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Planning for A/C in a heating climate?

davidbailey | Posted in Energy Efficiency and Durability on

Hi all!
I’m getting ready to do an outsulation project on a modest house in Montpelier, VT – climate zone 6a. The house was built in the 1970’s or 80’s. It’s 2×4 @ 16″ o.c. stick frame construction and is filled with fiberglass insulation. There seems to be a mix of foil faced and regular brown kraft paper fiberglass in the stud bays. The street level floor has 1/2″ polyiso on the interior side under the drywall. The exposed roadside wall faces south/southwest. The intention is to do all the work from the outside and avoid interior work as much as possible. 
My plan had been to use a pressure sensitive adhesive WRB (in this case Blueskin VP100) and add 4″ of mineral wool board insulation, leaving the windows and doors in place (window replacement isn’t a part of the scope). 
We consulted with a building enclosure specialist and he was keenly focused on air conditioning and expressed worry that a permeable wall assembly would allow vapor in which will condense on the drywall and cause no end of headaches. I admit to having a poor grasp on A/C and its building enclosure implications as very few homes I work on have A/C in their clients’ minds. The consultant wants to see 4″ of foil faced polyiso foam added to the exterior a la the Perfect Wall assembly. I appreciate his logic in this. The homeowner does not tolerate summer heat and humidity well and a/c is a likely scenario. But something is tickling my brain and background. I am worried that if/when water gets into the wall it will have no where to dry to, especially on the ground floor. I have replaced many walls the hard way that suffered from leaking windows or what-have-you that had wall assemblies that had no drying potential. This will be a rental property for some number of years before the homeowner moves in after they begin to age up and want to live in a more walkable place. I am having visions of a renter leaving the house for a day or longer with windows open and a torrential rain soaking the floor… but that’s my natural state of doom-and-gloom projecting and, anyway, how can one design around the possibility of negligence… But if water gets into the wall, which seems inevitable regardless of the source, where will it go?
What’s the safer or safest assembly here? Vermont appears to be on its way to being a mixed heating/cooling climate and it doesn’t seem too soon to start designing for this reality.
I appreciate any and all advice, and many thanks for your time!


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  1. Expert Member
    Akos | | #1

    Condensation in wall won't happen if the outdoor dewpoint is bellow room temperature. You have to be in pretty hot and muggy climate for the dewpoint to rise above 75F for extended period of time, in colder climate this might happen for a day or two but not long enough to matter.

    I'm in slightly warmer climate than you where summer time AC is the norm. Almost all walls have 6 mil poly on the interior and they work just fine, no issues with summer time condensation. These are houses with no exterior rigid, exterior foam or with exterior mineral wool.

    I would build the house with whatever insulation you are comfortable working with. I find rigid foam (EPS/GPS/plyiso) to be much easier to use install plus it is also significantly cheaper than mineral wool.

    1. davidbailey | | #3

      Thanks for your response. I appreciate you taking the time. My main concern here is the wall that might have polyiso inside and outside. Seems like a rot sandwich waiting to happen, but maybe I'm overthinking it?

      1. Expert Member
        Akos | | #5

        This was a worry when exterior rigid started to be installed on walls. Building scientists have look at it and find that it is not an issue. You can read more about it here (near the bottom):

        If you are not using sufficient exterior insulation for condensation control for your climate, air sealing the house at the sheathing layer, using a textured house wrap and not taping the exterior rigid helps.

        In your case with the thick exterior insulation over a 2x4 wall, I would not worry about it. You still have to get your window flashing details right. Any major bulk water leaks that make it into the stud cavity will cause issues. Than again, so do major bulk water leaks in standard insulated wall cavity.

  2. walta100 | | #2

    The way I see it upgrading R13 walls is a foolish way to spend anyone’s money.

    It sounds like the current wall is a R13 or so. Upgrading from R13 given to very high cost of replacing the siding will never recover the costs in fuel saving. I might feel differently if you said keeping the old siding is no possible for some good reason. Even if the siding is being replaced the return on investment is not likely to be in the walls.

    You say the client does not tolerate summer heat and humidity well. Do you have a feeling for the likely set point will be? I knew someone that that was dying and they had set the AC thermostat to the low 60s for comfort. I am sure it was bad for the house when it was hot and humid but short of that kind of extreme, I don’t see a problem given the short cooling season.

    The sky is not falling and Vermont is not becoming Louisianan. Yes, more and more people are demanding central AC but I think that has more to do with people’s expiations than the climate. I doubt you can document more than 2° F change in average the last 50 years.

    Seems like if they are not willing to do the windows this is not a real “deep energy retrofit”

    I challenge you to build the computer model of this home and look at the ROI for the wall upgrade. Seems to me this amount of money should be spent on upgrades with better ROI.


    1. davidbailey | | #4

      Thanks for replying, I appreciate that you took the time. I didn't say anything about the siding in my post, or about this being a true DER, but I can understand how you might have inferred that. The client has been thinking long and hard about this work and knows the arguments you've raised. The likely r10 walls the house might currently have (fiberglass is almost never installed well) won't suffice for switching the house over to mini-split heat pumps as far as heating is concerned. I think the impetus for this work isn't so much about ROI as it is about occupant comfort. An airtight, r30 wall is going to be significantly more comfortable than what's there now. The existing windows are passable double pane units. Could be better, I suppose, but talk about a terrible ROI!

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