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My larger cold greenhouse is 18'-0" x 12'-6" (5.49m x 3.81m) and is now lined with UV stabilised bubble wrap. I put most of it up myself with just a bit of help from a friend rearing the frames on the walls. I bought a make in the UK called Elite which is usually erected with the glass to the ground as normal. They however do a drop down door version which is the same glass to ground greenhouse with simply one different end where you can erect the greenhouse on top of walls up to 3ft high, but the door can then be lowered to floor level.

Mine is like the one below, but on 3ft high concrete block walls instead of brick. Being reasonably tall I don't like having to bend down to stagings so these are 3ft high and as I only ever used under the stagings for pots etc rather than plants the walls keep it warmer. Raising the greenhouse up means it's now 9ft to the ridge inside and the shelves in the eaves are around 4ft above the stagings, so do not block much light from them. I also have my old smaller 10ft x 8ft greenhouse still.

Dave,
that is excellent! My home-built greenhouse also has a 9 foot high roof in the peak. (or is it 9.5?) Anyway, I would say that a higher ceiling like you describe is absolutely better. Not just because you have more headroom as the grower or that you can grow taller plants - like columnars. There is a very practical advantage in that you have a larger volume of air which does not heat up and cool down as quickly and as heat rises, the hot roof is higher up off the plants and you don't have to be as worried about ventilation. Also, you can put some higher shelves up for plants that really like the heat like E. polycephalus while others can be put lower like Obregonia.

You also have to buy automatic roof vent openers though, since unlike when the greenhouse is erected glass to ground the vents are then too high to be hand operated unless you use a tall stepladder! Mine has solar powered automatic roof vent openers and auto louvre vent openers on the sides, as did my other greenhouse, so I have never manually opened a vent for years. (a viscous waxy substance in the cylinder expands and opens the vent)

http://www.greenhouse-vent-opener.com/ventomatic" onclick="window.open(this.href);return false;

http://www.greenhouse-vent-opener.com/sesamliberty" onclick="window.open(this.href);return false;

http://www.greenhouse-vent-opener.com/" onclick="window.open(this.href);return false;

Hi Dave, that is quite some space in there , but you do certainly have to use a ladder to look at the plants on the top shelves?
Did you the same garden glass as in the picture? The plants are a little bit more succeptible to burning than behind acrylic glass aren´t they?

Hey, I did not know about those automatic vents. Those are ingenious! I guess it would be properly called "heat powered" rather than "solar powered", right?

Anyway, I think I just might install something like that in my greenhouse this next season. My home-built greenhouse is not well sealed, but has lots of cracks, etc. that I left that way on purpose so that there is lots of ventilation. However, this is not good in the early spring and late fall when I would prefer to keep more heat in. Something like the vents you show would be ideal for me and I would then permanently seal up all the corners and cracks.

Yes mine is normal horticultural glass just like the greenhouse in the picture Jens. Some of the greenhouses do have acrylic, also some in the UK do toughened glass which breaks safer, but showers your plants with glass "confetti" when it breaks, rather than pieces chopping them in half as it falls! One of the problems with acrylic is being more flexible than glass it can bend and be sucked out in high winds. Some greenhouses are also made of twin wall polycarbonate, but that is hard to fix in conventional greenhouse bar sections.

This particular greenhouse manufacture listed the aluminum greenhouse frame and glass separately, plus provided a glazing list. As I had an account with a local glass merchant at the time I priced up their horticultural glass and that of the greenhouse manufacturer and saved about £80 buying it from my own glass merchant instead of with the greenhouse. If you want glass for a greenhouse always ask for horticultural glass (which is theoretically seconds since it may wave slightly when you look through it) rather than window glass as that is more expensive. Also horticultural glass comes in standard sizes pre-cut. As in the picture above mine is mainly glazed with standard 2'-0" x 2'-0" or 18" x 2'-0" horticultural glasses with only a few angle cuts needing to be made. You can also get the larger so called "dutch light" sizes too as a standard in the UK, and no doubt elsewhere.

Just had a look on American EBAY Daiv and you evidently can get the autovents in the US:-

http://www.ebay.com/sch/i.html?_trksid= ... &_from=R40" onclick="window.open(this.href);return false;

Can't see any for side louvre vents, but perhaps you don't use greenhouse louvre vents in the US?

http://www.allotment.org.uk/toolshed/gr ... indow-kits" onclick="window.open(this.href);return false;

By the way the hydraulic cylinders on the autovents can occasionally fail after a few years, but you can get replacement cylinders from places like the firm in the post I made earlier without having to fit a new autovent completely.

DaveW wrote:also some in the UK do toughened glass which breaks safer, but showers your plants with glass "confetti" when it breaks,

I call that Automatic top-dressing.

Anyway, I definitely will be looking into the vents and do not necessarily need louvre vents A couple of vents in the roof should make a huge difference.