Outside 17.4 inside unheated gh 45.9

Around here on a sunny winter's day a greenhouse/sunroom will heat up 50-70F warmer than outside. But that's only with clear sky and we are 30N lat so sun is pretty strong even now.

My sunroom will hit 120F with 50-60F outside. That's woven poly on the sides and a tin roof. With a clear roof it would get hotter. A clearer siding would also get hotter.

Any cloud cover drops that a lot. A few leaks in the structure has much less effect. It will still hit 100F+ with one end of the covering pulled half open.

You're looking at 1,000 watts per m2 perpendicular to sunlight path. That's a lot of heat, ~18,000 watts at peak into my sunroom. Enough to heat 1,100 sqft house from 50F to 80F in 6 hrs.

This post was edited by fruitnut on Sat, Jan 10, 15 at 16:02

Its 32 here in SE MA and the GH made it to a high of 38 all by its self.

Cloudy north winds 21mph inside GH 50* outside 23* propane heater,when temps drop to low teens to single temps I have to start wood barrel stove to keeps inside temps above 40*.

I have an unheated poly covered structure...8x12...
Inside, I have a very small area with a low tunnel. Outside temp 34, inside greenhouse 57 F.

Temp gain depends quite a bit on your climate. That is to say "sunny" varies a lot from place to place. Here in the CO mountains, with low humidity and low haze, the GH can gain 50 to 70+F degrees over outside temps. It often gets hot enough that some venting is necessary, even when only 20F outside.

In other places I have lived, this was not the case, although there was definitely heat gain on their "sunny" days as well.

So to answer your original question, I have to ask one in return, how sunny is your sunny? If you are in the SW, you'd probably be venting your GH in those conditions as it would likely get to 90F+. In cloudier and more humid areas of the country, you would get some heat gain, and likely not need your heater, but probably not as high temps as elsewhere (I'll let folks with more recent experience in those places comment).

Just got my GH enclosed. The north wall is insulated to R-10 and I'm still working on the north roof insulation. It was 47 and sunny at noon outside and the GH was 80. As soon as the clouds rolled in it immediately started to drop and got to about 55. Once there was no sun, it dropped to 50 and the heater kicked in. So in my limited experience, 30-40 degrees higher with the sun out and 10 degrees with clouds and probably not a whole lot different at night.

I have a 10x12 lean to with an oil filed radiator style heater. I have it set on a med. heat with a box fan always running on low hanging from the ceiling. It can get down to 3 degrees outside and it stays no lower than 55 degrees inside. Right now it is 22 outside and the greenhouse shows 59 degrees.

That is amazing,what type of floor to you have,you must really have it air type as well.

The floor has a few small pallets down the center as a walkway, the rest is bare ground. I have a couple wireless temp sensors out there so I can keep watch from inside. Pretty much all I have out there are tropical's, palms. burgs. some ferns. Mex. petunia. aloa. rubber plants. and assort. stuff.. Just got 7" of snow the outside temp is 22 and it's 56 in the greenhouse. Oh by the way I live in central IL.

Now that the sun stays out longer, the greenhouse in my backyard is hanging out at 70 during the day, drops down to 5 to 10 degrees F above outside temps at night. Today, in full sun very windy, 15 mph, outside temp is 22... Greenhouse is 70. The greenhouse is 6 mil reinforced poly plastic lined with bubble wrap.

I think the salient question to ask is not what daytime temps climb to, but rather how cold does an unheated GH get to at night? Basically, it will approach nighttime ambient temps unless you add some sort of protection, so if your temps drop below zero outside, Gh temp will too.