Replies

1. Expert Member
   BILL WICHERS | Mar 14, 2019 08:19pm | #1

   What do you mean by “blown closed cell”? Do you mean closed cell spray foam?

   A greenhouse is a VERY different structure from a regular house. In a regular house, you try to keep humidity levels low. In a greenhouse, you are going to have much higher humidity levels and you need that for the plants. You can’t build the greenhouse like a regular house. You have to design everything in a greenhouse to be able to get wet, handle high humidity levels, and handle condensation without problems.

   I would not use batts (except maybe mineral wool, but probably not even that), and I would not use any kind of blown in insulation. Closed cell spray foam could work (don’t even think about using open cell spray foam here), but it’s expensive. I would build the inside walls as open stud walls, and use only rigid foam insulation on the outside of the studs. Type II EPS is your best choice here, even better than polyiso since the EPS won’t care if it gets wet. Extend the EPS down into the ground at least 2 feet on the sides of the building to limit heat loss through the ground.

   I wouldn’t try to finish the interior of the walls. I’d probably use some plywood (NOT OSB, plywood handles moisture better) on the outside of the studwall to give it some structure, and the rigid foam would be on the outside of the plywood. I’d paint everything white to help keep light levels up. I wouldn’t try to enclose the walls at all since you’re going trap moisture no matter what you do. With open walls, there are no cavities to cause problems. The rigid foam insulation will never get saturated like batts or blown insulation will.

   For glazing, look into triple wall polycarbonate, but get the thicker 16mm kind which has better insulating value. Triple wall doesn’t cost much more than double wall, but has much better insulating value. The fancier variants (5 wall and the kind with diagonal bracing) don’t add a lot more insulating value, but they do cut down on light transmission.

   I’m assuming you’re already familiar with some of the passive solar greenhouse principles for getting maximum light in with minimum heat loss? Even if you plan to heat your greenhouse, being careful with how you orient the greenhouse and the shape of the structure can really help to cut down on your heat loss and thus keep your heating costs down. I’d also look into a hydronic heating system so that you can use a remote boiler (which can often times be a water heater), since these systems are the most durable in a greenhouse environment.

   Bill

1. 
   CJNL | Mar 15, 2019 06:27am | #3

   Bill Thanks for the thorough response. I have a few clarifications, questions and thoughts:

   - I have updated my post as closed cell foam insulation.
   - The greenhouse is not unlike an indoor swimming pool, a point elaborated in other posts on these forums. SIPs and PERSIST among the noted solutions.
   - Are you saying closed cell foam while expensive would work enclosed in plywood on the inside? Any need for WRB or other considerations in that case?
   - R20 of the pink stuff at my local home depot is running $4.70/sqft versus the $4/sqft I've been quoted for closed cell spray foam. Do you have a cheaper R20 of EPS in mind or a pricing benchmark?
   - Good idea on extending exterior insulation below grade.
   - No mention of roof approach?
   - I see many high end "solar greenhouses" and permie folks heralding SIPs for construction which amounts to an OSB sheet inward facing to the looming tropical thunder I'm about to unleash - could my painted closed cell sandwich last? How is it these SIPs survive - coating/WRB/HVAC?
   - Are greenhouses exempt from thermal barrier/ignition requirements seen in traditional housing i.e., no plywood or similar over insulation on the inside?
   - Triple Wall is the best solution for this climate. I am more remote than most so access to triplewall would be a + freight custom order paid on my dime, some local suppliers carry doublewall.
   - I am familiar with orientation, glazing angle and other passive and active means through the "Year Round Solar Greenhouse" book by Schiller and Plinke. Excellent resource but typically at the 10,000 feet level.

1. Expert Member
   BILL WICHERS | Mar 15, 2019 11:47am | #6

   Yes closed cell spray foam would work because it won’t soak up moisture. It’s not very economical though, especially in a wall assembly due to thermal bridging of the studs. CcSPF is really best reserved for a few niche spots where it’s the only way to go (hot roofs/cathedral ceilings for the most part).

   By “pink stuff” do you mean dow Corning’s XPS? Usually, for builders, “the pink stuff” refers to fiberglass batts. You absolutely DO NOT want to use fiberglass batts in a greenhouse!! (And a few more !!!! For good measure). The XPS will work fine, but isn’t the most green material. XPS is also one of the more expensive types of rigid foam. Type II EPS will generally be cheaper, and greener. You might also want to try to find some reclaimed EPS (or XPS) to save some more money. A greenhouse is a great place to use reclaimed materials!

   The roof on a greenhouse is usually mostly glazing, so there isn’t much of a “roof approach” to speak of. Make a frame strong enough to handle the snow load in your climate, make sure not to go too far between supports for whatever glazing you choose, and orient the glazing correctly (twinwall and triple wall need the channels running vertically so that they can drain, for example). I’d use the purpose-made glazing strips for those materials too, which tend to be an aluminum channel and a rubber seal. You really can’t do as good a job without using those strips, and they aren’t terribly expensive to buy.

   I don’t see a problem using SIPs, but they’re expensive. Any moisture sealed/proof assembly should work, but the easiest to build, and probably the cheapest too, is an open studwall with rigid foam insulation on the exterior. If you uses reclaimed foam I’m pretty sure that will be the cheapest way to go. I’d try to avoid OSB though since it’s not as moisture tolerant as plywood.

   Greenhouses and code. Now there’s an interesting subject! Yes, greenhouses are generally supposed to meet code, but almost every greenhouse is technically in violation of at least a few codes. The classic example is the crazy electrical wiring you see in greenhouses. The best way to wire your greenhouse is to put everything in PVC conduit.

   You can’t use drywall in a greenhouse, it simply won’t hold up. I’ve never seen anyone get in trouble using rigid foam the way we’ve described here, although technically it probably is supposed to have an ignition barrier. I think the big reason that inspectors tend to show leniency to greenhouses is that no one thinks of greenhouses as a potential fire hazard.

   Bill

1. 
   CJNL | Mar 17, 2019 07:07pm | #8

   Thanks again Bill, great details.

   The pink stuff is the dow corning and I agree that Type II EPS is a better product for the reasons you outlined. I found some reclaimed Dow cladmate, however, I was advised against it's use by a polyiso vendor as they indicated it would not be adequately breathable for this application. Do you share that opinion?

   As this is a passive solar greenhouse, the principle is to insulate unglazed roof areas which in my case is the north facing portion of the roof. So would you vent the air space / place WRBs on either plywood sheathing?

   With the glazing strips again due to available materials in my area I was considering sandwiching the panels with cedar coping and foam glazing tape (aesthetics and strength). I plan to use top seal and breather tape on the bottom edge.

   Ref the code items good input. I suspect issues only arise in instances where greenhouses are production and not public use / occupancy spaces. I second the conduit approach on any wiring.

1. Expert Member
   BILL WICHERS | Mar 17, 2019 09:25pm | #9

   Breathability doesn’t matter here since you’re not building regular walls. Polyiso isn’t a good choice though because it will wick water, and that’s bad news in a greenhouse. Try to go with type II EPS if you’re able to get it, XPS as a second choice.

   I would leave the interior side of the walls open, and paint it white. No drywall, no air or vapor barrier since it’s an open wall. Use plywood on the exterior of the studwall if you need it for structure, then put the rigid foam on the outside of that. If you want to use any exterior cladding put it over the outside of the foam.

   In a greenhouse, you can’t think about building things to dry — they won’t. You need to think about building everything with materials that don’t care if they get wet, and don’t care if they stay wet. It’s a very different way of thinking from building a house.

   You’ll be ok with the glazing foam and boards. You might want to try PVC trim boards though. They might hold up better.

   Bill

2. 
   PAUL KUENN | Mar 14, 2019 10:20pm | #2

   A little off subject but don't leave out solar thermal. You can get really inexpensive older solar thermal panels on Craig's list and heat a 2-4K gallon tank buried under the greenhouse. Might as well heat your hot water tank for domestic use as well. All summer that heat is collected and keeps it above 32 all winter. We can get through -25F days here in WI with that set up. Mid winter spinach is really happy and No electricity or fossil fuels used to heat the greenhouse. You can see my work on facebook for Olden Farms, Picket, WI.

1. 
   CJNL | Mar 15, 2019 06:39am | #4

   I have looked at the solar thermal and am considering in a phased approach, for now passive thermal mass. In my neck of the woods food grade 250 gallon tanks are readily available - I have 2 for a modest size greenhouse. As these are insulated no need to bury which would make maintenance a bit easier. I searched on FB - but did not find an exact match (Olden Organics, Ripon)?

3. GBA Editor
   Martin Holladay | Mar 15, 2019 07:53am | #5

   CJ,
   The PERSIST approach is a good approach for greenhouses. I like to see all of the insulation on the exterior side of the wall sheathing (that is, a traditional PERSIST approach, without any insulation between the studs). Closed-cell spray foam is expensive, and not very environmentally friendly.

   My greenhouse is finished on the interior with horizontal cedar boards (purchased from a local sawmill). After almost 40 years, the boards are still in great shape.

1. 
   CJNL | Mar 18, 2019 12:23pm | #10

   Martin,

   With regards to the exterior membrane does it need to be impermeable across the whole sheathing or just at the penetrations and seams for this type of application? I was considering taping penetrations and seams with a 6 or 9" Air and Vapor Barrier tape and Tyvek over. Not sure if the Tyvek would even be required if using two layers of EPS or similar on the outside with taped seams and a rain screen? Is it paramount that there is no vapor drive?

   Is there any special requirement / approach for treatment of the plywood edge to prevent water damage?

   Also can you point me to a schematic of the roof foam and sleeper arrangement noted here: https://www.greenbuildingadvisor.com/article/getting-insulation-out-of-your-walls-and-ceilings

   Thank you
   CJ

4. 
   PAUL KUENN | Mar 15, 2019 01:49pm | #7

   Here you go: https://www.facebook.com/OldenProduceFarm/
   Little barrels will not have the BTU output to last a cold season, that's why we bury a 2500+ gallon tank before the greenhouse is built. Great that you're going to build it the proper way.