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Community and Q&A

Insulate shop with polyiso between rafters?

trawson | Posted in General Questions on

Hi Folks …

[I am new here, but have read through enough posts to see that what I’m asking about seems to be within the normal range of discussion, but not answered directly by anything I could find.]

I am in MA (zone 5). I have a 12′ x 20′ prefab shed that is used as a workshop. It was insulated with fiberglass batts between the 2×4 (yes, 2×4) rafters. There was 3/8″ plywood below these covering the rafters, but not all the way up to the peak — top foot or so on each side was just the open insulation face.

I re-roofed it this summer due to shingle deterioration (it is 20+ years old) and found some wet insulation and some water damage. We removed all the old insulation and all the 3/8″ plywood, and replaced one 4′ x 4′ section of water-damaged roof sheathing. It’s not clear to me if the water issues with the insulation and that one sheathing piece were due to leaks, or condensation / lack of ventilation, but I’d guess it was a combination of the two.

There was no ventilation above the insulation, but when we did the roof we added a ridge vent and I have also installed continuous soffit vents.

At the moment it is uninsulated (and heat is off!) but I need to install something. My inclination was to put attic vent channels between the rafters, 2″ foil-faced polyiso under them, also between the rafters, then some kind of ceiling — perhaps plywood again, or Reflectix, on the bottom surface of the rafters. This would leave ~1/2″ of space between the bottom of the polyiso and the top of the ceiling covering. (The Reflectix might be subject to some damage in a shop but has lighting benefits and provides an additional radiant barrier for what that’s worth.)

If I am calculating correctly this would give me R-12 from the polyiso plus a bit more from the other stuff, which is adequate considering that I only heat it sometimes. I can’t really do more because the headroom is very limited so I don’t want to put furring strips below the rafters.

Here are my questions:

* Is this a good approach for my situation? If not, what should I change?

* Will something like this work in a sporadically heated building? At most I would tend to heat it 15 – 20 hours per week, often less.

* Do I need to seal the polyiso / rafter joints with foam to prevent significant heat loss?

* Do I need to either seal the polyiso to the rafters or install a moisture barrier below it to prevent moisture infiltration? Or can I just rely on the top side venting to remove moisture if it gets in there?

And here is (I think) a separate issue: In the summer I have a different condensation problem because the floor is poorly sealed and the building is over dirt — no slab (the corners sit on blocks). Moisture rises from the ground, especially on warm spring days, and condenses at the peak inside. I have remote humidity sensors, there can be a 20% difference between the top of the wall and the peak of the ceiling, just 18″ or so higher! I have seen it dripping. Just bought a humidistat which I plan to connect to an exhaust fan currently installed at the peak at one end; there’s a vent at the other end. Any other thoughts on what to do about this problem?

Thanks for any help.


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  1. GBA Editor
    Martin Holladay | | #1

    There are two ways to address these issues: the right way, and the "let's cut corners" way. This is a shed, so it's up to you to determine how much money you want to invest.

    Here are two articles you might want to read:

    How to Build an Insulated Cathedral Ceiling

    Cut-and-Cobble Insulation

    Here's some advice:

    1. You didn't really describe your floor assembly, but you should try to find a way to limit evaporation of soil moisture.

    2. Because your roof assembly has a history of rot, you should plan to install a vented roof assembly.

    3. If you want to insulate your roof assembly with polyiso insulation, install it as a continuous layer under your rafters instead of cutting the foam into strips and installing the foam between the rafters. Make sure to tape the seams of the polyiso so that it is airtight, and make sure to install gypsum drywall on the interior side of the rigid foam for fire safety.

  2. trawson | | #2

    Martin thanks very much for the quick reply. I read those articles as well as several of the others linked within them. I'm a science guy (former software engineer turned middle school science teacher) so I try to understand the science behind this stuff. It's difficult to sort out because the web is full of conflicting views, many of which I am learning are people's favorite ideas masquerading as science, and a lot of the "research" seems to be anecdotal. The most illuminating item was the long quote from Anton TenWolde, his view that the real science is much more complex than what the simple rules say matches my experience in many other realms.

    As for the shed, I can't use spray foam because I need the venting, plus it would be a really expensive solution for a shed. Unfortunately I also cannot put 2 inches of polyiso under the rafters because the ceiling is already too low. So I think I am stuck with cut-and-cobble between rafters, below vent channels created with wood strips or something like Durovent.

    If I do that, it seems to me that my focus based on the reading should be to provide the best possible air barrier below the rafters (e.g. taped drywall), and fit the polyiso between the rafters and not worry too much about sealing it super-tightly. My idea here is that given the limited use of the building in cold weather (compared to say a residence), doing things like using foam sealant between the polyiso and the rafters will have minimal benefit on energy use, and I suppose it might also reduce the drying ability of the assembly, which could be more problematic than the heat loss.

    Does this sound like it's going in the right direction? I'm trying to think of an ad hoc solution based on underlying principles, but since I'm new to this I don't have a good intuitive feel for the places where preventing heat flow out in winter and keeping moisture in the right place year round might conflict.

    The floor assembly is 2 x 6 PT joists over soil, two layers of plywood on top (3/4" PT then 1/4" luan intended to be "sacrificial" if floor needs replacement). There is some cut-and-cobble polyiso between the floor joists but it is poorly installed -- I did it before I knew anything about any of this stuff. There is no vapor barrier of any kind, and the edges of the 3/4" PT are very poorly sealed and have gaps.

    Thanks again for your help.

  3. GBA Editor
    Martin Holladay | | #3

    If you want to do a cut-and-cobble job in your rafters, the insulation won't have much value unless you meticulously air seal the perimeter of each piece of foam. The air sealing isn't optional -- it's a key part of the technique.

    If there is room to crawl between the soil and the underside of your floor joists, it would be very beneficial to install a layer of 6-mil poly (weighed down by bricks or rocks) on the soil under your shed.

  4. trawson | | #4

    Thanks Martin.

    If you have time I would love to understand why that air seal is important if the layer underneath (say sheetrock) is properly sealed. Is it to prevent heat transfer due to convection within the roof assembly?

    Plastic underneath as you describe might well be doable ... it's a question of what is more difficult, lifting the floor, or digging up the buried chicken wire we put in a few years ago to keep the skunks from living underneath ...

  5. wjrobinson | | #5

    Tom, making your shed good work space, which is not good work space, not even good lawn mower space, is as you know being a science teacher pretty much impossible.

    First stage when setting preassembled sheds is to clean off topsoil, add black poly, add stone. Moisture destroys the shed and the contents annually.

    And then to use the space for heated workspace, you need fire resistant insulation or you need to add fire resistance with drywall some would use plywood for a shop, Thermax insulation has an exposed to the interior rating, though I would cover it depending on the use of the shed. Office use different than using torches etc.

    You can solve your situation if you solve it. That means you will have to do. You can't stop ground moisture from above the floor, it has to be done below the floor assembly so as to protect the floor assembly from rot. remove skunk stop, jack up the shed on one end safey and place what should have been there to start with.

    To do otherwise, well there is no otherwise except to just leave it the way it is. Gutter might help with the downspouts taken far from base... Malcolm would say.

  6. GBA Editor
    Martin Holladay | | #6

    You are correct that a good air barrier under the rigid foam goes a long way towards stopping the problems of poor cut-and-cobble performance.

    However, if the cut-and-cobble insulation faces a ventilation channel above, then there will be a stream of cold outdoor air flowing above the rigid insulation. If the rectangles of rigid foam insulation are installed loosely, without any attempt to air seal the perimeter of each piece, then the cold air in the ventilation stream will mix with the air at the perimeter of each piece of foam, robbing the foam of its insulating qualities.

  7. trawson | | #7

    Decided to create a new thread on this topic here: Recap: Insulate shop with polyiso between rafters.

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