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Open Cell roof deck or attic floor

foamiscool | Posted in Energy Efficiency and Durability on

Done my research, open cell foam is my preferred choice. Building a 3,300 sf house in Fort Worth TX. Pitch of roof is steep [email protected] House is single story. Considering the following two foam options, would appreciate feedback. Additional info, going with two a/c units (4 and 2 ton, both units are two stage. Also using erv air exchange to supply fresh air.

Option A) Open cell foam 5.5 inches under the roof deck, 3.5 in walls, both hvac blowers are in attic. This will provide the closed envelope conditioned attic model. However because of roof slope, creating more surface area for roof deck foam than normal, translates to more $ for foam. Roof deck will be standard OSB.

Option A Benefits:
HVAC running in conditioned space.

Option B) Open cell foam 5.5 inches on attic floor, and 3.5 in walls. Same scenario with hvac blowers (in attic). Non-conditioned attic here, but on this option I’ll go with “OSB-radiant foil backed” for roof deck to help keep attic temperatures down. This option also give me the ability to add 5-10 inches of blown in cellulose on top of attic floor foam if I feel necessary in future.

Option B Benefits:
All ducts, vents and electrical housings cut at ceiling drywall are sealed with foam
Less expensive to foam attic floor than roof deck
I’m not having to condition my attic space.

Ok guys, A or B?

Thanks.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jeff,
    Option A is clearly better, by a wide margin.

    Here's how you can improve the performance of Option A: Include some way to address thermal bridging through the rafters, either by increasing the thickness of the rigid foam (so that it covers the bottom of the rafters), or by installing a continuous layer of rigid foam on top of the roof sheathing before you install your roofing.

  2. foamiscool | | #2

    Can you explain further why Option A is clearly better?

  3. homedesign | | #3

    bump

  4. GBA Editor
    Martin Holladay | | #4

    Jeff,
    Keeping your HVAC equipment and ducts within the conditioned space is extremely important. The penalty for leaving the equipment and ducts outdoors varies quite a bit, but can easily amount to 20% of your energy use. Most air handlers leak at the seams, as delivered from the factory, and are hard to seal well because of the need for panels to be removable for servicing.

    For more information, see Keeping Ducts Indoors.

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