I've been finding about the same difference in temp as you
inside my hooptunnel.
I found the biggest advantage is in keeping the air and soil
moist while the surrounding unprotected plants/soil dry up
from the cold winter winds.

Swanz

I built a minihoophouse last Oct and planted winter rye in it.
I unconvered the 4mm plastic today and the rye was 1 foot
tall. The rye in the rest of the beds were planted almost a
month earlier and are only 2 in or so tall. I tilled in the
rye in the hoophouse and covered it up again with the plastic.
It's a beautifull 70 degrees out, I got a remote thermometer
in the hoophouse and it's 105 degrees inside right now. Hoping
that'll speed up decomposition for planted in a few weeks.

Swanz

My husband is throwing together a "greenhouse" using 3x4 pieces of plexiglass, some lumber lying around and American ingenuity. It will be 4' sq and 3 ' tall. We cant afford to use electricity to heat it. We will use the dryer vent (Southside of the house) to provide a little heat. Also using leaves and grass clippings as mulch, insulation. Any suggestions on other means of providing heat? The goal is to over winter 4 Rocoto pepper plants that I can't bring inside.
Thanks all
Beth

This year I bought a plexiglass coldframe from Farmtek, which is light and seals well. I also sprung for a solar opener - it was difficult to install but very effective, especially since temperatures here in the mountains can range 40 degrees from high to low, even in winter months. We have carrots, spinach and chard ready for harvest in the frame now. My biggest problem has been keeping the frame together - even the lightest breeze picks it up and throws it across the garden. I'm working on some bungee cord anchors. But I do recommend the solar opener.