With enough supplemental heat, any greenhouse will keep the temp above 45 F. The real question is how much are you willing to spend on electricity/gas to provide the heat. The small single layer Weatherguard will require significant heat, especially in your relatively cold location.

cuestaroble: what you say is quite true, and perhaps I worded my question badly - I was hoping for info from actual users, their experiences and suggestions.....Elder

One method to reduce your energy use by about 40% is to use a double layered covering. It can be polycarbonate panels which have about a 1/4 inch air space or double-polyethylene, which has a very small fan injecting air between the layers. The link shows how to build an inexpensive poly house. It can be adapted to double layer with a little modification.

Here is a link that might be useful: hobby greenhouse design

I use the aluminet in winter amazing what it does holds heat inside in winter and cools down in summer amazing material

Here is a link that might be useful: useful

Although the aluminet may be a good product to use instead of plastic shade cloth in the summer, it does reduce light by 50-70%. This reduction during already low light winter conditions will significantly reduce growth of some plants.