembly to dry in both directions it is much more resilient to moisture drives from either side, including bulk water incursions from wind blown rain, less than ideal window flashing details, etc.

Kraft facers are not Class-III vapor retarders- they are variable. When the moisture content of the face hits mold-supporting levels it's vapor retardency is north of 5 perms. When bone dry the vapor retardency of a kraft facer is about 0.5 perms. That makes it something of a "smart" vapor retarder- it keeps the rate of moisture movement to a rate lower than that of latex paint when it's dry, but doesn't appreciably slow down the drying rate of the assembly if the humidity inside the cavity reaches mold levels.

When using unfaced batts both the air tightness of the wallboard and the application of latex paint become more important. It's impossible to detail a kraft facer as an air barrier, but even with cuts tears and punctures it's still a pretty good vapor retarder. If this were a wall to fully conditioned living space it would be worth either using kraft facers or a broadsheet variable permeance vapor retarder such as 2-mil nylon (Certainteed MemBrain), or a more heavy duty type of smart vapor retarder such as Intello Plus. But in a garage I wouldn't sweat it too much.

In colder climates such as zone 6, the fingers part of the MI mitt or zone 7, the colder parts of the da Yoop the moisture accumulation season is longer than in zone 5, and OSB or CDX sheathing would accumulate too much moisture over the winter with only 3-5 perm paint on the interior. But in Zone 5 it will be fine as long as the drying to the exterior isn't inhibited by siding that isn't back ventilated. Vinyl siding is inherently back-vented, but other types would need at least 1/4" to 3/8" of air gap, preferably vented at both the top & bottom to allow freer convection of outdoor air to the OSB/CDX sheathing.

Like kraft facers, #15 felt on the exterior of the sheathing is a smart vapor retarder, relatively vapor-open at moisture levels where the sheathing needs to dry or when wetted by wind driven rain, but ~1 perm-ish when dry. Housewraps tend to be much more vapor open, anywhere from 8-40 perms, which is fine.

Ok well I dont see any foam except at the sill and I dont believe the exterior is wrapped under the siding. I may not cover the walls with drywall for another year, but I at least want to insulate it soon.

Without the drywall you'll be able to pull the batts in few locations to test the moisture content before closing it all in. If this were fully conditioned space with ~35-40F dew point air in winter the sheathing would be saturated dripping wet after a month of cold weather, but a garage it probably won't. A sub-$100 2-prong wood moisture content meter is good enough. If any part of the sheathing is over 20% moisture content pull the batts and give the sheathing at least a week to dry out, and re-test before reinstalling the batts and installing the drywall. If it's under 15% everywhere it's fine to put up the drywall and paint it right away. Over 15% and under 20% install the drywall but give it another month of warm-ish weather for drying before painting.

Kraft facers would help keep the sheathing dry prior to drywall, but exposed kraft facers are a code violation due to the fire spread problem.

Also, if you ARE putting drywall in, you need to make sure the first course isn't just laying on the floor, or it'll absorb moisture.

Use 16 penny nails as spacers for the first course.
Then caulk the gap.

lol sounds like i'm back to face stapled kraft batts now, i was going to put construction adhesive on every cavity and sheathing joint this weekend either way

On every piece of drywall I have installed or seen installed, the first course on the wall starts at the ceiling tight against the ceiling drywall. Then the second course is held tight against the first or top course. This naturally leaves ~1/2" gap at the bottom if you are using standard pre-cut wall studs.

I've normally seen the ceiling installed first. Then the wall done from the bottom up. Mostly because we're talking about rooms with non-standard height ceilings. This yields nice, clean, tight seams at eye level. Then, if the seam for the last course aren't QUITE true they're up out of immediate visual range and a bit of care is taken with the tape & compound job.