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Do I need to reconsider open-cell foam vs. fiberglass insulation?

pure novice | Posted in Energy Efficiency and Durability on

I have a 16 X 28 work shop with a metal roof installed over felt and OSB Sheating with silver radiant barrier on the underside. The outside walls are covered by 7/16 OSB and Hardi Board planks.

I live in South Texas where it gets extremely hot and while building, I added soffits and a ridge vent with gable vents to help the air flow but it still gets fairly hot inside. I will install an 8 ft interior ceiling made of sheet rock and for the walls, I will use 7/16 OSB.

I have been told that open foam insulation is best for what I should use and the installer said he has to close off all the venting to make the building and insulation work right. After reading an article about needing fresh air I became concerned. Much of the time I can keep windows open and when needed, I have a small window air conditioner (12,000 BTUs) but I don’t leave it running all the time. Sometimes I am gone for 5-6 weeks during the hot summer and the building stays closed.

Will this cause problems? Am I making the right choice to go with the blown in open foam insulation vrs fiberglass batting? Am I creating a hazard by not having an air system? Lastly, will the metal roof cause condensation/heat thermal problems?

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  1. GBA Editor
    Martin Holladay | | #1

    There are three ways to insulate this building.

    1. You can have a vented, unconditioned attic above your workshop.

    2. You can have an unvented, conditioned attic above your workshop, with insulation installed along the roof slope, without any ventilation channels.

    3. You can have an unvented, conditioned attic above your workshop, with insulation installed along your roof slope, with ventilation channels between the top of your insulation and the underside of your roof sheathing.

    All three strategies will work if the details are done correctly. And two of the three strategies can be accomplished with either type of insulation you describe -- with open-cell spray foam or with fiberglass. (The exception is Option #2. That option will only work with spray foam, not with fiberglass batts.)

    It sounds as if your spray foam installer likes to install spray foam against the underside of the roof sheathing. But that is not the only way to use spray foam. It's also possible to install spray foam on the top side of your drywall ceiling, or to create ventilation channels with baffles between your rafters before the spray foam is installed.

    You have to decide what you want. All of these strategies will work. For more information, see How to Build an Insulated Cathedral Ceiling.

  2. pure novice | | #2

    Martin, thanks for the quick response. Sounds like your Option 2 is what my installer has recommended. The cost is about 2/3 more expensive and I'm not sure if I'll get enough difference in the comfort level for a workshop to justify the cost. I know it is more energy efficient to use the foam but it will never pay for itself with the amount of use I'll get. Do you think there will be a substantial difference in comfort?
    Also, since you didn't address the metal roof, air system or condensation parts of my question, does that mean you don't think I'll have problems assuming the installer does a professional job?
    Thanks again for your expertise.

  3. GBA Editor
    Martin Holladay | | #3

    You wrote, "I know that it is more energy efficient to use the foam [than fiberglass] but it will never pay for itself with the amount of use I'll get."

    Actually, spray foam isn't any more "energy efficient" than fiberglass. The energy performance of your building envelope depends on airtightness and R-values. It's perfectly possible to do a better job with fiberglass than with spray foam. All you have to do is pay attention to airtightness and to include a high R-value.

    In fact, many fiberglass jobs have a higher R-value than the thin layers of foam that spray foam installers like to recommend.

    Furthermore, it makes no sense to spray foam against the underside of your radiant-barrier roof sheathing, since you lose the benefit of the radiant barrier by eliminating the air space it faces.

    Concerning your other questions:

    Q. "Sometimes I am gone for 5-6 weeks during the hot summer and the building stays closed. Will this cause problems?"

    A. No.

    Q. "Am I making the right choice to go with the blown-in open foam insulation vs. fiberglass batting?"

    A. Not necessarily. I would be inclined to install a thick layer of cellulose on your attic floor.

    Q. "Am I creating a hazard by not having an air system?"

    A. I'm not sure what you mean by "an air system." If you mean a ventilation system, the answer depends on what type of work you do in your shop. Many types of shop work -- including spray paint work, gluing, varnishing, or sanding -- require adequate ventilation.

    Q. "Will the metal roof cause condensation/heat thermal problems?"

    A. No.

  4. pure novice | | #4

    Martin, Thanks again for the quick response and expertise. If I go with your inclination and put cellulose on the attic floor, then would I use fiberglass bats in the walls? None of the installers I talked to suggested cellulose--they either recommended fiberglass all the way or the open foam spray all the way.

  5. GBA Editor
    Martin Holladay | | #5

    In many areas of the country, cellulose is a very common form of insulation. In other areas of the county, insulation contractors don't offer cellulose. (I have no idea why this is so.)

    In New England, it's easy to find an insulation contractor who will install dense-packed cellulose in walls. In your area of the country, this might not be true.

  6. pure novice | | #6

    Martin, Thank you for the great assistance, I think I'm going to change from the open foam spray and go with your suggestion except I'll have to use the fiberglass in this area.
    Thanks again

  7. Expert Member
    Dana Dorsett | | #7

    It's unfortunate to have to go with fiberglass, since it's somewhat translucent to infra-red (radiated heat transfer), and less air retardent than cellulose. If you have to go fiberglass, the higher density goods like R15 (2x4) & R21( 2x6) are far better than the lowest density R11/R19 versions (which are barely more than air filters and pass more radiated heat too.)

    The big box stores are starting to carry rock wool batts (typically Roxul- branded goods, but sometimes others) in many parts of the US, which would be an uptick in cooling season performance from fiberglass (even at the same R), due to better infra-red opacity. While not quite as air-retardent as cellulose, it's not bad- comparable to the high density fiberglass batts.

    Never count on random air leakage through the walls & ceiling to ventilate anything like paint or welding fumes- you always need to at least run an exhaust fan. The infiltration difference between a spray foam solution and a batt solution can make a difference in energy use (particularly on latent-cooling/humidity), but is orders of magnitude below the ventilation needed to deal with those sorts of things.

  8. pure novice | | #8

    Thanks for your assistance. I'll show it to my installer.

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