I have a basic greenhouse, very much like this one. http://www.harborfreight.com/6-ft-x-8-ft-greenhouse-47712.html It is made with an aluminum frame and polycarbonate panels. Currently, it serves more time as a summer dehydrator than for season extender. I am not using it to it's full potential. I worry about my seedlings.  When we had 3 nights of below 32F weather, my seedlings came inside.

I found the R value of similar panels to be 1.43 from here: http://www.greenhousecatalog.com/greenhouse-insulation Practically speaking, what does this mean? What is the lowest outside temp in this style of uheated greenhouse is safe for frost tender plants?

Edit to add: We have it anchored to railroad ties with an earthen floor.

R value is a somewhat complicated measure of resistance of heat transfer used in industry. I don't fully understand it, but I know that wind seeping thru cracks in exterior walls greatly reduces whatever R-value insulative ability said walls may have. I believe you did the right thing bringing in your frost sensitive seedlings. I always err on the side of caution with seedlings that won't tolerate frost. I don't want to risk setbacks from losing 6 or 8 week old warm weather seedlings. I have discovered in my observations in my garden that some frost and freeze tolerant crops such as spinach, will survive and do rather well, even when the low is in the 20's, if merely covered with a row cover or even an upside down flower pot instead of being left uncovered to the wind and dew that settles and frosts the leafy tissue. You can improve thermal conditions inside your greenhouse with heatsinks like jugs or buckets painted flat black and filled with water. The sun will heat them during the day, and they will release this heat during the night. You can absolutely extend your season with your greenhouse.

Edit: The higher the number R-value, the greater the insulative qualities. It's my understanding that R-30 insulations has approximately twice the resistance to heat transfer as R-15. Your R-1.43 walls have very little resistance to heat transfer. I am unsure of what the lowest outdoor temperature might be that your greenhouse, currently as is, will keep frost sensitive seedlings safe from.

You would probably learn a lot by putting a thermometer in the greenhouse that can track the lowest or highest temperature it experiences.  Then you'd know how it does on a 32 degree night.

As usual, it depends on a number of things.  How leaky the greenhouse is will definitely affect how long it keeps the warmth in.  If that cold night was preceded by a sunny day there would be more heat inside to carry through the night.  If you have some thermal mass inside the greenhouse (water jugs, pots of soil, etc) it will also help store and then release heat.

A R value of 1.43 isn't anything to sneeze at, it does do a fair bit to hold the heat in. 

I'm sure other folks will chime in on the topic but I bet your seedlings would be fine down to 25 degrees in there as long as your day prior wasn't a 37 degree cloudy day.

Can't really give you any concrete facts or numbers, but here in my climate (USDA Hardiness Zone 3b/4a) I definitely get a good season extension on both the spring and fall ends of my gardening season.  Typically I do heat my GH with a woodstove for a few weeks at the beginning and end of the season, but can get about three weeks on either end of the season without supplemental heating.  Some things I otherwise would not successfully grow without the GH.  Tomato's being one of them. 

Hi Joylynn,

My thoughts mirror those above - R-value is a measure of how much heat loss occurs through a material. It is only one of the ways a greenhouse looses heat. Air infiltration is the other major one (air seeping through cracks) so sealing up the greenhouse with caulk would be a good move to get more out of it.
To put R-value in comparison, the walls in your home probably have an R-10 to R-20... so greenhouse materials are very inefficient / poor at retaining heat. There is a lot more about heat loss in the book below.

In my experience measuring a few polyethelene greenhouses they stayed a couple degrees warmer than the coldest temperature outside at night... usually 1-5 degrees increased temperature over outside at night. (During the day of course, they are much much warmer than the outside temperature.) As others suggested, I would measure on a cold night and then make a determination from there on what you can leave inside.

Another tip to get more out of it -- use two layers of protection, like a row cover or plastic bottles over sensitive plants inside the greenhouse. (What Eliot Coleman is well known for). You can also experiment with using thermal mass like jugs of water around plants to make pockets of warmer air.