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Conditioned attic ventilation

NEplumber | Posted in General Questions on

Hi there,

I will preface that I did a fair amount of reading on the subject over the years and that I am in the industry, not a home owner or a GC. 

I own a plumbing, heating and cooling company. With great service, high quality workmanship and a will to learn I get called to all jobs from homes to nursing homes to pharmaceutical plants. Boilers, hydronic heat, and combustion are my specialty.

I got called to a home that had a musty smell in the attic. It wasnt long before I realized he has a conditioned attic (spray foam) and no conditioning in there. Sealed well from the home to add to that.

I guess it wasnt sealed well enough as the smell permeated to his upper story with the guest bedrooms…..(sarcasm)

My goal now is to rid his house of this issue. The issue is only during the 5-6 months that surround summer (late spring, summer and early fall).

The fix.
I was planning on air movement and dehumidification. To kill two birds with one stone, I figured a Santafe dehumidifier in the attic with the supply ducted to a HArt and Cooley diffuser in the main center hallway. The return would pull attic air. Coupling the attic and the upper story would be a return grill on either end of the upper story (in the guest bedrooms). This would limit noise as well as prevent any “stale” attic air from polluting the bedrooms. The upper hallway is open to the floors below with a large staircase (and multi story tall chandelier).
Constant circulation and the use of a humidistat on the upper story would be the control theory.

Where did I go wrong. What would you do?

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  1. GBA Editor
    Martin Holladay | | #1

    This problem may be unfixable. For a few horror stories, see "Spray Foam Jobs With Lingering Odor Problems."

    If the spray foam insulation had been properly installed, there would be no lingering odors. My guess is that the spray foam installer made some mistakes, resulting in a bad mix.

    If I were you, I wouldn't suggest that this problem is fixable. The homeowners should contact an attorney.

    1. NEplumber | | #2

      Thank you Mr Holiday for the reply. I am told that the issue was there prior to the spray foam being installed. The foam was sprayed on the underside of the roof sheathing 3-4 years ago.

      Looking forward to your thoughts.

  2. Jon_R | | #3

    It would be informative to measure the humidity in the attic while the smell is occurring. If high, then reduce it to stop mold smells. Attic only dehumidification works even when the house interior isn't conditioned.

  3. Expert Member
    Dana Dorsett | | #4

    Mixing the potentially polluted attic space air into conditioned space is a bad idea. A small amount of exhaust-only ventilation in the attic so send that "bad" air outside seems like a safer approach.

  4. Expert Member
    Peter Engle | | #5

    +1 to #3&4 above. You can't fix a humidity problem without knowing you have a humidity problem. Now that it's fall, it will be harder to diagnose, but it wouldn't hurt to try. Since this is a summer problem, there's no reason to jump into a solution that might not address the problem.

    There are a number of reasonably cheap dataloggers available now that can be programmed to take temperature and humidity samples at regular intervals. Once an hour is fine. Once or twice a day is probably OK, too. The ones I use (from Lascar electronics) can store a year of data. Most come with charting software that shows Temp, RH, dewpoint and often others. This sort of monitoring will tell you right away if the attic is running wet. if so, dehumidification might be in order. If it is running wet in warm/hot weather, there might still be some big air leaks from the outside. Bringing high dewpoint air into a cooler attic can make it run wet. You can find that sort of air leakage with blower door testing. IR cameras and smoke pencils can help. I found a couple of baseball size holes and lots of smaller ones in my spray foam this way, even after a thorough visual inspection.

    I like the idea of ventilating the attic and maintaining it at a slight negative pressure relative to the house. This will keep odors from migrating into the house. However, if the problem is outdoor air leaks, depressurizing the attic could just draw in more hot/wet outdoor air. I would strongly warn against your solution, because that brings the smelly attic air into the house intentionally.

    If the air is musty/smelly due to high humidity and mold growth, a dehumidifier could be a solution. If so, I would just run it in the attic with a positive drain to a wet stack. If the mustiness is unreacted spray foam components, Martin's right-there's no cure except complete removal. Personally, I find uncured spray foam to smell more fishy than musty/moldy, but everyone's nose is different.

  5. walta100 | | #6

    Before you spend all the energy needed to run a dehumidifier. I think you should consider simply adding a supply and return vents to the attic. If you can keep attic at the same temperature as the house mold will not be a problem.


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