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HRV/ERV in woodshop – problems with dust?

J_Eric | Posted in General Questions on

Hi all,

I have an 800 sq. ft. in woodshop under construction in climate zone 4 in Virginia. It will have R-13 fiberglass insulation in the walls, and it currently has 5.5 inches of closed cell spray foam insulation at the ceiling level. It has a 13 ft. high cathedral style ceiling (manufactured roof trusses) with the ductwork in the webs of the trusses (all metal ductwork encapsulated in 5.5 inches of closed cell spray foam insulation) and the air handler/indoor unit below in the conditioned air space. I have a 5hp cyclone style dust collection system with a HEPA filter that will collect most of the larger saw dust particles at the tools, and I have a variable speed air handler on my HVAC system that accommodates a 4 inch thick filter to filter out any micro saw dust particles while the HVAC system fan is on and I’m doing heavy sanding/sawing.

I am concerned about condensation forming on the windows of the workshop during the winter months due to water vapor not escaping from the ceiling because of the vapor barrier of the closed cell spray foam insulation at the ceiling level. To circumvent this type of issue, I’ve looked at a Fantech HRV (HERO 150H), and I’ve heard good things about the Panasonic ERV (FC-10VEC1). For my situation, do I need an HRV or an ERV? My other concern is, can an HRV/ERV ventilation system work properly without issue when installed in a dusty environment like a woodshop (even though, most of the dust will be collected by my dust cyclone and the air scrubbed by my HVAC cold air return filter)?

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  1. this_page_left_blank | | #1

    An HRV will remove more moisture in winter. I'm not sure I followed though, what is the source of moisture you're concerned with? Normally indoor air is dry in the winter, and that's even with showers, cooking, etc. What is generating the moisture in your wood shop (are you drying green wood)?

    A dusty environment shouldn't hurt an HRV, you will just have to clean/replace the filters more often.

    1. J_Eric | | #2

      Trevor, thanks for your reply. The woodshop is on the second level of a two story structure. The woodshop will have a bathroom and utility sink, and in the future it could be converted to living space and would then have a kitchen and wash machine. I'm not drying wood, but I need moisture to be low considering its affect on wood.

      The bottom level of the structure has a bathroom, but could also later be converted to living space and would then have a kitchen and a wash machine.


      1. this_page_left_blank | | #5

        Humidity is going to be a much bigger problem in the summer, at which time an HRV is probably going to make things worse. An ERV may help, but its effect on humidity is going to be minimal. If you want to maintain a narrow range of humidity, you're probably going to need some kind of system with active dehumidification (and possibly humidification).

  2. Jon_R | | #3

    Always fully conditioned? Consider non ERV/HRV ventilation. It's easy to make it highly variable (eg, during higher VOC activities) and few problems with clogging.

    Ceiling/wall perms can make a difference in interior moisture. For example, if your ceiling were 5 perms, it might lose 27 pints on a Winter day just to vapor diffusion.

    A wild guess - it may be initially too damp (due to slab moisture) and then in Winter, too dry (almost no moisture sources). That suggests an ERV and a humidifier (in Winter). And that your low perm ceiling is a good thing.

    1. J_Eric | | #4

      Jon, it will always be fully conditioned.

  3. Expert Member
    AKOS TOTH | | #6

    For a dusty situation you want an HRV. These have a metal core you can wash in some warm soapy water if needed.

    I would measure the pressure after the 4" filter to your HVAC. If the pressure there is low (less than 0.1" water), connect the stale air pickup to there. This way the air going through your ventilator would get filtered twice. It does mean interlocking the HVAC blower and the HRV, which might consume a fair bit of electricity.

    Another option is to put an inline filter box on the stale inlet of the HRV, I use a fantech FB6 on my ERV, it does a much better job than the built in filter on the ERV.

  4. J_Eric | | #7

    The thing I'm most concerned about in my situation, apart from the healthy aspects of having fresh air brought into a tight space, is moisture not permeating through the sheetrock ceiling and into the unconditioned attic above due to the ceiling effectively having a vapor barrier in that of the closed cell foam. I live in a mixed-climate of zone 4, and from what I understand it's not a good idea to install a vapor barrier on walls and ceilings in this climate zone. I'm worried about moisture building up on my windows in the winter months and anywhere else inside the conditioned space that would possibly lead to issues with mold. I thought a ventilation system would bring in drier air during the cold months of winter which would eliminate any possibility of moisture on my windows, and my HVAC in combination with a dehumidifier would alleviate any excessive moisture build up during the summer months, all the while having fresh air brought in from the outside with the ventilation system.

    Am I being overly concerned for nothing? I would have rather used fiberglass insulation on the entire ceiling, and I wouldn't be as worried about potential problems that I'm reading about with vapor barriers in a mixed climate of zone 4 (closed cell foam being one of them), but the truss design was an oversight when I planned this build; I should have gotten raised heel trusses that would have accommodated R-38 insulation at the eaves. I had no choice but to use spray foam at that part of the ceiling. At this point, the closed cell foam is actually only sprayed into boxed in cavities where the top plate meets the heel of the trusses, in other hard to reach areas that would have been difficult to insulate with R-38, and over the air tight boxes covering the recessed lighting. That leaves about about 250 sq. ft of ceiling that currently remains uninsulated, and I could put R-38 in the rest of the ceiling if it would help moisture properly permeate from the conditioned space into the attic. Or, should I go ahead and have the rest of the ceiling sprayed with closed cell foam, observe it this winter and if there is excessive moisture then install a ventilation system or whole home dehumidifier?

    Thanks for your advice, I am not an expert on this issue at all and all your advice is much appreciated.

    1. lance_p | | #8

      Hi Jason,

      Excessive interior humidity during winter months is usually not an issue unless you are generating a lot of moisture. If you are using decent double pane windows they should be reasonably condensation-resistant.

      A recovery ventilator will dry the space in the winter and humidify the space in the summer; an HRV will do this more than an ERV since the ERV can pass moisture between the incoming and outgoing airstreams (i.e. ERV is usually a better choice in humid climates). Supplemental filtration on the intake is probably a good idea given the dusty environment as the small built-in filters in recovery ventilators may fill up fast.

      If you plan to use an ERV I see no reason why you can't seal the building up tight, similar to residential construction. This would mean taping all seams on your exterior sheathing to work as your primary air barrier. Adding 1" of properly detailed foam board insulation to the exterior would keep your sheathing above condensation temperatures and act as a vapor retarder to keep moisture from moving through the walls. It will also cut down significantly on heating/cooling costs as it will improve the insulation value of the wall assembly by more than 50%.

      Allowing moisture to permeate a wall or ceiling in winter months is generally a bad idea. Keeping humidity under control with ventilation (winter) and AC/dehumidification (summer) is a much better plan than having a permeable building envelope.

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