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Finished Basement Air Quality

MojoMan | Posted in Energy, Heating & Insulation on August 23, 2005 03:48am

I’m working on a proposal to finish a basement in a large, newer home in eastern MA. The homeowner is concerned with air quality, predominantly the musty smell that is encountered in damp basements. This basement is not wet, it’s just humid.

Vapor barriers on the wall and dehumidifiers are standard treatments, but are there better approaches? The house has forced-air heat and AC and we will be talking to their HVAC techs to see if we can tap that system. If the existing system can’t be used, electric baseboard heat is a likely choice for periodic heating. I’m thinking about something like a Fantech ventilator or air-to-air exchanger to help with air quality, but I don’t think that will help much when we have high humidity or freezing temps outside.

Any thoughts? Thanks!

Al Mollitor, Sharon MA

 

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  1. Billy | Aug 23, 2005 04:21am | #1

    Go here and you will get lots of excellent information.
    http://www.buildingscience.com

    http://www.buildingscience.com/resources/mold/Read_This_Before_You_Design_Build_or_Renovate.pdf

    Then search Breaktime for discussions about this subject and you will find mucho information.

    Billy



    Edited 8/22/2005 9:24 pm ET by Billy

  2. VaTom | Aug 23, 2005 04:41am | #2

    predominantly the musty smell that is encountered in damp basements

    If you control humidity and give frequent air changes, there'll be no problem.  We live in a "basement" with extremely good air.  Really no different from upstairs, other than the leaks.

    Good luck with the HVAC guy.  In my modest experience they generally don't know squat about the V.  Just HACs.  Dealing with the same issue currently for a friend having a very expensive house built here.  Much as I hate to get involved, between the contractor and subs and the client, there's absolutley no reason not to have good air.

    Rocket science, this ain't.

    PAHS Designer/Builder- Bury it!

    1. MojoMan | Aug 23, 2005 02:52pm | #3

      Thanks, Billy. I'll check out Building Science.

      Thanks, Tom. Fresh air makes sense, but my concern is what to do when the outside air is very humid, as we have here July-September. Is it best to simply seal things up and dehumidify? I doubt the typical homeowner is going to vary the ventilation on a day-to-day basis as the outside humidity changes.

      Al

       

      1. AndyEngel | Aug 23, 2005 03:33pm | #5

        Check FHB 169 for an article on this very subject. The key to air quality in basements is moisture control. Vapor barriers are not your friend here.Andy Engel

        Senior editor, Fine Woodworking magazine

        An updated profile is a happy profile.

        Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig

        None of this matters in geological time.

      2. VaTom | Aug 24, 2005 04:38am | #9

        Is it best to simply seal things up and dehumidify?

        No.  You absolutely need air changes. 

        Every dehumidifier I've seen will turn itself on when humidity gets high, and vice versa.  We use a dehumidifier in the incoming air stream.  Coupled with an air-to-air heat exchanger (HRV or ERV), it loses most of the added heat.  Same device works wonders during the heating season to conserve heat.

        What I did, and you want to do, is to make a system that requires nothing from the homeowner.  Other that to brag on your efforts, of course.  Our ventilation system is non-stop.  I followed then-Canadian code of .5 ACH (total air change every 2 hrs).  

        Make it automatic.  We monitor humidity and turn on the dehumidifer when required, but there's no reason not to use the built-in sensor.  Will require a bit of fine-tuning to determine the correct setting.

        Figure the cubic feet involved and go from there.  As uncommon as it is, a ventilation system is a small device that makes a world of difference.  I'm currently dealing with a friend, building here, whose HVAC guy actually told him that he'd get all the fresh air he needed by opening the door to go in and out.  That's HAC, no V.  The client knows better, having been here.  Cost is small, they'll work it out.     PAHS Designer/Builder- Bury it!

        1. Brian | Aug 25, 2005 01:20am | #13

          VaTom - I'd love to see a pic or diagram of your HRV/Dehumidifier setup, as I will be doing something similar in my ICF house soon.

          I will be drawing air from the bath and kitchen ceilings, and returning air to the basement, with the HRV in the (conditioned) attic.  If there is a way, I'd like to include a dehumidifier, since our Mid Atlantic air is so humid.

          BTW (if you remember our post a few weeks back) the second story pour went perfect - the trailer pump was much gentler - no form problems, straight walls, now that I am up the learning curve a bit, I think I'm gonna love my concrete home.

          1. VaTom | Aug 25, 2005 05:01am | #14

            Brian and Al,

            I have a pleated filter attached to the dehumidifier input, which lives outside on a retaining wall.  The output from the dehumidifier goes into the air-to-air exchanger.  It's hot if the dehumidifier's running, otherwise outside temp.  The filter also keeps the dehumidifier coils clean.

            The dehumidifier can be almost anywhere, not necessarily in the air stream.  Mine's in the air stream so we can dump the extra heat before it heats up the house.  Outside, so the majority of the generated heat doesn't ever enter the air stream. 

            Brian, you lost me.  You're using bath and kitchen air instead of outside air for your HRV?  If that's wrong, and you are bringing in outside air, you can use a dehumidifier like we do, in the intake air stream.  Before mounting it outside, it lived over our bath, with a plywood box covering each end (duct into both boxes).  If I'm not clear, yell.  Not pretty, but I can take a pic of part of the system.  Form definitely following function in this case.

            Congrats on the pour.  I've been pretty sure my pumping problems were due to excessive drop, even though I've been frequently told it shouldn't be an issue.  Your experience seems to confirm my conclusions.

            Concrete can, indeed, make a great home.  DW just returned from 10 days on the road.  She really missed our air quality.  And me, I hope.

            Watch out for successful concrete experience.  A GC friend talked me into supervising his crew for retaining walls and steps on a commercial job.  Once I get the rest of the dirt hauled off (60 yds so far) we'll be on to the forming.  Need any fill?PAHS Designer/Builder- Bury it!

          2. Brian | Aug 26, 2005 04:02am | #16

            VaTom - to clarify I am sucking air from baths and kitchen through the HRV to the downwind outside of the house, and sucking outside air in from the upwind side, through the HRV to the basement.  At least that is the current plan.  We do plan on AC, so summer humidity may be OK, if not I'll add the dehumidifier later.  It sounds like your air quality is something special - I hope ours is close.  We once had a rental with a doctor recommended allergy filter on the air handler.  Never had to dust - there was no dust. period.  We'd like to reproduce that in this house...

            VA is 45 mins from me... Can you deliver the extra fill dirt to Hagerstown cheap?

             

          3. VaTom | Aug 26, 2005 04:51am | #17

            Hagerstown I've been through, many times.  45 minutes from here it ain't. 

            I'm burning $15 of diesel per, 48 miles round trip, which nets me $110.  The only way that comes out "cheap" is that the GC is saving upwards of $50 tipping fee on every load I don't take to the land fill.  Guy getting the fill is tickled with his input to the pot.  70 yds so far, maybe another 30 to go.  A whole lot of dirt from a 50' wide lot.  And all I'm doing is creating an approved parking lot.  Starting the stone haul tomorrow.  Dirt one way, stone the other.  Engineer estimated $25k for the parking area, concrete retaining walls, steps, sidewalk, and trees.  Yeah, right.

            Thanks for the clarification, sorta.  Make up air for the baths and kitchen comes from the basement?  Dehumidifiers are far from ideal, but they do dry out the air.  As does our heat pump water heater, easily 50% of our dehumidification needs.  Any application there?

            Lotsa filtered air changes, controlled rh, ipso facto, great air.  Very simple.PAHS Designer/Builder- Bury it!

          4. Brian | Aug 27, 2005 04:57am | #18

            VaTom - your posts are always so informative -

            The basement idea is so we can have fresh air everywhere, the thought being drop it in the basement and suck it through the upstairs ceiling, hoping the eddy currents will cover the rest.  I really would like to keep it simple and not run a lot of ducting if possible.  I will be looking into the homemade HRV plans - you have convinced me they are pretty simple devices, and the cfm calculations are easy enough.

             

          5. VaTom | Aug 27, 2005 04:06pm | #19

            Thank you Brian.  Indoor air quality is something majorly missing from construction here.

            I won't pretend to understand your air system, but that's not important, long as it works.  Without really knowing what I was doing here, I ended up with a surprisingly functional system.  After a decade, the only change I'm making on the next place is to add a utility room to house mechanicals and contain some noise.

            I substituted corrugated aluminum roofing for the core.  Here's what Pop Science had to say:

             PAHS Designer/Builder- Bury it!

          6. Brian | Aug 28, 2005 12:31am | #20

            Awesome - I will build it in the mechanical room to isolate the noise - thanks for the plans. 

  3. User avater
    rjw | Aug 23, 2005 03:02pm | #4

    One of the national basement waterproofing co's has a humidity activated exhaust fan which draws air at floor level and discharges theough the rim joist.

    I have no idea how well they work, and am generally skeptical, but I know a woman who has one and says it has helped with her sons severe allergies.


    View Image
    Sojourners: Christians for Justice and Peace
    1. User avater
      BillHartmann | Aug 23, 2005 04:01pm | #6

      "One of the national basement waterproofing co's has a humidity activated exhaust fan which draws air at floor level and discharges theough the rim joist."Is the fan control differential so that it is not pulling in air that is more humid.There is one or two companies that are selling what is basically an expensive bathroom fan with a fancy chase for the exhaust pipe.It run continuesless list all kinds of claims about how it reduces humidity and how it is much cheaper to operate than a dehumidifier.And how it draws down nice fresh air from upstairs.But it not explain how it brings in outside air to replace what is exhausted. And how humid that air is this time of year.

      1. User avater
        rjw | Aug 23, 2005 06:05pm | #7

        Good points all, and that's why I'm skeptical about them.

        View Image

        Sojourners: Christians for Justice and Peace

  4. Tim | Aug 23, 2005 08:37pm | #8

    This is a classic problem. The difficulty is that, as most basements are below grade and the remainder of the house is above grade, there is a significant difference in the loads experienced in basement and where the thermostat is located. However, even with a dedicated unit or zone, the problem is that the cooling load is not there. Aggravating this condition is that the ground temps are low enough that the mostly below grade basement walls without a significant internal load, are at or below the dewpoint of otherwise good inside air conditions (air at 75 degF and 50% relative humidity has a dewpoint of 62.5 degF).

    The building scientists have more details on the best insulation details than I, but some construction details that help are exterior foundation wall and floors that are insulated on the outer side of the conditioned envelope.

    Two common ways to remove moisture from an environment that has a limited need for cooling. Condensation and displacement. Condensation like as in an AC coil, resultls in unneeded cooling, and then requires reheat (therefore, the dehumidifier is the common treatment). Displacment of the cool damp air with warmer dryer air can be relatively easy or not.

    You say there is an existing forced air system. Is it located in the basement? One option would be to take air from the upstairs, maybe even using the systems return ducting, use that as the supply for the basement and "return" that air to the unit return before it is filtered and cooled/heated to condition the space above.

    Another option would be to use an ERV to take air from the space above, exhaust it and provide somewhat tempered replacement air to supply the basement.

  5. thebozer | Aug 24, 2005 05:42am | #10

    about a year ago, i did a finished basement in Cape Cod and here's what i did in regard to air qualiy.... 2x4 construction with fiberglass insulation (no Vapor Barrier), standard dehummidifie with little hero pump (runs continuous/as needed and homeowner never needs to empty pan), and also installed Fantech's HRV which runs continuous. I've been back several times and this seems to be performing very well.

    1. MojoMan | Aug 24, 2005 02:36pm | #11

      Thanks Tom and Bozer!

      How do you use the dehumidifier in the in-coming air stream? Any special connections, or do you just dump the fresh air in the direction of the dehumidifier?

      How much did the Fantech HRV cost?

      Al

      1. thebozer | Aug 25, 2005 12:49am | #12

        i wan't too concerned about that. the square footage was about 350 for my project; it may be something to think about in a larger room?! i had two supply vents and the dehumidifier was pretty close to both. the HRV goes for around $750.

  6. User avater
    constantin | Aug 25, 2005 12:24pm | #15

    I second taking a closer look at the wonderful resources that buildingscience.com puts at your fingertips. Things like insulating the exterior of the foundation wall/floor to keep the warm in but the water out. French drains at the bottoms of foundations to carry water away, etc. Once you institute these basic steps, the basement will never get musty unless you have a lot of infiltration.

    My new basement replaces a old rubble/brick mix that leaked for years. The old basement was a haven for Termites (in MA!), water, and critters. The new foundation (cast concrete) is dry even when there is standing water outside (clay soil) thanks to Rub-R-Wall and 2" of XPS insulation. Another big plus is leading the roof water away from the foundation (dry-well, daylight, whatever) as it takes away a big chunk of the water that can infiltrate your basement.

    Once the basement is reasonably water proof, the only worry is the high humidity of summer air conensing in the basement. If you seal the infiltration opportunities and then treat the basement as just another zone on your AC system (or install a dehumidifier), this should not be a concern.

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