Bringing the plants inside would be the easiest solution, but I don't have a whole lot of appropriate space in my house to grow anything, and my dogs and cat would destroy them. This means that the best solution is a greenhouse. I don't have space for a big walk-in area, but I also don't need that much growing area for a simple herb garden; a 2'x4' space with a couple racks inside would be plenty. I didn't find many commercial solutions for my needs, and nothing I wanted to pay for, so I did some research on designing and building my own and I'd like to share the basics I've found with you.
Greenhouses are designed to trap heat and light, two things that are in short supply during the winter in northern latitudes. If you have the capability to recess the floor of the greenhouse into the ground to a point below the frost line, this makes the heat portion of the equation far simpler. Unfortunately, the area where I'm looking to site my micro greenhouse is on concrete, so I'll have to design a bit differently.
Both light and heat in greenhouses are primarily supplied by the sun. This means you need a glass or plastic wall that faces the sun for the longest possible amount of time during the day. For me, that means a southern facing with about a 30 degree slope, and a white or other light colored material on the northern wall to direct heat and light back into the growing area.
In order to trap the most possible heat, you need to insulate your greenhouse. Insulation should be placed into the roof, sides, and back. This can be residential fiberglass batting, foam board, spray foam, or whatever other material you have available. If you're building on concrete, elevate your plants a few inches and insulate under them -- concrete is wonderful for retaining heat, but it takes a ton of energy to get warmed up. My plan is to use half of a wooden shipping pallet as a floor and then pack it with straw or some other fill material.
Either glass or clear hard plastic will work fine for the sun-facing wall. Glass is great for trapping heat, but it can be expensive and fragile; plastic is light, inexpensive, and durable, but it doesn't insulate well. I haven't decided which to use yet, and probably won't until I'm actually laying down cash, but that's one of the last things I need to put in place as summer is just starting and I won't need to worry about insulating my plants until sometime around October.
I'll keep you updated as design and building progress this summer. With a bit of luck and skill, I'll have fresh rosemary and cilantro to put on steaks in January.
Lokidude
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