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Community and Q&A

Insulate the ductwork or warm the crawlspace?

Dan Verbeten | Posted in Energy Efficiency and Durability on

I work in an old church that has been repurposed that has a forced air heat system. The ductwork capacity is large and when the heat kicks on the initial blasts are pretty cold. A work colleague thought it would be better to put some type of register off the ductwork to heat the crawlspace than to insulate the ductwork. The crawlspace is about 2500 square feet and has a vapor barrier with spray foam insulation on the exterior walls. The thought was that heating the crawlspace would help the furnace work less hard to get warm air pumped into the building. Thoughts?

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  1. Expert Member
    Dana Dorsett | | #1

    What is the goal- to get rid of the initial bursts of coolth?

  2. Dan Verbeten | | #2

    I would say the primary goal is to have the most efficient heating system possible. Secondary to that would be to get rid of the cold air blast.

  3. Expert Member
    Dana Dorsett | | #3

    Keeping the crawlspace heated would lower the efficiency but might mitigate the cool blast, if only slightly. Insulating the ducts wouldn't be as effective- the initial puff would be just as cool, but the system would be more efficient.

    A modulating air-handler with a multi-stage furnace would be expensive, but could mitigate the chill of the both the initial and subsequent air movement considerably.

    Most church heating systems are ridiculously oversized for the actual heat load to be able to take advantage of deep setbacks for when the building is not in use. Being capable of raising the temperature 25F in less than an hour makes deep setbacks possible, but reduces comfort due to higher air volumes (= wind chill) , and more cycling.

    Getting to the "right" compromize may be beyond the scope of what's possible on a web forum, and would need a lot more detail about the building, patterns of occupancy, equipment sizing, and the actual budget for fixing the comfort issue.

  4. GBA Editor
    Martin Holladay | | #4

    In general, raising the temperature of the crawl space will require more energy than leaving the crawl space at a lower temperature. So the simple answer to your question is: If you want to save energy, insulate the ducts -- don't raise the temperature of the crawl space.

    However, that doesn't mean that installing duct insulation is the most important measure to improve the performance of your church. There may be opportunities to perform air sealing work, either in the crawl space or attic, and this type of air sealing work might be a better use of scarce dollars than installing duct insulation.

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