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Community and Q&A

Building for high humidity indoor environments (bathrooms, greenhouses etc.)

Alex Halpern | Posted in General Questions on

Hi I am wondering if people have helpful tips for designing buildings intended to withstand higher levels of humidity indoors. 

We are building in a cold climate zone 5. After installing insulation  I was going to put in a smart interior vapor barrier.  However I am not sure what would be best for the interior wall design. (Moisture resistant drywall?) poly siding ?

I am also planning to install an ERV ventilation unit to help me control humidity levels and ventilation.

The space will be used to grow plants year round in a colder climate. The humidity shouldn’t be extremely high. However there may be periods after watering with heaters on in the winter where humidity levels could get relatively high.. 

I want to design this well so I can set myself up best to not have to deal with harmful moisture issues, mold and mildew. 

Any thoughts or suggestions would be greatly appreciated. 

Thank you,

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  1. Expert Member
    Malcolm Taylor | | #1


    Bathrooms and greenhouses pose separate issues which are best dealt with in different ways.

    Like any other room within the conditioned area of a house, bathrooms should be designed to control and exhaust humidity so that no other measures are necessary.

    Greenhouses should be isolated from the rest of the conditioned area, and do benefit from building assemblies that are moisture resistant. I don't know much about their design and will leave it to others with more experience to comment on what works.

  2. Expert Member
    Peter Engle | | #2

    If you want a well-insulated wall with little risk of internal condensation, a vapor barrier on the inside is a must. I've designed similar spaces using Dow Ag Board. This is a polyiso insulation board that has an aluminum facer that is left exposed on the interior. You can get versions with embossed and painted surface that looks sort of like an older-style refrigerator shell. Seams are either taped or finished with Dow-provided T-moldings. The facers are rugged enough for powerwashing, and could be painted if that's what you want. You would want continuous Ag Board on the interior, taped to be airtight. Outside of that, you can use standard wood studs with fluffy insulation. I would go with vapor-permeable exterior sheathing and/or insulation. Plywood sheathing would be fine. OSB not as good, but still OK. Sawn lumber sheathing or wood-fiber sheathing (Gutex) would be awesome.

    1. Alex Halpern | | #3

      Thanks Peter,

      This is helpful information.

      Would drywall not be recommended? I don't anticipate the walls getting exposed to water. Humidity yes. But actually getting wet from water not so much. I'm wondering if I can get away with moisture resistant drywall and good ventilation.

      However eventually if water will in fact be getting on the walls occasionally I'm wondering if there are some other interior siding alternatives that would be better suited, just to be safe. I am not sure I understand what you been by the polyiso Ag board as an interior wall finish ?


  3. Walter Ahlgrim | | #4

    Alex if you think this place is likely to end up over 55% humidity for much of the time when it is below zero outside I think good windows, fluffy insulation and a wood structure is a very risky set of choices.

    Generally the cost of triple pain windows keeps most people out of trouble. With lesser windows when it is cold outside the inside of the glass ends up being the coldest condescending surface and collects most of the water out of the air some place where you can see and remove the water before any harm is done.

    Consider your window choices carefully and stone window sills would be great.

    With better windows other things could be colder than the glass and collect water inside the walls where things could rot and mold.


  4. Expert Member
    Zephyr7 | | #5

    I absolutely agree with Malcolm here: greenhouses need to be built very differently from bathrooms. Bathrooms have OCCASSIONAL high moisture levels, and can be built very simlarly to the rest of the home, with only a relatively few special measures (a fan, mold resistant drywall, etc.).

    Greenhouses are VERY different, and will be seeing high moisture levels ALL the time. There really isn't a way around this, if you're growing plants, you'll be watering those plants, and you'll see high moisture levels as a result. You ideally want the greenhouse as a stand alone structure, but if it has to be attached to the home, treat the wall between as an exterior wall with zero drying potential to the exterior. I would actually put a vapor barrier on the exterior (greenhouse) side of that wall, and only allow drying to the interior. In a cold climate, that shared greenhouse wall will behave more like a wall in a tropical climate where the exterior is warm and moist all year.

    I would use at least twinwall polycarbonate for the "clear" parts (roof and walls), and triple wall is better. This gets you some insulation value in your clear parts, which will save you a lot on heating the space. You can use aluminum supports, which don't care if they get or stay wet, or wood. Cypress is a good wood to use, but not commonly available. Some used cedar. Some use pressure treated (if you want to do certified organic growing though, that might rule out the use of pressure treated wood). You can use "regular" framing lumber if you paint it heavily, usually with a white paint. Don't use any fibrous insulation anywhere in the greenhouse, stick with rigid foam.

    I've always thought the best part about having a greenhouse, or visiting one, is going inside in the winter and pretending it's not winter. I highly recommend anyone in a Northern climate try visiting a commerical greenhouse in the depths of winter if you can -- it's almost like a tropical vacation. I used to have a family friend who was a commerical orchid grower with several acres of enclosed greenhouse space. I always loved visiting that place in January/February.

    Note that hydronic heating is the best way to heat a greenhouse. It's durable and doesn't care if it gets wet. Some water heaters are rated to be used like small boilers that can work for this application, then you just use fin tube under your benches. Next best is probably a garage-style gas heater, but be sure it's mounted inside -- if it's outside, moist air will condense inside the heat exchanger after every heating cycle and will RAPIDLY rot away the insides of the unit (ask me how I know :-)


  5. Alex Halpern | | #6

    This is all incredibly helpful information for me to consider , Thank you everyone.

    This greenhouse space would be the size of a small bedroom. I'm hoping It may still operate well enough to keep or maintain an average humidity at or bellow 40% with proper ventilation. If we are careful not to get drywall wet , and use a smart vapor barrier behind the drywall is this still to risky of an undertaking ? Should I consider some kind of plastic wall panels ? Or heavily painted plywood instead of drywall?

    Curious if rigid foam ok to use inside walls with an interior smart vapor barrier ? And why it's recommended in this case? Low permeability is the cold climate ?

    This is quickly getting a bit complex for me, but my girlfriend is depending on me! And I'm determined to not mess this up !

    Thank you,

    1. Expert Member
      Malcolm Taylor | | #7


      I don't think there is any real chance of keeping the humidity in a greenhouse anywhere near 40%. You need to plan as though it's going to be a lot higher than that.

    2. Expert Member
      Zephyr7 | | #9

      You're probably going to find average humidty levels are in the range of 60-80% in a greenhouse. Don't use drywall in a greenhouse! You'd be better off using some kind of T and G plank wall covering, with a vapor barrier behind that.

      Rigid foam tends to be immune to moisture. Most insulation is at least somewhat affected by moisture, or will at least allow the moisture to pass through (mineral wool, for example) to far-side surfaces where that moisture can condense and cause problems.

      The suggestion to look at ideas for indoor pools is a good one too -- there are similar moisture concerns with pools.


  6. Brian Carter | | #8

    I would visit the archives of Building Science Corporation and read what Joe Lstiburek has said about building details for indoor pools.

    Part of the info missing is what sort of climate your greenhouse will have. Tropical? Cool, as in high temps in the 60's? The cooler the less moisture the air will hold. Another consideration might be using LED's rather than glazing to provide lighting. It would allow you to insulate all the surfaces and limit condensation. Energy use might even be lower. You might also consider a separate air handling system so you can manage humidity apart from the rest of the building. It might be as simple as running a dehumidifier periodically. On the other hand, plants will appreciate the CO2 that people put into the air.

    It probably is best to leave room for different solutions for managing humidity so that you can adjust as you get more familiar with the level of control you need.

  7. Alex Halpern | | #10

    Ok, thank you everyone.

    Sounds like my best option for success might be to just use rigid foam insulation-smart vapor barrier - and rigid pvc interior wall panels.

    Now with the ceiling - does anyone know if you can do an unvented roof assembly from bellow the roof deck ? R 35 rigid foam in cavity with R10 foam bellow rafters ? Or did I just make this up? And the only correct way for unvented assembly is to have the rigid foam above the roof deck?

    In that case Am I better off just having it vented above the insulation? Still using rigid foam?

    Thanks everyone for helping me not do anything stupid...


  8. Walter Ahlgrim | | #11

    If this is really going to be a green house then I say do right and go full greenhouse aluminum frame glass roof and walls on 3 foot block stem walls that way you can hose down everything and any water will run to the floor drains.

    The way I see it if the room has as much glass as I am imagining it seems to me keeping the insulated roof and some small parts of the walls will not lower the heating costs much if at all and the plants will do better with more light.

    I think trying to go halfway puts you in the worst possible position.


  9. Alex Halpern | | #12

    Thanks Walter,

    Sorry, to clarify we are likely going to use artificial lighting.. Some windows or portion of the wall will let some supplemental light in. But I may potentially limit this if it will cause more problems. I was hoping for a hybrid plan with some sunlight in combination with grow lights.

    So, It will primarily function off lights. My main concern is keeping things between 50-80 degrees year round in zone 5 and avoiding mold and mildew issues from higher humidity levels.

    Thank you,

  10. plumb_bob | | #13

    I just yesterday pulled the BSC article "In the deep end" to address high humidity interior in a colder climate zone as my friend is looking to re-roof his carwash building, climate zone 7a. Good article and relevant to your project.

  11. Expert Member
    Zephyr7 | | #14

    You can do hybrid, just treat the greenhouse as "outdoors" from the home's perspective. Seal the wall between on the exterior side with a vapor BARRIER (not a smart vapor RETARDER). The purpose of the vapor barrier is to keep moisture from the greenhouse from migrating into the wall. Make sure the wall can dry to the interior of the home. You'll need to seal up your soffits so that they aren't bringing humid greenhouse air into your attic. Sealing your soffits may complicate the attic venting for the rest of your home, so you need to be careful with this.

    The easiest way to go might be a lean-to style greenhouse on one of the exterior walls of your home. Build it out with triplewall polycarbonate which both insulates much better than glass, and is far more durable if something falls onto it (branches, squirrels, hail, etc.). Yellowing isn't too bad with modern materails, you can expect maybe 15+ year life. I have some twinwall on an old greenhouse here that is around 30 years old and still fine. It's been my expierience that glass breaks more often than polycarbonate gets replaced for yellowing, so the "glass doesn't degrade" argument isn't really a fair comparison...

    BTW, this being a green building forum and all, you really SHOULD try to use natural light as much as possible. It's a form of solar power after all ;-) Try to use the artificial light to supplement natural light when the days are short. This will save you lots on your electric bill too. Trust me on this one. You'll want to work out your operating costs before getting going on this project. Some years back I was planning on a 450 gallon reef aquarium setup, and one of the reasons I ended up not going through with the project was the over $100/month it would cost me in electricity for the lights and pumps. Note that a lot of that was for metal halide lights, LEDs are cheaper to run -- but they still add up.


    1. Alex Halpern | | #15

      Awesome Bill,

      Thank you. The greenhouse is actually free standing from the house. But this was helpful info. I would love to use as much sunlight as possible.

      I am mostly trying to wrap my head around vapor Barrier vs Retarder in this install as well as insulation and wall finish .

      Thank you ,

  12. Karl B (Zone 6A) | | #16

    Alex, if you haven't already seen Joe Lstiburek's "BSI-106: This Bud's For You" (concerning residential and commercial grow operations; i.e., indoor greenhouses), it's worth taking a look at:

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