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Will fiberglass insulation retain interior heat that a thermal barrier (foil) will not?

user-7026765 | Posted in Energy Efficiency and Durability on

I have an unheated shed for gardening tools, equipment, and chemicals that has foil backed plywood walls and roof sheeting. I plan to install standard plywood to the interior walls. Summer temps average 90 to 100 degrees max so the thermal barrier of the foil should provide some insulating value to keep the interior somewhat cooler. I am concerned that winter temps that typically dip to the 20s but occasionally could dip lower, could cause something in the shed to freeze. Although insulating the walls could render the foil thermal barrier ineffective, should I nevertheless insulate for the winter cold? Is there an insulation that could be used in conjunction with the foil backed plywood that would retain the air gap needed to make the thermal barrier effective?

Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name?

    It's hard to keep a shed from freezing when outdoor temperatures "dip to the 20s and occasionally could dip lower" unless the shed has a heat source. Most such sheds will freeze, although it may be possible to design a shed (in a sunny climate) with south-facing glazing and a well-insulated thermal envelope that doesn't freeze.

    If you have paint or drywall compound that you want to keep from freezing, don't store those materials in your shed.

    The foil facing plus the adjacent air film has an R-value of about R-2 or R-3. Adding real insulation will improve the R-value of the wall assembly. Be careful, however: if you add insulation between the studs, the aluminum foil will become a wrong-side vapor barrier. If you intend to add insulation between the studs, you should try to rip the aluminum foil, make holes in it, or remove it, if possible.

  2. user-7026765 | | #2

    Thank you.
    I am a new member (joined last night) and did not realize that my question was posted without my name.
    Rather than ripping, making holes or removing completely the foil on the back of the plywood, could I eliminate problems created by the wrong-side vapor barrier by placing vents in the interior plywood walls I intend to attach to the studs?
    Wes Moody

  3. GBA Editor
    Martin Holladay | | #3

    It's hard to know how far to go with the details on these walls, since this is a shed. There probably is no heating system, and there probably aren't any occupants generating interior moisture. So any worries about wintertime moisture accumulation are minimal, even with a wrong-side air barrier.

    But sometimes, a shed evolves, and eventually someone puts in a heater and turns the shed into a guest bedroom. So whether you need to worry depends in part on the likelihood of that scenario.

    So, I'll answer your question as if this were a small house.

    Q. "Could I eliminate problems created by the wrong-side vapor barrier by placing vents in the interior plywood walls I intend to attach to the studs?"

    A. What do you mean by "vents"? Do you mean holes in the wall filled with some type of metal grille (like a retrofit soffit vent)? If that's what you mean, the answer is no. You don't want to put holes in your sheathing. Ideally, your wall sheathing is airtight. That's how you help your wall assembly retain heat and limit heat flow through the wall.

  4. user-7026765 | | #4

    Thanks, Martin. I think I will forgo the insulation and just rely on the thermal barrier of the foil.
    Wes Mooddy

  5. Expert Member
    Dana Dorsett | | #5

    Foil faced sheathing isn't a "...wrong-side vapor barrier..." unless it's in US climate zone 5 or higher. A location that will " ...dip to the 20s and occasionally could dip lower..." is warmer than zone 5, more likely to be zone 4, but could even be zone 3.

    The interior-side plywood is a "smart" vapor vapor retarder that would limit the amount of moisture going into the wall cavities even if the interior of the shed were more humid than the outdoors, and more than sufficient to limit wintertime moisture accumulation in a zone 4 or warmer climate.

    I don't quite understand what would even constitute a "wrong-side air barrier".

    In any event, there is NO risk to insulating that wall with fiber insulation, with or without foil faced sheathing on the exterior, as long as there aren't any vapor-impermeable layers on the interior. Unfaced or kraft faced batts are fine, foil faced batts are risky. Latex paint on the plywood is fine, if desired.

  6. Debra_Ann | | #6

    If your below freezing temps occur mostly at night, and it gets warmer during the day, then it might help to include some mass inside your shed (concrete, stone, water, etc). We had a 12' x 24' x 8' high green house covered with just a heavy clear plastic sheet. The floor was covered with crushed stone, and we had a lot of concrete blocks holding up waist-high growing containers.

    Anyway, the stone and concrete retained heat pretty well. It could drop into the low 20's at night, but not freeze inside. A few times it dropped to the single digits outdoors at night, and only dropped to the mid-teens inside. Of course, we did gain solar heat during the day. After a few cloudy days, it got colder inside. But in our climate, I wouldn't store anything there that needed to avoid freezing.

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