In article ,
Michael Evangelista wrote:

I'm building a small plastic hoop greenhouse (very small, 5'w x 20' long)
over an existing garden bed. Using the gray pvc conduit, painted where it
will touch plastic, with rebar stakes. Structurally I'm basically good. But I
have questions about types of plastic, keeping it heated, problems to watch
for, all of that.

There's lots to read online but i'd rather have a conversation about it.
Looking for any advice, tips or warnings, thanks!

Use 6 mil greenhouse film, not "vapor barrier plastic" or other hardware
store plastics. The greenhouse product is treated to resist UV with a
rated life of 4 years, while the vapor barrier will go to heck in less
than a year in the sun.

For an extra penny per square foot, "anti-drip" or AD coating and IR
(helps keep heat in) are worth it to me. That's one penny for both (not
each) at my usual supplier.

If you run power to the greenhouse, using a double-layer of plastic with
a small (50W or less) blower to keep it inflated helps plastic life (it
does not flap against the structure in the wind as much) and is slightly
better insulated.

Don't know about heating, I don't - well, I do know about heating, which
is why I don't, more accurately. You might try the "Eliot Coleman"
technique of putting cold frames/row covers inside a hoop house
(double-layer coverage) - basically greenhouse heating is simply
expensive - it's an inherently poorly insulated structure. One source
gives an R-value of 0.83 for greenhouse plastic, and R1 or so for
IR-coated plastic (roughly double that for two layers, inflated) - if
your house is a cramped 5 feet high, you have at minimum 176 square feet
of surface area - if it's 0F outside and you'd like to maintain 40F
inside, you'll need 7040 BTUs per hour or roughly 2 KW electric heat (or
about half that for double-layer plastic.) How expensive that is will
vary with how harsh your local climate is, your electric rate or other
fuel source, and what temperature you attempt to maintain overnight
(daytime with the sun shining will be different.) VERY roughly it might
cost me nearly $1000/year to heat your little greenhouse in my climate,
with electricity from my provider as the fuel source.

If you get some solar heating during the day, unless you can figure out
a way to remove it to "storage" VERY effectively, you soon hit a point
where you need to vent it or the plants will cook. I have thought about
solar PV panels running fans that would pull air from the peak down into
pipes in the soil, but never actually implemented the idea, so I'm not
sure how well it would work, if at all. I know for long stretches in
certain months it would have no useful sun to speak of so it would not
be much of a heat source. At least the coldest weather here is usually
associated with sunny days, but they are also pretty short days.

Have you thought about ventilation?

Depending how far along you are, if your BED is 5x20, you may want to
provide some room so you can walk, not on the plants.

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