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Community and Q&A

Best Material for Greenhouse Floor With Water Drainage and Capillary Break

Trevor_Lambert | Posted in General Questions on

I’m getting ready to assemble a glass greenhouse. Not the most efficient design, but my wife really wants to sit in there in the winter and have 360deg, unobscured views.

The perimeter foundation is going to be 2×6 pressure treated wood, per the manufacturer’s instructions. The other option is a full, concrete slab, which I don’t think is worth the cost. For the floor, I was thinking of this from bottom up:
leveled, sandy soil
1.5″ comfortboard 80
2″ patio stones

I think the comfortboard can take the place of a gravel base for the purpose of water drainage and capillary break. The instructions for an insulated floor specify 36mm XPS, which is out of the question, plus a 6mil poly under the foam. I don’t know why you’d want or need that poly there, but maybe someone can enlighten me. Seems like it would be a catch basin.

Anyone see major flaws with this plan?

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  1. Expert Member
    BILL WICHERS | | #1

    The traditional greenhouse floor is crushed stone if you’re going to be growing plants in there, often with precast pavers or poured walkways for foot traffic.

    I’m not sure about the poly either. In large commercial greenhouses, a geotextile fabric is usually used under the stone. The geotextile material keeps the stone and ground from mixing together into a muddy mess. If you pour a slab, you don’t need to worry about this, but the poly might help with moisture control — maybe. Greenhouses tend to be so humid anyway I’m not so sure a vapor barrier on the floor really accomplished anything.

    I would look into reclaimed XPS under the floor. Hidden applications like this are perfect places to use reclaimed insulation.

    I’m assuming by “not the most efficient design” you mean glass in comparison to something like triple wall polycarbonate? The multi-wall materials do significantly reduce energy use in the winter. You can use double pane glass too, but it gets really costly. Maybe you could convince your wife to consider triplewall on the roof but glass sides? Just a thought.

    BTW, hydronic heating systems work awesome in greenhouses and are probably the most reliable way to heat one due to the humidity. I’m assuming this will be a greenhouse for plants and not people :-)


  2. Trevor_Lambert | | #2

    If I was designing the greenhouse, I'd have three conventional walls and roof, and just the south wall fully glazed. But that wouldn't meet the criteria. We're getting a kit, so I don't really have the option of substituting materials, like PC for the roof. At least I don't think, I guess I could ask. There aren't plans to heat the whole interior, just heating mats under seedling trays.
    I'd like to use reclaimed foam, but I don't think there is a place around here. Don't they usually sell them by the truckload or at least pallet anyway? I just need a few sheets. I could use some for my future shop, but I doubt I'd be allowed to store a massive quantity of them on our property for an extended time.

    1. Expert Member
      BILL WICHERS | | #3

      You can get triple wall in sizes close to thicker glass panes. The best stuff is about an inch (25mm) thick in terms of R value. It is a A LOT better than glass in terms of insulating performance. There is a 6mm twinwall variant available that is about the same thickness as 1/4" glass and can probably fit in the same framing since greenhouses usually use a sort of "clamp it between aluminum extrusions" sort of glazing method.

      In my expierience, tree sap sticks to the polycarbonate better than glass, but algae sticks to glass better. Either way, just be careful what cleaners you use with polycarbonate since some (usually ammonia-based cleaners) will cause the surface to haze.

      Craigs list often carries ads for reclaimed foam. There are national-level dealers, and those guys usually want to sell full pallets and truckloads, but the more local operations will usually sell by the sheet.


      1. Trevor_Lambert | | #4

        Here in Canada we have no such dealers, as far as I've been able to tell. I did find one guy on our equivalent of Craigs list, and sent a message. I've found a lot of people selling on sites like that are pretty unreliable.

        I floated the PC roof idea roof idea past my wife, and she thinks it will look bad to have a different roof than walls. She also thinks it yellows over time, which is a major reason it got nixed from the start. Do you know if this is true?

        1. Expert Member
          BILL WICHERS | | #5

          The multi wall stuff has a ripply look to it when new (see here:, a little bit like some kinds of textured privacy glass. I've never really found that part to be objectionable. I have some solid (not multi-wall, but it's the same polycarbonate resin) polycarbonate panels in the roof of my old greenhouse that have been in maybe 15-20 years and they show ZERO yellowing. I put them in after several squirrels fell out of trees and broke roof panels. After replacing the glass with polycarbonate, no more broken panels :-) Note that solid polycarbonate looks just like glass, but it is easier to scratch it.

          I think some older resins yellowed with time, and acrylic ("plexiglas") may be more prone to that than polycarbonate ("Lexan") is. We have some very old (30+ years) twinwall on one end of that same greenhouse that also shows no yellowing at all.

          I would try pitching it to your wife as the roof only obscures the sky a bit, like a hazy or cloudy day, but keeping the clear stuff on the sides keeps a nice view of your yard and gardens. If she's worried about light transmission for her plants, that link I posted has data on that too, and polycarbonate is only down a few percent from glass. In my own experience, you never notice the very slight difference. The roof will probably also get you the most bang for your buck in terms of heat savings too.


          1. Trevor_Lambert | | #6

            She's really only concerned about the aesthetics (insofar as the roof goes). I read that acrylic (Plexiglas) is less prone to yellowing, but for whatever reason they don't appear to make it in multi wall sheets. Too brittle, maybe.

          2. Expert Member
            BILL WICHERS | | #7

            Acrylic is much more brittle than polycarbonate. Acrylic will shatter, polycarbonate will bend. It's really noticeable if you have two thin pieces to play with.

            Personally, based on my own experience with polycarbonate in greenhouses, I wouldn't worry about yellowing since I've never seen it even a little. What I would check into is the ripply look of the multiwall stuff to see if you find that objectionable. The ripply look is VERY noticeable on the multiwall stuff. If you go with rigid polycarbonate sheet, you have no ripples -- it looks like glass -- but it's not as stiff so thin pieces will sag over the length of a span. "thin" here means 1/4" over a 2' span so be careful.

            Another option for you is to put some poly sheet over the greenhouse in the winter and inflate it from the inside with a small blower. This is common, and all the stuff needed to do it is available from greenhouse supply places. The poly sheet forms an air barrier between the glass and the poly that gains you some insulation. It's a bit of work to set up, but makes a big difference in terms of heat loss.


  3. Expert Member
    NICK KEENAN | | #8

    You may find that an uninsulated floor works best in a greenhouse. You have to worry about both overheating and getting too cold. The ground acts as a heat sink when it's hot and as a source when it's cold. Remember, the center of the earth is at 5000F and during the winter it is releasing heat toward the atmosphere.

    What some people do is dig the floor down below the frost line. If you don't want to do all of that excavation, you could raise the frost line by insulating the ground on the outside perimeter of the greenhouse. The rule of thumb I've heard is an inch of foam is worth a foot of soil, and it should extend horizontally a foot for each inch of thickness. So if your frost line is down 36" you'd want 3" of foam extending out 36". If the greenhouse itself can't provide enough warming to keep the surface of the soil above freezing on the inside then you're going to get frost killing things off anyway. For the most part all you need is to keep the inside above freezing.

    1. Trevor_Lambert | | #9

      "What some people do is dig the floor down below the frost line."

      Really? So their greenhouse is partially below ground? I've never seen such a thing.

      I see your point about the insulated floor. I'm not so sure that insulating the perimeter would be enough of a benefit to be worth the effort, and the real estate it would chew up. What would make more sense to me, but I probably won't do this either, is to dig a 3' deep trench at the perimeter and have 3" of insulation there in a vertical orientation.

      1. Expert Member
        BILL WICHERS | | #11

        >"Really? So their greenhouse is partially below ground? I've never seen such a thing."

        Look up "solar greenhouse" online. Lots of examples of such things, often dug into the sides of hills or otherwise partly below ground. The idea is to take advantage of some of the natural heat you get a little below grade.

        Insulating around the perimeter trench-style like you describe is pretty common and DOES help. Think of it like insulating a foundation or crawlspace wall.


  4. Jon_R | | #10

    Greenhouses typically over-heat during the day and then need lots of heat at night. Plants also like it cooler at night. All of which means that interior thermal mass would be of some use.

    Might even be worth it to extract and store heat from the interior with a air->water heat pump. Setup in a way that the same heap pump could also extract heat from the exterior.

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