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Supply placement in highly insulated home with vaulted ceiling

user-1063388 | Posted in Mechanicals on

SETUP AS FOLLOWS: climate/region – Washington, DC, (More cooling days than heating) – Deep Remodel – R-45 roof – dense packed roof joists – with double roof for ventilation. R-36 wall values – 5.5″ dense pack with 2 inches of rigid on exterior. Double glazed windows with proper solar shading. PROBLEM: I am struggling with mechanical sub about location of supply vents for three rooms on my second floor that have vaulted ceiling (2nd floor stairwell and landing zone with library and master bedroom – all on east side of house. – All other rooms are on the low side -south west – of the house and have drop ceilings. Contractor wants to put insulated ducts in ceiling joist bays of the vaulted room. I want to put them high on the interior walls of adjacent rooms that have drop ceiling and throw the air across the rooms with vaulted ceilings. He is concerned about the traditional heat loss/gain issue at the windows. My feeling is that throwing the air across the room at the windows is fine because this is such a tight house and outweighs the thermal loss I get by sticking an insulated duct into what would otherwise be a fully packed joist bay.

Assuming there is general agreement with my approach, should I also consider putting ceiling fans in the vaulted rooms to recirculate rising hot air in winter or will a well balanced supply/return set up do that for me.

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  1. user-659915 | | #1

    Patricia, I think your strategy is undoubtedly the better one. HVAC ducts should never be run in an insulated ceiling unless there is absolutely no alternative.

    As far as the fans go, by all means install them to provide an active airflow assist to your summer cooling but the supposed benefit of recirculation of rising hot air is pretty much a myth in a properly insulated home in a temperate climate. Unless, that is, your vaulted ceilings are ridiculously high.

  2. user-600397 | | #2

    I'm in complete agreement with James and you Patricia. In my experience, the HVAC subs are a bit behind the curve in insulation strategies and are more comfortable sticking with what they've been doing all along. We've been helping a lot of clients undo installs like the one he's suggesting!

  3. user-1063388 | | #3

    The ceiling high point is 12 feet (high side of shed roof assembly) - low side of vaulted rooms is about 9 feet, so not a huge rise and not really that high. Expand on the myth of recirculating hot air in a well insulated home if you don't mind. Won't hot air collect at the high point of the assembly regardless of the insulation and air tightness. Thanks

  4. user-659915 | | #4

    Yes, you will have a temperature gradient with the warmer temperatures at the top - this is true even with an 8' ceiling, which you can notice even when you stand on a stepladder to change a light bulb. However so long as the space is properly insulated and not too tall this hardly matters, it'll still be plenty warm at the occupancy level. If you do run the fan, even set in reverse, the forced air movement will generate a degree of wind chill largely cancelling out the benefits unless the fan is set very high in a very tall room. And of course the fan also costs energy to run.

  5. user-723121 | | #5

    I will second what James says on the temperature gradiant in a well insulated room. I measured my living room today with an 8' ceiling. Near the ceiling was 72F and on the floor was 70F, our home is fairly airtight at the ceiling level. Major temperature differences form top to bottom in residential buildings is mainly due to infiltration and the stack effect. Warm air leaving the building up high and cold, outside air replacing it at the lowest leakage points in the building envelope.

    I maintain a house with a open floor plan that is well insulated and airsealed will have little temperature gradiant at all due to the natural convective loops that occur. This was the principle of the double envelope house of days past.

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