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Community and Q&A

Retrofitting an apartment in an existing metal-sided building

John Peeper | Posted in GBA Pro Help on

Greetings and thank you in advance! I’ve been an avid reader of many articles on this site, and now it’s finally time to put them to practical use…

I will be constructing a 700 sq. ft. apartment with a loft area in an existing wooden pole barn structure with metal siding. I have some questions on the best way to insulate the retrofitted space. Here are as many pertinent details as I can think of without burdening you with unnecessary details:

Climate Zone 3
The building is framed with 6×6″ wooden posts spaced 10′ apart.
The rafters are 2×6″ spaced 5′ apart, so I will be using 2×4″ strapping spaced 24″ O.C. for ceiling finishes.
There is no insulation currently.
Once the space is enclosed, it will be possible to provide roof ventilation entry at the gable eave. The ceiling in the apartment will slope with the rafters (I’m not putting in a drop ceiling in order to have space for a loft).

Since the metal siding and roofing is already there, I will essentially be constructing from the outside – in, rather than vice-versa. I’m looking for the best way to ensure a tight building envelope and prevent moisture damage. My biggest concern has to do with the ceiling.

I’m currently planning on buying used Poly-iso sheets in 2.5″ and 3″ thicknesses for the ceiling insulation. This will fit perfectly inside the thickness of the 2×6 rafters. It will be cut-and-cobbled basically, but shouldn’t be too difficult since the rafters are 5′ apart. The roof purlins running the length of the building above the rafters will create a 1.5″ air gap between the insulation and the metal roofing, which, once the eaves are vented, should create a sound design against moisture trapping, correct?

But if I understand correctly, If my roof is designed to dry to the outside (it would be), then I would want to take steps to avoid moisture traveling through the walls from the inside, correct?

I can air seal all of the joints of the Poly-iso with tape, but that would be fairly expensive. Using the air-tight sheetrock method would be another option. The thing is, I was hoping to use sheet metal on the ceiling underside rather than sheetrock, which of course is NOT airtight.

What about using an air and moisture barrier like Grace Ice and Water shield on the underside of this assembly? Thus making the design go from top to bottom:

Metal roofing
Purlins/ airspace
Rafters with 5.5″ Poly-iso rigid insulation
Grace Ice & water shield air & water barrier
2×4″ ceiling strapping/ air space
Sheet metal ceiling finish

Would that not ensure that no air or moisture could ingress/egress through the sheetmetal ceiling, thus allowing the the whole assembly to dry to the outside?

Grace is expensive, but it seems like this would be a more cost effective way of air sealing than buying several hundred feet of expensive tape and cans of spray foam, based on my calculations. Is there some reason this would NOT work or be a bad idea? Do you have any suggestions that would be better suited to my application?

Many many thanks!

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  1. Steve Knapp CZ 3A Georgia | | #1

    John. Curious if you have discussed permitting and code issues with your local municipality. You wouldn't want to get to far along and have an official or inspector raise concerns about converting a pole barn to living space. Of course, you may be in a part of Zone 3 where there is no oversight.

  2. Brendan Albano | | #2

    Hopefully the pros will chime in here, but the impression I get is that in these weird retrofit situations, it's often better to use a nylon "smart vapor retarder" type product like Certainteed MemBrain or Pro Clima Intello, rather than a full vapor barrier like grace ice and water shield.

    I feel like your assembly isn't quite right, with your wood rafters in a potentially wet area with an impermeable membrane below them. The expensive solution to insulating roofs from the inside is just to blow in closed-cell spray foam to create an unvented roof.

    I don't know much about them, but I think you can make a vented cathedral ceiling work in your situation by using a thinner layer of rigid insulation with an air gap to create a continuous vent from soffit to ridge vent (can you install a ridge vent?), then use dense packed cellulose for the majority of the insulation. It seems like this might be cheaper. This article touches on it:

    I've read that the challenge of cutting and cobbling, in addition to the labor, is that as your building expands and contracts, the seal between the rigid foam and the rafters can break.

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Your suggested approach raises several issues. The most important issue concerns summertime performance. If outdoor air can circulate freely above the polyiso, and through the unsealed seams between the polyiso panels, you are setting yourself up for moisture accumulation in the cracks between the polyiso if this building is air-conditioned.

    Moisture will condense against the cold Ice & Water Shield under those circumstances.

    If you are going to attempt the cut-and-cobble route, I think you have to seal all of the polyiso seams, on both layers. As an interior air barrier, I would use a smart vapor retarder rather than Ice & Water Shield.

  4. John Peeper | | #4

    Okay, thank you all for your answers.

    I did forget to mention that codes, permits, and inspections are a non-issue due to my geographical area. That doesn't mean that I want to build junk though of course.

    What you said makes sense Martin. The moisture could condense in-between the cut-and-cobbled seams, and the issues I would be inviting would not harm the insulation necessarily, but rather the rafters.

    I had thought about dense packed cellulose, however I'm leery of not getting a good job done with it. I feel like I would probably save money buying Poly-iso seconds and doing the job myself. Cut and cobble won't be too bad seeing as how the rafters are 5' apart - I will be able to use fairly large pieces.

    Supplementary question: If I DO tape and seal every joint of the two layers of Poly-iso insulation, would I need a smart vapor retarder at all?

    Thanks again

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Cut-and-cobble jobs sometimes result in air leaks. (For more information on this topic, see Cut-and-Cobble Insulation.)

    Having a good interior air barrier is good insurance.

  6. John Peeper | | #6

    Thank you Martin. I had read the cut-and-cobble article, but I just now re-read it. What are your thoughts on insulating with mineral wool and using 1" Poly-iso on the bottom side of the rafters to eliminate thermal bridging? So the layers would be:

    metal roofing
    purlins/air gap
    mineral wool
    poly-iso taped and sealed
    strapping/air gap
    sheet metal ceiling

    Would that be any better of a design than cut-and-cobble?

  7. User avatar GBA Editor
    Martin Holladay | | #7

    Mineral wool is an air-permeable insulation, so your suggested approach would be much improved if you could figure out a way to install an air barrier -- for example, taped housewrap -- above the mineral wool.

  8. John Peeper | | #8

    Hmmm... Between the impermeable layer of poly along with the impermeable housewrap, wouldn't that create trapped air sandwich? I suppose that's not a bad thing.
    The housewrap would be easy enough to do - it could be stretched tight and tacked to the underside of the roof purlins.

  9. User avatar
    Dana Dorsett | | #9

    Housewrap is highly permeable to water vapor, even though it's a reasonable air barrier. Most grades of Tyvek are north of 25 perms, Typar is about 10 perms, give or take.

  10. John Peeper | | #10

    Oh that's right, thanks Dana. Talking about so many different products gets my wires crossed occasionally - I'm certainly not a professional. So any accumulating water vapor would still diffuse to the outside. Good deal. I have much to think about now..

  11. User avatar GBA Editor
    Martin Holladay | | #11

    Trapped air sandwiches are good.

    Leaky sandwiches are bad.

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