Return on an outside wall! Keep it, or shut it off and insulate the bay? Help?

Discussion in 'HVAC Heating & Cooling' started by lithnights, Jan 31, 2010.

  1. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    .
    Dana; The above 'quote' is from an earlier posting of mine to the poster" lithnights". Again,INTERIOR walls areBEST but if it really comes down to the last resort an exterior wall is ok as long as it is weather tight/insulated.

    Note; the proper installation of a Return air passage via studs, joists, etc, can be done quite sufficiently air-tight if a little care is taken by the installer. And this can also be accomplished without the use of any caulking or tape,etc, by just a bit of good workmanship on the part of the installer. Panning and blocking of the joists using metal can be just as air-tight as any supply ductwork if it is done by a skilled tradesperson.
    Been there , done that.
  2. lithnights

    lithnights New Member

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    143
    Location:
    PA
    For those that are curious, the house in question here is in the Philadelphia area.. southeast PA.
  3. Dana

    Dana In the trades

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    An interior vapor retarder is required if the dew point of the interior air is found for extended periods of time within a moisture-susceptible layer of the wall structure. In most of Canada that will be for weeks/months on end, but in Tennesee, a few hours on the coldest days of the year, in which case air sealing is sufficient.

    In hot humid cooling dominated zones (Florida, and most of the gulf coast), air conditioned buildings need a vapor retarder on the exterior of the structure to keep water vapor from EXTERIOR air from condensing on susceptible materials within the wall structure, and placing vapor retardent materials on the interior keep the wall from being dried out by the mechanical cooling systems.

    In much of the moderate zones of the US the wall structures need to be able to dry toward the exterior in winter, toward the interior in summer, and vapor retarders do nothing but creat problems that otherwise would not exist in a decently air-sealed wall.

    In sountheast PA where this particular house is located vapor retarders aren't required, but the more highly vapor retardent materials are best placed toward the interior. It's a mixed climate, but still strongly cooling dominated with 5000HDD to ~1000CDD. The average Janary temperature is 30F/-1C but the average daily high is well 37F/5.5C which is over the dew point of 68F room temp with 30% realtive humidity. Some amount of interior vapor retardency is desirable, but not required. It's nothing like the issues you'd find in Ottawa or Calgary. Philadephia's vapor issues are more simliar to what you'd have in Vancouver (which also doesn't really need sub 1-perm interior vapor retarders.) As long as the structure can dry toward the exterior for at least half of the hours in a month, it isn't likely to develop mold issues from simple vapor diffusion through walls. But from air transported moisture, YES. Air tightness, both of the sheathing and the interior wall gypsum counts. A cross-sectional square inch of infiltration into a wall cavity is worth a whole 4x8 sheet of (highly permeable) gypsum.!

    And mold potential from a bigass air flow onto uninsulated sheathing from a cavity being used as a return duct, YOU BETt!
  4. Hube

    Hube New Member

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    156
    Location:
    Ontario
  5. hj

    hj Moderator & Master Plumber Staff Member

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    Return air is very "smart". It will find a way back to the furnace. In most homes here, there is only ONE return grill and all the air from the house finds its way back to it. That being the case, reinsulating that stud space and eliminating the direct return would be more efficient. In addition to the heat loss through the uninsulated bay, the return air is being chilled so it adds to the load on the furnace.
  6. lithnights

    lithnights New Member

    Messages:
    143
    Location:
    PA
    First off, thanks for everyone's comments and suggestions. As always, I learn a thing or two everytime I post a question. I think what I am going to do is what Hube suggested in posts 11 and 14. By installing a return grill on the floor that links into the existing chase going downstairs to the furnace, I keep a return. By then blocking off the old vertical chute (running from the floor to the existing grill) and filling it with insulation, I insulate the wall cavity. I'll then sheetrock over the space where the old grill was, thus cutting off any open air into the room.

    I think this will resolve my problem.

    Thanks all.
  7. dlarrivee

    dlarrivee New Member

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    1,172
    Location:
    Canada
    You want more than insulation in that void, you want an air seal as well.

    Get something to fill in the holes that make it so you can see your neighbors from inside the house.
  8. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    I agree with Hube...running return air thru the studs or joist space has been done for years. The only negative in my experience is that you have to have enough cross sectional area to return the air....sometimes you may have to take in more than one stud space to get the area required. This would be most critical if you have a furnace that typically does not have the external static capability.
  9. lithnights

    lithnights New Member

    Messages:
    143
    Location:
    PA
    Agree. I already used mastic foil tape to cover up the tiny holes I saw. I can't see any others from that vantage point (looking through the hole where the current grill is) so short of ripping down the wall, I have to assume there aren't any other air gaps.
  10. Dana

    Dana In the trades

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    If you use dense-packed cellulose to 3lbs/ft^3 or higher density to insulate it those tiny holes (even some bigger ones) basically go away packed tight with long-lasting cellulose plugs, an even when the plugs give up in a coupla decades, air can't move through the hole at nearly the rate it might with standard-density blown fiberglass or batts. Depending on the joist spacing and the thickness of the wallboard you have you might be able to get away with slow-rise half-pount foam too (with some risks...).

    If the stud bays are 24" wide and the gypsum is only 1/2" thick standard-density cellulose would still slow down any air leakage by 90%, but dense-packing might bow out the wall over time. If it's 3/4" gypsum or 16" stud spacing dense packing should be fine.

    Success with slow rise foam depends lot on the experience of the installer, but you have a bit more margin against blowouts with narrower bays and thicker wallboard. Foam would air-seal it perfectly, but for the price & risk differences my personal inclination would be on the dense-pack cellulose side. YMMV.
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