Greenhouse within a greenhouse?

I was looking at a photo on another site and noticed in the background somebody had a greenhouse, maybe 6 x 8 feet, within a much larger greenhouse.  Is anybody on this site doing that?  Can you bestow your wisdom about how you are using it, or point me towards some info someone else has written about this?

It can certainly work, although I'm used to seeing people put a low tunnel inside a high tunnel which I think would be a lot less permanent and easier to manage as needed. I wonder how a full greenhouse inside another would work in the hot summer months, or if both are just not used outside of winter?

I wonder if people are using it to create a microclimate within the greenhouse or for something else like to bring extra heat in that is disbursed to the larger greenhouse.  No idea if that's enough volume to make an impact on the larger space.  I could not see for sure in the photo if there were plants inside the greenhouse - it kind of looked like there weren't.

I've seen it here from our very own Mike!  Hopefully we can draw him here for comment.  
Mike, Mike, Mike (it might work?)
Mike's Passive Solar Greenhouse

I'm here!  Oh yeah, I did that...  kinda



Mine was a "next best thing" to a greenhouse that would hold tropical temps in northern WI.  I still have citrus that I don't want to have die so I had a space with a roof over it within the greenhouse.  I closed it in with poly for two winters and was able to keep it warm enough with a little milk house heater.  So this year it got a better glazing system.  I'll remove it in the summer and reinstall when it gets cold.

My longer term heat plan is to suck hot air off the ceiling of the main greenhouse and blow it through a radiator to store the heat in water barrels in the small greenhouse.  That might allow me to avoid heating it at all.

The solid roof over the mini greenhouse isn't ideal but it's what I had to work with.  I should get an inside pic one of these days...

That Salad greens guy in Maine(Elliot??), pardon me here, I think said that each layer adds move you a USDA zone. so if you're in zone 5 adding two would make it about zone 7....again I think i heard this from his presentation.(I saw it on video online)

A raised bed with bottom heat and a low tunnel over it is something I want to emulate.
The lady at The Dirtpatch Heaven TY channel has lots of nice videos on a compost heated bed that she sets up just like this.
I think there will obviously be a point of diminishing returns but how about cloches, under a low tunnel, inside a green house?
3 layers of cover!
Might be hard to check on the plants...

What I like most is the idea that heating the  inner most greenhouse will heat the greenhouse that  surrounds it.
A incandescent light source in the very middle could give heat and light, with virtually no wasted energy.

Mikes plan seem to actually capture the ambient daytime heat and shove it deep into the bowls of the green house, so it can come out latter when its needed most.

I visited a guy years ago who had an open-ended high tunnel used to produce fruit trees, in the middle of the structure there was a little greenhouse in which he had fruiting potted papaya and mango trees. The high tunnel was a lot warmer than ambient during the day, but cooled down to ambient at night, while the little greenhouse did not get any hotter than the big high tunnel during the day, but retained heat surprisingly well at night in winter.  This is in mild temperate New Zealand climate where the extreme air temperature low for the year is probably around 34F so results might be less effective in continental climates.

Thanks so much, everyone  This is exactly the kind of input I was looking for.  
I saw a youtube video where someone was heating a greenhouse with a hot tub.  Heating it up with electricity and uncovering it in the greenhouse at night, but keeping it covered the rest of the time.  This was to scavenge heat off a hot tub they were going to use anyway.  Maybe you could put a hot tub in the little greenhouse and keep the temperature up so you didn't have to use as much electricity.   I also saw that someone was scavenging heat off a sauna.

Ben Gorski wrote:That Salad greens guy in Maine(Elliot??), pardon me here, I think said that each layer adds move you a USDA zone. so if you're in zone 5 adding two would make it about zone 7....again I think i heard this from his presentation.(I saw it on video online)

Yes, Elliot Coleman talks about this in a couple of his books (The Winter Harvest Handbook in particular, and I believe it's also mentioned in The New Organic Grower). He uses row covers inside the (unheated) greenhouse to grow cold-hardy winter greens through the winter in Maine (I believe he's USDA zone 5).

EDIT to add: I was just looking at the book again, he said each cover you add is a zone and a half to the south...so row cover inside of greenhouse is 3 zones southward