Efficient roof insulation design for large metal building
We are preparing to build a 12k sf building. Wood framing, all corrugated metal sheeting. Some builders have recommended low E sheeting products to trap between the framing and metal sheeting to supposedly achieve an R15 or so. The physics of this just does not make any sense. Because the building will be climate controlled, 24/7, we are further looking to install 8″ batts in the walls and ceiling throughout but that only gets me to an R25-30 in the ceiling in which we would like to see an R40 in the ceiling.
I realize that creating an air gap is most beneficial in which if we further insulate with batts, I cannot see how the low E sheet would do anything. Do you have any test data for a building similar to what we want to build or can you recommend a solution for our roof insulation? We might be able to just get more glass in there but thought there might some added benefit to the Low E products that just might help.
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Replies
Bob Smith,
You are right to be suspicious of the usefulness of a radiant barrier or low-e sheeting product for your building. (For more information on radiant barriers, and why you don't need one in your building, see Radiant Barriers: A Solution in Search of a Problem.)
You have explained that the building will be insulated and conditioned. In most areas of the country, that means that the building will need to meet local energy codes, including requirements for insulation in walls and ceilings that meets minimum R-value requirements. Consider these code requirements to be the bare minimum of acceptable insulation levels.
One detail worries me: if you are building a wood-framed commercial building with steel-panel siding, don't forget an air barrier. It's very important for your walls and insulated ceiling to be as airtight as possible.
Yes, there will be an air barrier installed and our insulation requirements far exceed the local code requirements. Can you recommend anything for the roof other than good old glass batts that would be cost effective?
Bob,
Sure. Cellulose.
Are you insulating a flat ceiling or sloped rafters?
The roof trusses are kind of a hybrid design that would mimic a vaulted ceiling so approx a 3.5:12 exterior pitch and a 2:12 interior. I do not think cellulose will be practical for this application. We would not have any interior ceiling treatments to capture the loose fill. The plan for the interior is simply to install the unfaced batts and cover with a durable, thick mil plastic as an air/vapor barrier. There are some concerns right now of having a vapor barrier on the outside of the wall between the metal sheeting and studs as well as the interior plastic but some people have indicated that as long as moisture is not really getting in, there should not be much condensate concern. The verdict is still out on that.
Is the recommendation from you to abort any use of a low E product under the roof sheeting and concentrate on an air seal and thicker insulation media?
Bob,
Most building codes do not allow the use of polyethylene sheeting as a ceiling finish material because it is a fire hazard. Be sure to check with your local fire marshal and building inspector to be sure you don't need a more durable (and safer) ceiling.
You don't want a vapor barrier on the outside of your wall studs, but you should have an air barrier, especially if you are insulating with fiberglass batts. Either plywood or OSB would be a good material to use in this location.
No, you don't need a radiant barrier. You should do your best to improve the building's air barrier -- your ceiling sounds particularly poorly designed -- and to invest in thicker and higher quality insulation.
I should mention that the building is an agricultural building and it is common here to see exposed plastic as the facing of the wall. What do you see as poorly designed in the roof?
Bob,
It will be hard to make the polyethylene airtight. Fiberglass batts are air permeable, and are usually the worst of all available insulation products. And there is no air barrier above the batts to reduce the penalty of wind-washing.
What do you recommend for a proper air barrier? I assume this would install between the metal sheeting and studs.
Bob, from the sound of it, the type of truss you are using is what we call a "scissor" truss. One pitch for the roof, a lower pitch for the ceiling. I assume you need the headroom inside that the ceiling pitch allows.
It's going to be hard to insulate between the trusses without ceiling paneling of some sort. Drywall is typical in houses, but I suppose not in a barn. What about installing a plywood roof deck (or purlins), then rigid foam insulation (or nailbase panels) over that, and then the roofing? Place the insulation above the trusses and skip the ceiling. In that case your challenge is to have the air barrier at the walls continuous up to the roof.
Around here, people are using a banding system to band the insulation in place. I could probably deck the roof but that is a pretty thick roof to achieve an R40.
Bob,
If you don't want to insulate above the roof deck, and if you are set on using fiberglass batts, you could greatly improve the thermal performance of your ceiling by installing a layer of rigid foam at the ceiling level (under the trusses).
The best air barrier options for your wall would be either taped sheathing (OSB or plywood) or drywall installed according to the Airtight Drywall Approach.
One of the problems we see around here is condensation on the underside of metal roofing in utility buildings--structural steel frames with light gauge purlins, metal siding, and metal roofing. The walls and roof are usually insulated with a thin layer of fiberglass covered by white plastic sheeting. Warm air carries moisture up through the seams in the plastic and it condenses on the bottom of the roofing, eventually forming ponds of water at various places in the plastic.
Bob, I would make sure you don't have this problem. You need an effective ceiling air barrier. Simply pushing batts against the metal and strapping them in place is really risky in my opinion. I would either install drywall or plywood on the underside of the trusses, tape it together into a monolithic barrier, and then insulate with loose fill on top, or insulate over the roof deck.
Here's an IR image showing a pond on top of a plastic insulation cover sheet...
Bob,
I share David's concerns and have had the same experiences. (We are in zone 5).
You don't say anything about your location or the intended use of the building but I am guessing that you are in a heating rather than cooling climate. If that is the case you have a structure with a high potential for condensation issues - and the more fiberglass you install, the worse your problems will get.
- the exterior metal skin is a vapor barrier
- fiberglass insulation does not provide an air barrier between the inside conditioned air and the metal but it will lower its temperature
- air moving from inside the building will pass through and around the fiberglass and reach the metal with possible condensation resulting.
If you want an energy efficient building, you need an air barrier which can keep outside air out of your insulation. As designed, your building does not have one.
Both of these problems would be solved effectively by the application of a layer of closed cell spray foam, which can go directly against the metal. Closed cell foam will form a continuous air and vapor barrier which will allow you to add r-value behind it using other, less costly materials. How much foam you need depends on the total r-value of your assembly and the climate you are in. Your foam contractor should be able to advise you.
(full disclosure: my company install spray foam as part of our insulation mix)
Building is being located in the midwest where temps will vary -20F-110F. Building will be used as a machine shop with offices so cooling it is as big of a concern as heating it. Spray foam seems not as good an idea as a complete underlayment due to all the thermal breaks at each of the roof trusses.
is there not a proper air barrier that can be installed between the metal sheeting and framing? I do recognize the issues with condensate but figured the air barrier created with the interior plastic sheeting would block that migration of moist warm air towards the cold metal sheeting.
Let me also ask that if the spray foam is deemed a perfect line of first defense against the corrugated metal followed up with some batts, How expensive would this be as a DIY system? I would end up doing this myself most likely. If we also do some in the walls, I would need to cover about 21K sf.
I am against spray foam as the only insulator simply because I need to keep to building somewhat modular and you won't be adding any conduits in the walls with spray foam in them. however, adding 1" of foam followed with batts sounds like it could give some good noise reduction as well as an air and vapor barrier.
That's a huge temperature range. It's easy for me to say, since it's not my money, but I would abandon plans for a metal building and either build a conventional wood framed building with "outsulation", or a SIPS building (including SIPS for the roof). There is no way I would try to adapt a typical metal building (which is really intended as a utility structure IMO) to the climate and use you're talking about.
There are literally thousands of them in place around here that seem to do fine. maybe not as "green" as they could be but they do function. Typical temps would be from about 20F-100F but there are those extreme days here.
From the prices I found on foam, I could easily spend 20 grand on 1" of foam!!! I could get a LOT of drywall for that... Where would a guy get foam if that is a decent direction to look?
My mistake, I was forgetting the details. The title of the thread says metal building, but your original post says wood framing, metal siding and roof. I would use no spray foam at all on a project like yours. I would sheet the ceiling and blow loose-fill in over it to R60. For the roofing, a solid deck, 30# felt, and the metal. Make sure to get energy heel trusses. Frame with walls with 2x6, dense pack with cellulose, and install 2" of rigid foam over the sheathing before installing the metal.
This would not be an energy-code-minimum building, but no one who comes to this site is thinking in those terms.