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Avoiding condensation in detached garage/workshop (Climate Zone 5)

AMcNett | Posted in General Questions on

Hi GBAers –  a while back I asked a broad question re: insulating and air-sealing a detached garage as I convert the building to a hobby woodworking shop. Here’s the link: https://www.greenbuildingadvisor.com/question/air-sealing-and-insulating-detached-garage-with-flat-low-sloping-roof

I got a lot of excellent responses to that question and based my approach on them, along with info from this GBA article: https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs

Now I am approaching the completion of my project and want to ensure I get the final building envelope details correct (and tightly control costs while doing so). Hopefully someone can point me in a reasonable direction.

This week I had an insulation contractor install a ~1.5″ coat of closed cell spray foam in my 2×4 stud and 2×6 rafter bays. Now I am specifically concerned about preventing/managing condensation in my walls and ceiling, which will be clad in 3/4″ plywood and 7/16 osb respectively. I am in climate zone 5 (Cleveland, OH area).

The workshop will only be used a couple-few hours a day max. My hope is that I will only need to heat and cool the space just enough to control condensation on my large cast iron surfaces and not need to wear gloves while working in the cold months. 

I believe my final steps are:
– install mineral wool in my 2×6 rafter bays
– cover ceiling with 7/16 osb
– install fluffy insulation (mineral wool or, preferably from a cost perspective, fiberglass) batts in my walls
– cover walls with 3/4 plywood
– install weather seals around the rollup door, man doors, and windows

My “thin” wall and foam coat lead me to worry that flash and batt in the wall and/or ceiling could backfire. I also wonder if I will actually see appreciable insulating performance beyond what’s delivered by the foam. Should I simply forgo additional insulation in the ceiling and/or walls and simply heat/cool more aggressively when I am actually using the space?

Due to budgetary constraints, for the time being the space is going to be heated with a 110v oil radiator heater, and cooled with a portable AC. I am also considering simply situating 100w work lamps over each of my cast iron surfaces when they’re not in use. 

Thanks for any help you folks can provide. A couple pics of the current state of the garage are attached.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    >"This week I had an insulation contractor install a ~1.5″ coat of closed cell spray foam in my 2×4 stud and 2×6 rafter bays. Now I am specifically concerned about preventing/managing condensation in my walls and ceiling..."
    ----
    >"The workshop will only be used a couple-few hours a day max. "

    Putting the closed cell foam between the studs was a waste:

    https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste

    Even in a residential home with respiration cooking bathing type humidity sources installing anything more than the minimum necessary for dew point control would be a waste. For a zone 5/Cleveland climate would be about 27% of the total center-cavity R value of the walls, or 40% of the total on the roof. In a mostly unoccupied shop building it doesn't even take that much.

    >"I also wonder if I will actually see appreciable insulating performance beyond what’s delivered by the foam."

    With only 1.5" of framing that's less than an R2 thermal bridge through the insulation, and HELL YES it's worth filling in the rest with fiber, even if you're only heating the place to 50F! Compressing a cheap low density R11 batt into 2" will add another R8 to the 2x4 wall, a compressed R13 would be good for R9, but significantly the R-value of the thermal bridging framing has more than doubled to over R4. "Contractor roll" R11s and R13s are dirt cheap.

    In the 2x6 rafters you're left with 4" to fill. A compressed R19 delivers about R15. So with R9 of foam and R24 total you'd be at about 38% , not quite 40%, but this isn't a house- it's nothing to worry about.

    Using kraft faced batts is fine (but not necessary), and will offer a bit more dew point margin since they are "smart" vapor retarders- so is the OSB and plywood. The humidity levels in a shop won't be very high in winter even with fairly minimal ventilation rates, but the low vapor permeance of dry OSB and dry plywood (and possible kraft facer) give it even more margin.

    1. AMcNett | | #4

      Thank you. I appreciate the advice.

      Re: the closed cell foam between studs I hear you. My primary concern there was air sealing considering daylight was readily visible between the 1920s skip sheathing and presumably similar vintage painted cedar shingles.

      I am going to fill both the stud and rafter bays, and most likely with fiberglass to appease the family CFO.

  2. Jon_R | | #2

    In cold weather, use a humidistat controlled heater to keep it just warm enough to prevent corrosion and boost temperature as needed for comfort. In warmer weather, use a dehumidifier and AC as needed for comfort.

    1. AMcNett | | #5

      I was trying to remember the word “humidistat” all day. Thank you. Solid plan.

  3. walta100 | | #3

    I found this sensor on the web a few years ago I thought it would be perfect for a shop like yours if it controlled a heater and or dehumidifier while attached to a large chuck of iron.

    https://www.airtest.com/support/datasheet/EE46.pdf

    1. AMcNett | | #6

      Cool find. They look to be ~$200 per sensor. I’d have to wire it to something that could switch power to the heater or AC/dehumidifier (for me, an arduino or raspberry pi or similar given my rudimentary electronics knowledge). Could be a fun project. Thanks!

    2. Jon_R | | #9

      I use a $16 humidity controller. Full title is "US AC 110-220V STC-3028 Dual LED Temperature Humidity Thermostat Controller Tool".

      1. AMcNett | | #10

        Thanks, Jon. I will need to find one that's capable of 12.5 amps for my existing equipment (the one you use looks to support 10 amps). Great idea though.

  4. walta100 | | #7

    I see no reason for a computer all you need is a relay or a SSR.

    E bay has a few Siemens Condensation Monitor for less money.

    Walta

    1. AMcNett | | #8

      Great, a new rabbit hole for me to fall down. :) Thanks Walta. I will take a look.

  5. Colin63 | | #11

    Easy fix is to run a fan on moist wet days

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