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Heating ducts in vaulted ceiling

bzwiers's picture

I live just outside the Seattle area and am building my home. It is a three story, on grade, 3750 sq ft structure. I'm planning on using two separate furnace units - an electric heat pump for the third floor and a gas fired unit for the other two - so that I can better control the zones as well as avoid ducting chases through the open floor plan of the second floor. The third floor is mostly vaulted ceilings and my plan was to run the ducting in the 2x12 rafter bays but I'm wondering if condensation would be an issue.

Structure from the outside in = standing seam metal roof, underlayment, 5/8" CDX, 1 1/2" air gap, 1" foil faced poly iso as a baffle edges and seams sealed with foam, Blown in John's Mansville BIBS insulation on 2x12 rafters.

If anyone has suggestions or experience I would love to hear ideas or input.

Heating (post #210145, reply #1 of 3)

May I suggest that you take a look at baseboard heating. Baseboard heating, devided into zones, means that every room, has its own thermostat controlled motorized valve. This means that every room will run within one half of a degree from your chosen setting all the time. This system is totally quiet and economic as it prevents overheating in any room and you can close doors without upsetting the system. As the sun moves round the home, each room thermostat lowers or raises the heat required in that room.

Thanks for the suggestion. I (post #210145, reply #2 of 3)

Thanks for the suggestion. I did consider this as an option but unfortunately it does not address the C portion of HVAC. Other things I've considered and ruled out and why:

1. Mini split - wife cannot live with the visual appearance

2. Radiant - expensive and no cooling - unless I run cold water through the system in Summer but condensation will almost certainly be an issue.

Any thouhgts, input, or suggestions on things to consider when installing ducting insdie the roof bays would be appreciated.


Condensation (post #210145, reply #3 of 3)

Water vapor in the home, is caused by people, cooking, washing, drying, breathing, sweating and animals, indoor plants, fish tanks.

In heated areas in winter, having and using exhaust fans in the kitchen and bathroom, pushed this warm wet air outdoors, and replacing it with colder drier air from outside lowers the amount of water vapor in the air and the amount of condensation that will form.

The warmer the air indoors the greater the amount of water vapor it can hold, however once this warm air meets a cold below dew point surface condensation forms. This is usually on cold windows in winter, or on the outside of windows in humid areas in summer.


Condensation doesn't form on warm surfaces (above dew point), obviously all trunking that is not insulated, or insulated with an insulation external surface temperature below dew point (all of it) will be covered in condensation.