Replies

1. Expert Member
   Dana Dorsett | Apr 19, 2019 11:28am | #1

   >" Do I still need to be concerned with condensing vapor on the concrete walls? "

   You never need to be concerned about condensing water vapor on concrete walls. Concrete is highly water tolerant, and can take on a large amount of water as adsorb before appearing physically wet.

   Regarding wall insulation, cut'n' cobbled foam between studs puts the cool edge of the studs at risk. Installing continuous foam held in place with 1x4 strapping through screwed to the foundation would be far better- even 2.5" of foam would outperform 3.5" of cut'n'cobble. The IRC calls out R15 continuous insulation for foundations in zone 6. Since the garage is being actively conditioned (even if only part of the time), that would be the right thing to do, even if there is full insulation between the garage and the living space. Setting it up like Option 3 would beat code-min with some margin.

   Alternatively, a minimalist 2" + 2" EPS insulated concrete form would get you to code min with a hind of margin if it hasn't already been built. Or, 4" of reclaimed roofing EPS (cheaper than batts) on the exterior side extending from the footing to the foundation sill would work, protecting the foam from UV damage above grade with a cementicious EIFS. Another possibility is 3" of reclaimed roofing polyiso on the interior side either trapped to the wall with a 2x4 studwall or 1x4 strapping onto which the gypsum gets mounted.

2. 
   Aedi | Apr 19, 2019 11:41am | #2

   I'd follow Dana's advice on the insulation. But no matter what you do, be sure to include insulation between your garage and your basement, and and air sealing as well. I also think it is a good idea to include exhaust ventilation in any garage workshop.

3. 
   airfix | Apr 19, 2019 01:04pm | #3

   Dana,

   The comment about condensing water vapor on the concrete was poorly worded. I was worried more about it being a condensing surface then the water dribbling down the wall to the wood framing.

   When you say the cut an cobble approach would put the cool side of the studs at risk, what do you mean? Are you talking about condensation like I mentioned above affecting the studs?

   The walls are already up.

   I'm going down the continuous layer of foam approach then. I might do option 3 with a smaller stud cavity say 2x2 that well help maintain maximum workable space in the garage.

   I am looking into getting garage ventilation but I've not found a solution yet.

   Steve

1. Expert Member
   Dana Dorsett | Apr 19, 2019 02:03pm | #5

   >"I was worried more about it being a condensing surface then the water dribbling down the wall to the wood framing.

   When you say the cut an cobble approach would put the cool side of the studs at risk, what do you mean? Are you talking about condensation like I mentioned above affecting the studs?"
   ---------
   First, there is never going to liquid condensation dribbling down the surface of bare concrete from humid indoor air reaching the cold concrete. The moisture gets taken up into the concrete as adsorb- the surface appears dry even when the moisture content of the concrete is substantial.

   The risk for studs in direct contact with below-grade concrete is that it will wick ground moisture from the concrete into the wood. If there is low-permeance foam board limiting it's ability to dry toward the interior, and no heat on the foundation side to drive it out, the cool, ground-temperature edge of the stud can become moist enough long enough to rot. Even with BIBS cavity fill there is some potential of stud damage over time if the footing is in wet soil with no capillary break between the footing & foundation wall, but there would still be orders of magnitude more drying capacity toward the interior than with cut'n'cobbled foam.

4. 
   airfix | Apr 19, 2019 01:07pm | #4

   A builder friend of mine says that in all his homes his garages have 2x4 studs mounted direct to the concrete wall and blown in BIBS into the cavity with the insulation in direct contact with concrete? I thought it was not a good plan to have insulation in contact with concrete? We have the outside of our concrete wall waterproofed.

   Steve

1. Expert Member
   Dana Dorsett | Apr 19, 2019 02:11pm | #6

   >" I thought it was not a good plan to have insulation in contact with concrete? "

   That's right. Fiberglass will wick ground moisture, and on the above grade portion frost can build up in the cold side of the fiberglass over a winter in a 6A location if the wintertime garage humidity is high enough. With most garages the infiltration rates are high enough that indoor humidity in winter is usually too low for that sort of problem, but the conditioned basement the risk is real. In a heated tightened up garage it's hard to say, but the risk is still likely to be low. A 2x6 BIBS wall would also meet IRC code minimum for R-value, but that's not what I'd recommend. (R19 is the minimum if thermally bridged by studs.)

   Even if the wall has exterior side waterproofing moisture still wicks up from the footing. Waterproofing is rarely going to remain perfect over the long haul too.

5. 
   airfix | Apr 19, 2019 03:11pm | #7

   Dana,

   Thanks for helping me understand the science so that I can make more educated decisions in my build.

   Steve