the dead space acts as a big insulator, but the plastic transfers the heat too quickly. you can end up giving some of the heat back to the outside during the heat/daylight hours. sometimes, (so much of this is climate specific). in short cold cycles it is worth exchanging this heat to rock bed, or soil or water. for night/cold cycle. i have pulled through 4 inch ads in soil and in rock pathways. in one house i transferred the excess eave temps into recycled hydronic floorboard heat exchangers. all part of the madscientist creed.
i meant to add to the above; now that the goodies are in beware the mouse invasion.