Every year I check figures on this topic so I can readily tell you that if you heat the house you live in you already are saving by growing plants there also. The shop lights are a fraction of even one seasons heating expenses- I have 24 currently in my house lit over plants.

Within a few weeks I know I'll run out of practical in-house space and I'll move plants to a greenhouse that will require overnight heat but until then I'll take every advantage of the "Free" heat from my house.

I wouldn't need to change anything right away, until my seeds turn into seedlings. I would never try to keep some plants in a greenhouse in winter. I just need to convince my mom, who's bed is near a south facing window, that some plants must be near the window near her bed.

Well, I liked the idea of overwintering some of my semi hardy annuals, ivy topiaries and tender succulents in my unheated greenhouse because some of them ,like the foxtail ferns, are messy inside, and hate the dry indoor air. So, I use my seed propagation mats as gentle bottom heat, and I constructed tents out of sheets of bubble wrap that go over various plant groups. I made the tents, which are actually blankets, so that they have a tarp zipper that goes down the middle. Laid the blankets on my plants and tucked them around the pots and secured with clothes pins. I'm also in zone 6B, and right now with temps in the teens outside at night, temps in the greenhouse are in the twentiesat night. But the soil temps in my mini tents are about 45 at midnight. in the morning, When the GH warms up, it's a simple matter to unzip the tops of the tents and push back the covers. end of the day, just zip them back in. Our GH is vented, so if temps get too warm, and I am away, I am pretty confident the plants would not "cook" under the bubble wrap. But I am collecting data everyday to see how things behave! because it's a new GH for me. BTW, the soil thermometers are attached to a digital thermostat, so when the soil warms up during sunny days, the mats turn off. I think they are the cheapest way to get heat where you need it in the GH.

Are you able to post a pic? This might be a good idea for a house in a house..

Good ideas Amy, I agree about the benefits of heat mats with a soil temp thermostat. I do the same things, using either plastic dome covers, frost blankets, or both. Here in chilly zone 4, I do this in March and April. Before that, it's too cold to bother with it.

For the OP question: overall it's much more efficient to do grow lights instead of heating the whole space. The energy in one heat mat is similar to that of one grow light, but of course with a heat mat you are not heating the air, just the soil below the plants.

i would NOT mess with putting them close to a window ...

in my MI ... its too cold by the windows .. at night .. and seedlings like it very toasty.. i am talking media temps ... cold damp media.. can be a problem ....

the very biggest key .... is timing them out from last frost date ...

and for my ann arborish MI ... if i started them.. say before 3/1 ... they were so leggy as to be near useless .. by the time i could get them out in late may ...

common shop lamps is all you need.. maybe a heating pad in the basement ... and i ran them at night for the little heat generated.. when the furnace kicked down ... enclosing them in some plastic ... the plastic being a dollar store paint tarp ....

in no manner.. can i see heating a stand alone structure as being more cost effective ...

ken

common 4 foot fluorescent lights ... with common shop bulbs.. never more than 2 inches from the plants ... 4 bulbs better than two ... and some hardware chain to make the lights adjustable ...

the whole light set up.. should run no more than 40 bucks ...

I'll post a picture tomorrow of the green house with plants under these bubble wrap blankets i made. Tonight, a real cold snap, our temps are already in low teens, expected to go to zero(that is farenheit!), so I added actual fleece blankets from the house over the bubble wrap tents. Soil temps in the tents still hovering above 35 degrees. The plants are in a kind of dormancy, no new growth, but no dead foliage. I think they are hanging in.
Karin, your greenhouse is an inspiration! Went through your photos. terrific work.

Here are photos of plants in the unheated glass greenhouse which are spending the winter on propagation mats, under homemade/recycled bubble wrap blankets. The red stripe is a tarp zipper that runs down the middle of the blanket which is about 8 feet long and six feet wide. The skirt of the blanket is held snugly around base of plants with a clothespin. When the greenhouse is in sun, and it warms up, I unzip the tent and let them catch some rays. Last night, with outside temps below zero, the tents kept the soil in the pot surfaces from freezing. I added fleece blankets over the tops at night for added insulation.

That looks like something I can definitely try. My gh isn't as nice as yours, it is just a polycarbonate one but I'm thinking this is worth a shot.

I have industrial sewing equipment so making the blankets should be a breeze. touching wood here...

Thank you for posting your pics.

Lori

Bubble wrap blankets were quickly made with packing tape, that adhered very securely to the BW, and the 7 foot tarp zippers( 2 to a pkg) were purchased at Home Depot very inexpensively. They also were easy to attach, as they have a self adhesive edge, meant for sticking to plastic. I made them in about a half hour with nothing but 3" packing tape.
Good Luck!

I make my own custom zippers for my business - in theory I could make one as long as the roll but I don't need a zip that is 1000 yards long!

Thanks for the heads up about this unique tent/gh within a gh.

Do you know the wattage of the lights and the wattage of the heaters, the hours per day each would be on, and the cents per kilowatt hour for electricity? It's pretty easy math if you have this info.

A friend of mine who has a greenhouse would agree lights indoors are cheaper - however:

1. Greenhousing is something you do "because you CAN" in many cases
2. Hardening off to move outdoors is easier in a GH, esp if you're talking about things like veggie/flower transplants or tropicals that summer outside. It's tougher taking from a place that never drops below 67 degrees (most modern homes) to a mid-spring environment outdoors that still dips into the 40s at night than it is from a GH - since unless you heat your GH to the nines, your GH night temps are closer to ambient outdoor (if you let them be) and makes the transition easier.
3. Even with the light reduction of glazing, a GH still usually gets more light than artificial lights, (short days and winter clouds do reduce this quite a bit, however) unless you're using high-wattage HPS or MH and really going all out like the pot growers do.