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What’s the best strategy for saving energy in my unconditioned attic?

stuartbrodsky | Posted in Energy Efficiency and Durability on

I’m in zone 5A and have 6″ of insulation between the floor joists and no insulation between the roof rafters. The HVAC system and all its flex duct work are in the unconditioned attic. The building is a town house with 40″ common walls running east west and the 18″ roof ridge running north to south. There is eaves and ridge venting. The roof rafters are irregularly shaped: some are standard widths, others are not.

The HVAC unit and flex duct work are in the attic, in the unconditioned space that is bitter cold when it’s bitter cold out, and roasting hot even on moderately warm sunny days from late spring to early fall.

Is it better to focus on reflective material or insulation in the roof, or more insulation on the floor to reduce the HVAC run time in the conditioned on the two floors below?

I have a ventilated crawl space under the house. The joists are insulated with original (30 year old material. The exposed concert walls have rigid insulation. In the winter the floor above the crawl space is cold, and drafty in some spots.

Thank you.

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  1. wjrobinson | | #1

    Hire a blower door person that also does air sealing. If you have N gas heat, great. If not time to add a mini split. Also if you tighten your home you need to move toward sealed combustion hot water and heat systems. Tightening a home up too tight with the older style units is not safe.

    I would start with finding the blower door gent... through a utility program or state energy program.

  2. slopecarver | | #2

    You have 2.5 options:

    1 - Bring the HVAC inside the conditioned space. Insulate and air seal your existing rafters. I'm not sure what the existing practice for this is regarding cost effective high r assemblies that control vapor permeability.

    2 - Insulate the ductwork and blow in to R60+ above your existing insulation. Make sure you airseal your attic floor so no hot air escapes into the attic space.

    2.5 - combination of the 2 with a utility room that is insulated to the rafters with the rest of the attic floor being your R60+

  3. Expert Member
    Dana Dorsett | | #3

    Reflective insulation at the rafters is pretty useless during the much more important heating season, and less cost effective than adding more insulation at the attic floor to bring it up to current code min.

    Air sealing both the house (particularly the attic floor), ducts, and air handler are an all important first step.

    If the attic venting is soffit-to-ridge it may be easier to air-seal using highly vapor-permeable housewrap (~30 perm Tyvek is better than ~10 perm Typar here) stapled to the rafters, and caulked to the framing. Any seams joining sheets of housewrap has to happen at framing, and must be caulked between the sheets, and the edges need to be caulked to something solid, not just stapled. That will at least bring the HVAC within the PRESSURE boundary of the house, limiting the infiltration-drive effects of duct leakage, without blocking the moisture from getting out of the cold attic in winter.

    In the crawlspace, istalling a ground vapor retarder mastic-sealed to the rigid insulation, and held in place with 1-furring throuh-screwed to the foundation with TapCons would allow you to seal the venting without creating problems. Air-sealing the foundation sill & band joist is also important too. If the wall foam is only an inch thick, it's worth considering bring it up to code-min with another layer of foam, or a non-structural studwall with R11-15 rock wool/fiberglass insulation. Code-min for crawlspace walls for zone 5 is R10 continuous insulation, R13 if between studs:

    With both the attic and crawlspace air sealed, you've taken away the bulk of the "stack effect" air infiltration drive, which is the most important air leakage to deal with. With the insulated and sealed foundation walls with a ground vapor retarder you've also reduced the moisture levels in the crawlspace during the summer season, where the joist edges can easily be cool enough in an air conditioned house to take on moisture, leading to chronic mold issues.

    If local codes require vented crawlspace for flood drainage purposes, you can seal the vents with operable doors or removable weatherstripped hatch covers, insulated with R10 rigid insulation.

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