Mostly Unconditioned Barn/ Workshop
Farmer_Wes
| Posted in Energy Efficiency and Durability on
We’re in zone 4A. The county next to us is 3A.
We are trying to insulate a barn roof to reduce solar radiation in the summer and make it possible to lightly heat in the winter just to keep things above freezing (like 40 degrees). Summer time we will be relying on fans.
The roof, from the outside in, is face fastened metal corrugated roofing, synthetic roof underlayment then plywood. Purlins are 2×8 24 oc and trusses are open web steel. There is no ridge vent so I think it will essentially be an unvented cathedral ceiling but with very limited heating in the winter I’m not sure if we have all the same issues that a typical cathedral ceiling in a house has. Does anyone have any suggestions on how to insulate and perhaps how to cover the insulation to make it reasonably attractive?
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Replies
Farmer Wes,
The best approach would have been to install rigid foam on the exterior side of the roof sheathing, but that train has evidently left the station. It's a little late in the game to figure out an insulation plan.
For an unvented roof, your only option at this point is to install closed-cell spray foam on the underside of the roof sheathing. You will need supplemental framing to have some way to install drywall (which will be necessary for fire safety). Exterior rigid foam would have been simpler and cheaper.
Thanks Martin. Is that the case even in a barn space like this where there aren't significant sources of moisture?
Insulating these after the fact is always a challenge. Martin is spot on spray foam, this is how typically these get insulated up here in the great white north.
You have one big thing going for you is that you have a roof deck so the condensation under the metal roofing is not an issue. In your milder climate, as long as there is no major interior moisture source, you can insulate with batts. I would stuff the cheapest faced batts you can find into the 2x8 purlins and cover with either drywall or osb. This gives you an assembly with some drying capacity to the inside which should avoid moisture issues.
Thanks for the input! I thought about trying to do the exterior rigid, but I have done it once before on a tiny house and being a DIYer I really struggled with it. I just couldn't bring myself to tackle it on this scale on my own and I had trouble finding a contractor who would consider it. Spray foam is at least easier to outsource.
One additional thought that comes to mind, If we thought air conditioning the barn in the summer was something that could potentially happen down the road does that influence the decision between batts and spray foam?
"between batts and spray foam?"
Batts will require air and vapor barrier on the interior side of the insulation to prevent the warm moist air from moving thru the insulation and finding the cold surface. When the warm moist air gets to the cold surface water will condense and stuff gets wet, given enough water mold will grow and things start rotting.
If the plan is to not install drywall closed cell spray foam is the only real option be it expensive ungreen and risky. Note exposed CCSF is considered a fire hazard and not allowed in living spaces as should a fire get to that foam the toxic smoke would kill everyone very quickly likely before they can escape.
These types of building are optimized to produce the lowest cost per dry square foot. That is great to store your tractor and RV but absolutely everything that lowered the building shell costs works against you when you later decide you want to make it energy efficient living space.
My guess is it insulating to R39 and covering the insulation with drywall will cost as much as the shell did.
Seems to me you are trying to fool us and maybe yourself. You start your question will insulation make this place cooler when the sun is beating down on it and two posts later the place is fully air conditioned.
At this point it looks to me like you have a huge cathedral ceiling to insulate.
You might find these articles interesting.
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
https://www.greenbuildingadvisor.com/article/insulating-pole-barn
https://www.greenbuildingadvisor.com/article/insulating-a-metal-building
You will find a lot of people on “Garage Journal” forum that like your project.
https://www.garagejournal.com/forum/
Walta
"These types of building are optimized to produce the lowest cost per dry square foot. That is great to store your tractor and RV but absolutely everything that lowered the building shell costs works against you when you later decide you want to make it energy efficient living space. "
Unfortunately, spot on.
With the price difference between spray foam and batts, and some people thinking batts will be okay in my climate and others saying spray foam, I'm trying to think is there a way to justify the expense of spray foam if it gives us more options in the long term. I could justify a couple extra thousand for spray foam if it gave me or somebody down the road, the option to insulate further and then air condition. I'm not trying to fool myself or anyone else. I'm just trying to make the best long-term decision given the resources and expenses that exist.
I can have the whole ceiling spray foamed with 2 in of closed cell for $7,000 which is a lot less than the shell. Enough rigid foam to cover the roof deck with 2 in would have cost $5,000 for materials alone, not including labor. It seems like this topic makes a lot of people a bit agitated. I'm here because I'm trying to do the best thing for the building and the environment with the resources I have. These buildings serve an incredible function for a farmer and many others and they will continue to exist whether or not we are able to get good advice on how to build them or how to retrofit them. There are a lot of different buildings for different uses. I'm not trying to live in mine. I am just trying to be reasonably comfortable and keep things from freezing. I'm generally more comfortable outdoors than I am in a conditioned space, but I've found that the barn as it is now is significantly hotter inside than it is outside. I'm looking for a way to remedy that that might be somewhat future proof.
If you look at the article that Walta linked to in #5, Five Cathedral Ceilings that Work, number five is an unvented flash-and-batt -- spray foam with fiberglass batt below it. Number four is just spray foam. If you spray foam now, you could add fiberglass batt later for more insulation.
If this is a dry place to keep the tractor that is great but if the plan is to repurpose the space over time I think it is a poor plan.
My wife insisted on insulating the garage and got her way what I found the garage is almost always warmer than the outdoor 365 days a year. In the winter that is great but, in the summer, not so much. At 9 PM it feels 30° warmer than outdoors and 10° warmer at 6 AM.
Walta
Walta,
I don't mean to hijack this thread but this "greenhouse effect" you experience in your unconditioned garage also happens with unconditioned attics. I know you like to advocate for keeping attics vented and all the insulation along the attic floor because the surface area is smaller, which is true of course, but you have to balance the smaller surface area with the larger delta-T across that surface area. I'm speaking from warm, cooling-dominated climates. In colder climates vented attics make perfect sense: they require less square footage of insulation, and the hotter attic temperature is a benefit to the delta-T in heating season. But in warm climates it can make a lot of sense to insulate at the roofline instead to cut down the delta-T. The optimal solution is based on the floor area of the attic, the pitch of the roof and the roof shape (gable vs hip). Steeped pitch gables make the least sense and low pitched hips make the most sense to insulate along the roofline.
The delt-t is the same from the back of the shingles to the living space no matter where the insulation is!
The only factors are the R value and the surface area.
If your plan is to not vent or condition the attic insulating both the roof and the floor of the attic, you are playing a risky game with rot and mold.
Walta
Definitely not suggesting to put insulation in both places. The temperature in a vented, non-conditioned attic will pretty much always be hotter than the outdoor ambient air temperature. And in that situation the thermal boundary is between the ceiling of the living space and the attic above it. In a cold climate this is a net benefit, but in a warm climate this is a net negative.
In a conditioned attic, the boundary is between the outdoor air and the attic. In a cold climate this is a net negative but in a warm climate this is a net positive. I see your point about the roof material itself heating up and maybe causing that outdoor temperature to be higher than ambient but you can get around some of that effect by having furring strips or some other way to ventilate just the roof without ventilating the attic. Or by using a reflective roof material, like galvalume. But I may need to consider this point more - perhaps the situation is not as simple as I thought.
"In a conditioned attic, the boundary is between the outdoor air and the attic." No this is flat out wrong. The same number of BTU are absorbed by the roof the unvented roof must be the same or warmer than the vented roof.
The only reason to condition an attic is if you are going to live in it like a cape cod home or if you insist on putting your HVAC equipment in the attic. I think both are silly but to each his own.
Walt
[replying to #15]
Yeah, when the attic is hot the heat is coming from sun hitting the roof.
Thanks to all! I would say this is a dry place to work on the tractor, and other projects, occasionally store feed in a pinch. Like a workshop/ barn. It's not a house but that doesn't mean I want it to be terribly uncomfortable, since I'll be spending some time in there. I imagined it would be "shady" in the barn and therefore cooler, but I have found that it is a sweat box that heats up significantly hotter than the outside ambient air. I had a energy consultant type person come and look at it and they suggested that if I could stop the radiation from the metal panels the space would feel a lot cooler. ie. Being surrounded by surfaces that are not radiating heat will be more comfortable. Made sense to me. So the conversation about insulation begins. If I am going down the insulation path, cheap is one consideration, but flexibility to accommodate future hypothetical needs is another consideration. We're planning to rent the farm for weddings during spring and fall when the temps are nice. I've seen portable ACs for rent. If weddings really took off I could imagine extending our season so that having AC could be nice, but that is not a primary goal. Just an incidental bonus option if the insulation solution allowed it someday.
I see plenty of metal shops, with heat but no AC that are lightly insulated which I assume is done to create a slightly more comfortable space for staff and keep things above freezing for pipes. Perhaps since mine has a combination of wood and metal it is more problematic. If AC is throwing a wrench in things lets just leave that out. Can I insulate this building with anything other than spray foam at this point and not have a problem? I like Akos solution with batts and osb. Any objections?
Walta, It sounds like in your experience the insulation has not made the garage more comfortable?
I was hoping to open the garage door, get a good fan set up and be able to have a covered area that feels similar to sitting under a shade tree on a hot day. Expensive shade tree I know, but there are other practical reasons why working on projects under a shade tree isn't ideal. Do you think your garage would be more comfortable in that circumstance?
Is doing an air/ vapor barrier on the inside really an option? Is drywall serving this purpose? I thought it was bad to do vapor barrier on the inside.
Has anyone had any experience with something like this: Simple Saver System Metal Building Insulation from Thermal Design?
https://www.youtube.com/watch?v=28_1g706IbQ
Its a membrane ceiling material that seems designed for retrofitting insulation to metal buildings.
“Walta, It sounds like in your experience the insulation has not made the garage more comfortable?”
It is a mixed bag when it is very cold outside the garage seems comfortable but less comfortable on warm afternoons.
Walta
A barn is not the same as a house. As long as there is no major interior moisture source (ie people living there, livestock or green wood), the dewpoint inside the structure is about the same as outside air. About the only condensation you'll have is on the back side of the roof (and sometimes wall) metal panels from night time radiative cooling. Having a wood deck bellow the roof means this moisture source is dealt with so you can insulate with batts without issues. I would avoid anything non-permeable over the batts such as poly or non-perforated radiant barrier. CDX/OSB/drywall or house wrap over the batts all work.
As long as the assembly doesn't have any warm side vapor barriers, you'll be fine to run AC.
"Having a wood deck bellow the roof means this moisture source is dealt with."
I'm a little confused on your thinking there. Isn't the danger that condensation under the roof wets the sheathing and framing of the roof and over time causes it to rot?
There are two moisture sources here.
Lot of times these metal buildings don't have anything under the metal roof panels. The problem with this is if you try to insulate with batts, that condensation that forms under the metal can drip down and saturated the batts over time. By having a roof deck with an underlayment, this problem is avoided.
The condensation the underside of roof deck is a worry only with houses in cold climate where interior dewpoint can be much higher than outside temperature. If this warm air hits the roof deck it will condense and if there is no venting it will saturate the wood over time.
This is not an issue with one of these buildings since it is not lived in, the dewpoint in the winter never gets high enough. So even if the interior air makes it to the roof deck, there is not enough moisture in it to condense. It also helps that it is a warmer climate.