Replies

1. 
   FredL | Mar 05, 2002 04:59am | #1

   *
   Dear David,

   There are 3 concerns with attic mounted ductwork. From the cold climate POV:

   1. The ducts create insulation voids that promote heat loss from the house.

   2. These same voids chill the ducts themselves.

   3. Duct leakage either sucks insulation fibers into the house, or allow warm, moist air to blow into the insulation.

   Loose fill insulations will conform much better to framing and the ductwork. You MUST seal all the duct joints to NASA standard. Finding fibers in the core of your new HRV will not be a happy day.

   Keeping my ducts indoors, Fred

1. 
   Tim_Rice | Mar 05, 2002 04:17pm | #2

   *David,You can install ducts anywhere you need to, though I am with Fred in that if you can avoid putting ducts in an unconditioned space, do so. If you cannot avoid it, seal the duct joints with mastic, insulate with 1-1/2" of foil faced fiberglass duct wrap and tape the joints. Since the HRV ducts will not see the conditions that supply ducts (55 degree cooling and 120 degree heating) will, should not be a problem.

1. 
   Tim_Rice | Mar 05, 2002 04:18pm | #3

   *Fred,I didn't realize NASA had standards for ductwork. Part of the space program we never hear about?

1. 
   FredL | Mar 08, 2002 05:53am | #4

   *Dear Tim,I suppose they do. "NASA grade" is weatherization slang for doing insanely great work. In terms of duct sealing, I would mean leaving less than a few parts per million of leakage area.Not watching my words, Fred

1. 
   Tim_Rice | Mar 08, 2002 04:16pm | #5

   *When dealing with ductwork, especially the sheetmetal kind, SMACNA is the folks to reference. As far as parts per million of leakage area goes, I don't think that is a measureable quantity in terms of duct leakage. But as I posted before, I learn something new every day.

2. 
   FredL | Mar 08, 2002 06:01pm | #6

   *Dear Tim,We usually see duct leakage expressed in units of square inches or CFM25 when there is pressure testing. There are guys out there trying to build perfect systems and they are getting them down below 1 square inch of effective leakage area. A man named John Tooley runs Advanced Energy and they train techs to seal really tight.Sometimes I use the 'part per' system. Most houses leak at rates of around 1 part per thousand of surface area. But when we try to build something as tight as a swimming pool liner, ppm is not an unreasonable thing.Do you measure duct leakage routinely and how to you record the figures?Learning new stuff too, Fred

3. 
   Tim_Rice | Mar 11, 2002 03:34pm | #7

   *FredL,I do not measure or record duct leakage. The units seemed odd. I would have expected that air leakage would be measured in CFM or as a percentage of overall system flow. Parts per thousand/million/billion are units that I'm used to seeing in concentration measurements, such as water hardness or dissolved oxygen. This whole "weatherization" business, has got my interest. I'm not trying to be disagreeable, just curious.I do design and specify forced air systems, among other things, and usually reference the SMACNA manual for fabrication and installation. Medium and high pressure systems are common in large, variable air volume systems. Up to 3" in the supply is not uncommon. Leakage at this pressure would not only be costly, it would make a lot of noise. Double -walled, spiral wound duct is the norm. I would not expect anyone to use such in a low pressure system, though. Not very practical.So how do you seal duct to "NASA" standards? I am used to seeing low pressure ducts sealed with foil tape, mastic or polyeurethane/silicon types of sealants.