Double layering plastic for Greenhouse?

brandond(6)December 19, 2009

Hey everyone. I want to insulate my greenhouse better. Its a wood framed greenhouse 12x12, with ridgid poly carbonate on 3 sides. The north wall is insulated quite well with the reflective insulation and foam insulation underneath that. I bought some 6mm thick plastic and want to layer that on the inside of the studs where the polycarbonate is. Should is just take my small staple gun and staple it to the wood. That seems like the easiest route. I will be taking it down int he spring when things warm up. These cold nights are killing me with my electric heater on cost. A friend who has a greenhouse told me this should help a bunch in terms of heat loss and insulation purposes. So my ? is should I just staple it to the wood, or what should I do. thanks,brandon

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Steven Laurin & Company

For a tighter seal, you might consider using thin, wood batten strips over the poly seams. Depending upon the capacity of your staple gun and staple size, that may do the trick. Otherwise, galv bugle-head screws and a screwgun could work.

A more finished system could be designed by using stainless T-nuts inserted in pre-drilled holes in your studs. Machine screws could then be used to secure the wood battens over the inner glazing material. This avoids unsightly staple or screw holes in the framing material.

    Bookmark   December 19, 2009 at 1:18PM
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Dan Staley

What archdiver said. You'll want to think about having an air space of at least 10mm and likely not more than 30mm for insulation. 6mm poly from Home Depot has lower light transmission and combined with your polycarb you may very well get only ~60% light transmission in there. If you have a few extra bucks I'd consider a greenhouse film with, say, 90% light transmission as an alternative. But certainly your first year figuring it out the plastic is cheaper to play with to get your system down.


    Bookmark   December 19, 2009 at 2:16PM
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I plan to do a double layer of greenhouse plastic- one on the exterior and one on the interior of a 2x6 rafter. That's a net of 5 1/2" space. Is that too much space between them? I bought a film that prevents infra-red heat loss and condensation. Will it also prevent heat gain?

Mark Mahlum

    Bookmark   December 19, 2009 at 2:50PM
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Mark, if you are using poly only, you may want to consider buying a small air blower to inflate the two layers of film if there is a common channel where it can all receive air. Air is a wonderful insulator. I can't picture what your set-up looks like, but no......if you get a 5.5 inch space, it shouldn't hurt anything.

No, it will be negligible on loosing solar gain, and what gain you may lose will be more than compensated by cutting your heat losses by trapping IR radiation. Be aware on IR-AC film there is an inside and an outside or the anti-condensate will not work properly. Does your roof have an angle? The better a g'house is insulated, the more snow load it'll accumulate, if you are in a snow zone.

    Bookmark   December 19, 2009 at 4:05PM
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Hi Calliope,

The south wall is at fifty degrees. That sits on a pony wall of 3 1/2' with operable windows. I live in the foothills of the San Juan Mountains, SW Colorado at 7400'. Average snowfall is around 125".

The greenhouse is still under construction and is insulated with windows and door installed. I have thrown a piece of construction grade plastic over it ( not sealed anywhere) and at 1 pm today the temp was 98 degrees. Outside temp was around 36 as we're are running several degrees below normal right now. I plan to install the permanent greenhouse plastic on Monday. I'm trying to find some kind of barrels for water containment. I need 1,000 gallons or so, I think.

Is there any way to post a picture or two?

Take care,

Mark Mahlum

    Bookmark   December 19, 2009 at 9:43PM
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sfallen2002(z5 IA)

Would love to see pics esp of wood framing details. Will you paint them? How will you deal with condensation? Curious, not a flame!

    Bookmark   December 20, 2009 at 11:38AM
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Since your beam sounds flat, as opposed to concave, any weight from snow upon the plastic, until it slides off, will be pretty heavy. That's why I was thinking that inflating it would provide a more solid surface and distribute the weight more evenly over the surface area. But, Fifty degree angle? LOL. That should be more than sufficient to want to slide off.

I've had eight inches snow on my structures, and they withstood just fine, but I have square steel construction and all but one are arched or gothic arched. The one with a straight top is rigid twin wall polycarb.

Plastic is more brittle in cold temps, and forces on it can cause it to fail, or worse yet, take down your structure. I am way in over my head making suggestions for Colorado. It's an whole different ballgame than g'houses in my area. I do not want to steer you wrong. I really can only tell you what works in my area.

    Bookmark   December 20, 2009 at 2:11PM
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Actually, I think my follow up posting has been confused with the initiator of this thread, brandond. Sorry if I caused confusion.

My roof is a 14:12, or 49 degrees. I design and build homes for a living (31 years now) so I'm fluent in the structural aspects of wood construction. The design criteria at 7400' here is 76 # live load and another 10# dead load. My design exceeds those requirements.

My concern is the 5 1/2" air space between the two layers of infra red and condensation resistant poly. Insulation is trapped air, basically and pressurizing the air space shouldn't help it's R- value, or more important, lower it's U value. In fact, using heated air to pressurize the between layer space would be self defeating, I think.

Am I overlooking a detail here?

By the way, two weeks ago, I received 25" of snow in a 24 hour period. I have had as much as 41" in 48 hours here. A foot of snow overnight is quite typical. The San Juans are very snowy. These mountains are also recipients of abundant, very intense sunlight. The snow will slide off until it builds up at the bottom.

Thanks for the input,

    Bookmark   December 20, 2009 at 4:31PM
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Dan Staley

Us here on the Front Range want your elevation and sunlight for our structures, Mark, but there are other things here to occupy us when we can't go out in the wildnerness or visit our family down there.

Nonetheless, the air layer provides a gap to break up the conduction happening by dint of close atoms and molecules in the plastic-poly-material.

Any air layer will increase your insulation, and my well-worn second copy of The Solar Greenhouse Book speaks well of blowers and layers. You don't want the two layers to touch if you can't maintain tension so a little squirrel blower using little energy will work.

Any air gap between two material layers will increase R- value.

Lastly, your elevation there will wear out your fabric sooner, as I'm sure you know.


    Bookmark   December 20, 2009 at 8:22PM
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Thank you Dan, for explaining that. The pressure is to maintain the integrity of the air gap and keep your plastic from sagging. Who said anything about it having to be heated air? No I didn't confuse you with Brandon. You asked a question, and I responded.

I certainly didn't mean to suggest any ignorance on your part about building a structure what might fail under snowloads. It's an all too common occurance in g'houses, even professonally built ones, and there was no way to tell from your initial post if your structure was built by a master craftsman or out of old pallet wood and chicken wire. ;-)

Since commercial growers routinely use double poly I can tell you it will not significantly reduce your light quality. And I can think of no reason why a 5.5 inch spacing would be contraindicated or cause a problem. I suspect I have close to that on the roof areas of my g'houses.

    Bookmark   December 21, 2009 at 10:26AM
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I finished insulation, Drywall (water proof, of course) and poly installation and the temp the other day was 98 inside, 33 outside (we are having significantly colder than average temperatures this month). I don't have water containers installed yet (1,000 gal or so) nor was the blower that forces heated air into the rock beneath running, but that's a lot of BTU's to hopefully capture and store. This will be quite interesting, kind of like a physics and biology lab experiment.


    Bookmark   December 23, 2009 at 9:16AM
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I need advice on fixing greenhouse plastic sheeting to the inside wooden frame of our new polycarbonate greenhouse. See photo.

Should the entire inside lining of the roof be one solid sheet of plastic?

I've cut narrow strips of plastic for fitting between each the beams, securing the edges with sticky "cloth tape" -- but realise I need advice:

I know thin wooden battens would be best to fix the plastic to the wooden beams, but I am in a hurry -- it's the middle of winter here and I want to start growing things.

I tried using double sided tape but it is not strong enough to hold the plastic up to the roof. I hope to have about a 3" air gap.

For heating I will be using old oil barrels (cleaned out) with water as passive solar collectors; parked on small stones at the back of the rear wall of the greenhouse (out of view)

Greenhouse dimensions: 5 meters long and 2 meters wide.

Our winter is cold (overnight down to zero degree centigrade, winter midday gets to about 12 degrees C. We live in the south of Tasmania (island south of Australia) -- posting a question here as there are more greenhouses in N America than in this neck of the woods, it seems.

Thanks in advance :)

    Bookmark   July 13, 2013 at 4:03AM
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Plastic sheeting: it should not matter if you use one large piece or several smaller ones attached to each of your support boards.
The thin batten boards would be best, especially if you are planning to leave the liner plastic up all year. If it is only for the winter, you may get by with just using staples to support the smaller sheets of plastic, and plan on taking them down in the summer, provided you do not have water collecting between your roof and the plastic.

For your passive heat water storage, the recommended amount of water is 150 liters per meter square of floor area. You would therefore need 1500 liters for your 10 sq. m. greenhouse to have much effect. Anything less and the inside temperatures on cold nights will probably be the same as outside by about midnight.

    Bookmark   July 13, 2013 at 5:52PM
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Thanks, cuestaroble : there should be no water collecting between the sheeting and the roof.

Re the passive heat water storage, you'd said I'd need
> 1500 liters for your 10 sq. m. greenhouse to have much effect.

OK thanks: I have 7 x 200 litre drums -- 1400 litres ... ready to go, so maybe I should get one more, just to be on the safe side. I plan to use the drums as the base of a bench to grow seedlings on, shall have to be careful how I drain that surface, but that is another question ... THanks for the input.

    Bookmark   July 14, 2013 at 12:17AM
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