posted 8 years ago
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.
"Study books and observe nature; if they do not agree, throw away the books." ~ William A. Albrecht