You answered the question yourself in your second paragraph.
You want a coldframe. Minimal heat and ventilaton non-mechanial. Plants come out in the summer.
But in your area you also need a load baring structure. A hoop house is ideal for large amounts of snow because it slides off. They are also more resistence to wind because of the round shape.
The reason you may have to add some heat and ventilation is that the seasons do not change overnight.
It gets hot then cold then hot again.
Too hot in the daytime and too cold at night.
It's easier to add a little heat and venitlation than it is to move plants in and out of the coldframe everyday.
Also as tempertures stablize you can remove the plastic covering altogether and replace it with a shade cloth according to your needs.

Not so much a difference but a usage. A large cold frame is used to over winter plants. It's kept cold (the opposite of a green house that is kept warm). You keep your thermostats on say 40 degree's and this insures the temp does not get above 40. It is also covered with a white plastic instead of a clear to help prevent heat build up.

A green house is the opposite of the above.

Can you turn a "cold frame" into a "green house"? Yes. Just add heat and clear plastic.
But there is the snow load thing and safety issues that go along with that. Typically G/H frames are stronger since you have people in them working ect.

So you would want a heated "greenhouse" for now.
Hope that helps.

Thanks, cactusfreak. So I am thinking on the right track. The fancier greenhouse packages are for all you guys lucky enough to use one year round. I know they say the hoop house doesn't hold snow but around here it does. My hubby used to work at a theme park around here and they had to pull the snow off the hoophouse they had (inflated double poly). Could depend on the width & steepness of your hoops, I guess. Just have to make sure my greenhouse isn't near my house. It has a 12/12 roof pitch and snow comes off like an avalanche. :)

If it doesn't cost too much to operate them I would like the automatic vents & fans. I figure the roll-up sidewalls would be for later in the spring season when temps level out more.

Have to think this all out. A perennial grower around here seems to have a greenhouse for starting plants, and hoop houses that the plants go into which later on her removes the plastic from and either leaves them open or uses shade cloth (depending on the plants). And I think he puts the plants back in the hoop covering it for winter. He might help me with decisions because he has gone entirely to wholesale now and doesn't want to deal with the public anymore. But I also want to have annual plant arrangements...and the guy who does that around here seems to keep all of the hanging baskets, etc inside his operating greenhouse for the entire season (with all the humidity, fans, etc). He wouldn't give me advice because I would be trying to clash with his business.

Now I have to research how efficient heat would be with a hot water heater & pipes system or if it would even work around here. I guess I should find out what our lowest temps were in March and April somehow.

(had to change the subject so it would let me post again).

Thanks, Mylu. I guess I need a coldframe to be a greenhouse for about a month. :) I have seen greenhouses/coldframes listed with the same specs...pipe sizes, # of purlins, bow spacing, etc. But I have also seen some listed that seem alright but say "not rated for snow loads". Guess I have to ask a lot of questions while shopping. I have a lot of time as this would need to be built before fall so it would be ready for early delivery of plants in the spring. Can't build it in the spring cause of the snow.

All the houses around here seem to be double inflated clear poly...which is cloudier than clear but clearer than white. We don't have as much sun as everyone else I guess. (Which makes no sense really when you think about how much closer to the sky we are sitting on top of these mountains!) ;)

I just saw a new sales greenhouse built up here that is the double or triple wall panels and it is very clear! Almost seems too clear. Looks like it would be a giant magnifying glass...reminded me of frying bugs when I was little. Couldn't help think it might fry the plants too.

Daisy, I don't know what city you are in so I just picked one. Scan down. Go to left hand side of page and type in your city. This will give you the weather for the past year. Near bottom of page.

Here is a link that might be useful: Weather for New Hampshire

"I just saw a new sales greenhouse built up here that is the double or triple wall panels and it is very clear! Almost seems too clear."

That sounds like polycarbonate. Yes it's clear but it does a terrific job of scattering the light instead of magnifying it.

Sounds like you trying to open a nursery. Is this the case?

If you want to heat the house I would also suggest double layer inflated also. I can only imagine how darn cold it gets there. And I hate cold weather.

BTW check out the Atlas Snow Arch. They shed snow like your house too.

"not rated for snow loads"
From memory here so I could be a little off on the numbers.
4ft on center = 30lbs per sq ft snow load
5ft on center = 20lbs per sq ft snow load
6ft on center = 10lbs per sq ft snow load
Ratings are for 6mil cover.
The closer the bows the stonger the snow load. My suggestion for your climate would be 4 ft.

Here is a link that might be useful: Atlas

Here in the heart of PA the researchers came up with a different term "High Tunnel" to describe the cold frame. Penn State has close to 40 structures set up at their Hort. research facility. The name was chosen to seperate the structure from greenhouses for TAX purposes as a non-permanent structure. My point is that you can go to any degree of complexity that you want with a covered structure. The Haygrove greenhouses, developed in Great Britian and now covering hundreds of acres there, has a plastic cover that pulls clear up to the tip of the arches, exposing all.

I really like the roll-up sides which I think will replace ventilation fans in years to come. Mine are somewhat automated now in that I raise and lower them with an 18V cordless drill in less than 1 min. ea. Heat sources vary to extremes. Figure on the sky being the limit on costs involved. I support the previous suggestions but recommend that you shop around for a reasonable price, even watching the farm classified adds for a used frame (my 30x96 cost less used than a 20x96 purchased new). Visit a few people that have them erected to get their perspective. I highly recommend a wire-lock(wiggle wire)plastic fastening system. Its cheap, fast and you can use a second wire in the same base channel if you want another layer of plastic.

cactusfreak, that worked great for the Portsmouth area, which is where I used to be from, but when I put in my town and the largest town near me it only gave this week's forecast. Too remote to bother I guess. I am beyond where the eastern state border dips way in then bulges back out. Thanks for trying though. I probably would get a relatively close temp chart if I used the first location across the Canadian border, since it is only about 45 minutes away.

Mylu, right on all counts. The "super clear" house is polycarbonate panels; yes, I hope to open a nursery; and cold up here is putting it mildly... -30F to -40F in the winter. Even though it is now June our forecast for this week that I just looked up says we will range from a high of 80 days to a low of 40 at night. So you can see how operating a greenhouse/nursery on a small scale would never work with a heated greenhouse. My house oil bill in the winter is about $400 a month and we keep the thermostat on 63 and supplement with wood heat. :) Such a long way from where I was raised (Oklahoma). BTW... I hate cold weather too.

I figured I needed a 4-foot bow spacing too. Even then I could probably add center support posts and/or cross bracing (rafters) too just in case we got another ice storm year like a couple years ago that collapsed a lot of things up here. Better safe than sorry I guess.

I liked the Atlas greenhouses. One was mentioned in Taylor's book "Secrets to a Successful Greenhouse and Business".

bmoser, I like the drill idea for rolling up the walls. In a newsletter from Griffins Supply I remember reading an article by some guy with a ton of greenhouses and they had been replacing all their automated greenhouses with ones with roll-up sides, etc to cut their expenses. I like the idea of roof vents too cause it only makes sense that if the heat gets trapped up at the roof...then let it out up there. Roof vents and roll-up sidewalls would create a good airflow.

I will keep an eye out for used frames. I am in the planning stages now, trying to think it all out in advance, decide exactly what I want to do, gather as much info as possible, etc. I am going to talk to the perennial nursery in the area that just went wholesale only. He is a really nice guy and I will be able to learn a lot from him for this area. Plus I will be able to get larger perennials from him in smaller quantities. May even find that I could supply him with stuff that he needs but didn't grow. Hmmm... now I can't wait to go talk to him.

Also my nephew has a maintenace/landscaping company that services one of the largest condo/resort places up here. I am trying to educate him that you don't have to always use the same old bushes that everyone uses... that there are some new shrubs etc with a lot of color and character that work well in this area. I have come to hate arborvitae, forsythias, burning bushes, and the common red barberries and common daylilies! Anything that only gives you interest for one week of the year is a waste of real estate. :) Okay... off my soapbox now.

Anyways, I have a lot of planning to do and you guys have been a lot of help. I think I am narrowing in on a niche for me... and I should probably forgo growing the annuals but I love designing mixed planters/baskets so much. May have to change my mind down the road though.

I have to go search for the Haygrove greenhouses now. Will be interesting to see.