Wish mine got hot,daytime is no problem, at night it gets colder inside it than it is outside.

How is the temp at night?

A tarp will help lower inside temperatures, but as you mentioned, also reduce the light. A better option would be shade cloth, which comes in varying degrees of shade. As an example,a 50% shade cloth would reduce 50% of the light as well as reduce the heat load in your greenhouse. The required amount of light getting to the plants depends on the type of plant. Tomatoes do best in full light, whereas many houseplants do well in much reduced light.

Sometimes people build lath houses or patios where one sheet is solid and one sheet is poly, alternating. That very idea is to provide some shade and relieve the heat to the plants. So you could try shading parts of the roof. You might also consider getting a shade cloth, which will cut down on the heat/light passing through.
Another option is to whitewash the roof, it will rinse off when the rains come, and then you put a new coat on next spring/summer.
You may also want to install a swamp cooler, or build one.

At night it gets a bit cold but I have a small electric heater that does pretty good. But during the day if the sun it out, and with fans on and the door open, it will reach 100+ quickly. I have come home from work to find the GH thermometer peged at 120+. I think I will try to shade part of it with cloth since I can remove that pretty easily. I'll keep everyone posted.

It the heat is up in the daytime it can burn you plant leaves use shade cloth. I have too on all my greenhouses and shade houses in Galveston. Will also give you a frost barrier

Here is a link that might be useful: call Frank

Wow, I thought I had problems with Temps in my Greenhouse. I had Temps like that last year, but with the Heat sinks, I am not having the over heat problems this year, that water really makes a difference.

We did get our First Freeze last night, and my Heater had to Kick on to hold the GH temp at 55. But with air temps getting so low and sucking all the stored BTU's out of my water at night, the Cool water is keeping the greenhouse from over heating during the day, even to the point that the Roof Vents did not even have to open.

So you might want to look into Water Heat Sinks

Kittyl mentioned "whitewash" the roof. Is this the product you are talking about?

http://65.172.200.75/index.cfm?page=_productdetails&productid=2035&learnmore=1

It is called Shade 2 and it is Greenhouse Shading Paint. Sounds exactly what I need and the price is reasonable to try I think.

You might want to at least try the shade cloth before the paint. I had planned to get a custom shade cloth roof cover made but I got distracted before I could order it so....I just bought a length of 50% shade cloth, cut it in two (needed two widths to cover the roof), lapped it over the roof and secured it at the eaves with spring loaded clamps.

Our gh holds mix of plant types and I found that the two sections I used could be folded back from a corner (or a entire end) to offer some plants direct morning light while shading others all day. In the Fall, when the heat receded I removed the eastern most section of the shade cloth (thereby shading only half the roof) and only last week end pulled the last shaded cloth sections.

It worked so well (quick, flexible & cheap) that I've given up the idea of the custom one-piece-cover and will use the two piece approach in the future.

Hello
Yes shade cloth and for paint just use watered down indoor latex 50%-80% water (thin use more coats)... it is very similar to the real thing. Vent it good, make sure the vents are huge or the roof opens right up and have those water sinks too...so they relase heat at night an store heat in the day time. Small greenhouses under 100x20 feet are very quick to change temp..... best to build big(or have large heat sinks) The right way for the right climate. I have ebooks coming.

Here is a link that might be useful: greenhouse answers

I am looking into GH plastic to replace the single layer 55% white plastic that came with the polytex garden mart GH. It does cut out more than enough heat in summer but is too cool and doesn't let in enough light in winter. I plan to go with Klerk's koolite 380 or the K-50. This a garden center 24X30 foot greenhouse that was set up by the owners that only thought about the seasonal sales window, not year round applicability. So I am working with them to amend this situation. Black shade cloth over a greenhouse to cool it just doesn't make sense to me. There has to be better ways to diffuse the light and let the plants get all they want and still not have the greenhouse absorb heat from the black shade cloth. For the science types on this forum I would love your input on klerk's hi-tech plastics.
Just go to Klerks.com.
The price to me is not that much of an issue for a 4yr plastic it is cheap in my opinion. I did use the koolite 380 on my first greenhouse in Hawaii and it really made a difference on the quality of the plants I grew in it as opposed to the same plants grown outside. I never enclosed this greenhouse because it never was cold enough to be a real issue. Here in Texas I have the extremes of 100F + and 20F - so I have to look at this from heat and cooling properties and well as maximising the growable light sprectrum to get the most from the plants inside the greenhouse both summer and winter.
Any one using the white pigmented shade cloth?
I agree with Jimmy that heat sinks are something to look at for moderating temp extremes as part of any GH design.

Hi
I've been redoing my shadehouse and since I live in zone 10 Florida HEAT is a real peroblem. I went with the four inch lattice as i don't dare cover the walls in this climate lol Eliminated the shadescreen this time and will grow vines on the roof. I once covered the walls with plastic during a cold spell and when i came home the temp inside was 120!! while 58 on the outside. Over the last several years I've used a partial covering of frost cloth pumped heat from the house and let it escape. Worked but had cold spots and is very wasteful.
This time I'm using a heated aquarium and water lily pool The rising heat will make it frost proof but figure I'll still have cold spots. The low so far this year has been 42 for 2 hours so not much of a test lol.Water stayed at 70 but air fell into the low 50's.
If we get a frost going to try spraying with the warmed water. 90 percent of the year it's too hot but of course those few hours can be fatal.
Heat is sure a lot harder to deal with lol gary

Hello, I have a greenhouse with some kind corrugated material for green houses. I moved and I am in full son East to West all day. I had to take all my potted,hanging,vegetables out...I do have a small,low ceiling,area with glass door. I put elect. Heater in the winter. No problem since I live in the Gulg area of Louisiana. The sun,heat is killing me...is there a paint for the poly. Walls I have?