A few thoughts; the Harbor Freight 4mm twinwall polycarbonate panels (made in China) are lower quality than twinwall polycarbonate you would buy from a greenhouse supplier. They don't seem to have UV protection. I have a HFGH 10x12 and my panels last 3-4 years here in the sunny south before they deteriorate (I am replacing them.) Most twinwall poly from greenhouse supply places has an approximate 10 year warranty and one side is treated for UV protection.

Single pane glass will lose the heat much more quickly on cold nights than twinwall poly, so you might save on panel cost, but you might also then be having to pay more to heat your greenhouse. This heating calculator might be helpful in comparing the performance of different greenhouse materials:
Heat Loss Calculator

For my two cents, I'm wondering how much you can depend on adequate heat being gained into the greenhouse through your house exterior wall, but others will probably have better insight on that question.

The idea of storing water in black barrels for heat mass gets mixed reviews in this forum; many folks say that it's not effective, especially in small greenhouses, where space is better used for plants. I've read that the heat stored in the water can help raise the greenhouse temperature for a while, but by the critical coldest hours before dawn, the heat benefit has been lost. You might seek out archived threads on this topic (just giving you a heads up.)

I have not purchased the HF vent openers, but I recall that past posts here have not been very positive about the quality. My guess is your idea of spending more to buy better ones might be a wiser move.

From a cosmetic point of view, I'd think your carpentry skills would allow you to build an attached structure that would look more like an actual improvement to the house than attaching the HF structure. It would allow you to install a more traditional door, too. My Harbor Freight sliding doors work OK for my purposes, but if you'll scan the archives here, you'll find that some folks don't like them in winter weather (the tracks can fill with ice and freeze shut if conditions are right.) This has only happened once to me during a record winter storm, but I live in a warm climate.

I don't think you'll get enough heat through the house walls to allow citrus culture. I've got an enclosed porch on the south side of my house. It's used mainly to heat my house which it does very well on sunny days. But even after a day at 100F it's within a few degrees of outside by morning. That's near 20F on many mornings recently. You'll need an open door or large open window to the house to keep a greenhouse warm enough for citrus.

I've thought about putting citrus in my sunroom. But even an open window won't keep it warm enough unless I had a fan to circulate the air between house and greenhouse. I'd have to leave the door open. That allows enough height that cold air flows though the bottom and warm air the top. An open window doesn't usually have enough height difference to allow heat transfer by natural convection.

This post was edited by fruitnut on Sun, Jan 20, 13 at 9:59

I agree that HF is junk and heat doesn't go through walls enough to make a difference. A permanent addition to your home will cost more, but also increase the value of the home. People without plants call it a "sun room."

From what I read about Seattle, it hardly ever gets below freezing, right? If all you have to do is keep it just above freezing a few nights a year, just wire an outlet and plug in a cheap electric space heater. That plan would be cost-prohibitive in most of the US, but if your winter is mild and your heater hardly ever runs, then it won't matter.

Cole Robbie, I may have left the wrong impression in my previous post. I never stated that Harbor Freight was junk; in fact, I maintain a blog about them, and answer questions for folks across the country as they build them.
Building our Harbor Freight greenhouse

Mine has been an excellent investment, for the money; it has allowed me to enter the hobby of greenhousing without spending so much it frightened me away from trying it. It has met my needs beautifully for over five years, with a few inexpensive modifications.

The panel problem is very annoying, but the HF panels are also so inexpensive that I can pretty easily replace them every 3-4 years if I don't want to invest in more expensive polycarbonate. I just try to help folks by being honest in my descriptions of the shortcomings of the kit; that way they can weigh out the pros and cons themselves.

That said, I still hold the opinion that attaching a HF greenhouse to a house is not going to add to the cosmetic appeal or long term value of the house. I don't think it would look like a quality addition at all.

I also agree that it would be a good idea to try to construct a permanent greenhouse so it could be viewed as a "sunroom" by future owners. I don't think there's a very large percentage of the population that automatically views a greenhouse as a definite advantage when buying a house.

Which makes us all very special people, of course. ;-)

Mudhouse, Fruitnut and Cole_Robbie,
Thank you all for your input it has helped convince me to go with building a twin-wall polycarbonate myself. I have been doing more research and found another local supplier for the panels with lower prices (8mm Triple R2 for $50) so I should be able to build it for $500-575. I am unable to make a permanent addition to the south side of my home since it is within 12' of the property line - it would violate code to do so.

Mudhouse - thanks for the calculator link, it works out to 3648 BTU Hr assuming 66F GH temp and a 28F night which is about as cold as it gets here.

I have been working on a separate solar domestic hot water project and after everyone's comments I have decided to integrate the two projects

see: http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Main.htm

Basically I am going to build a system similar to the link above to heat hot water for my home, the thermal storage tank will be in the greenhouse against the wall of my home, heat lost from the tank will therefore heat the greenhouse and in addition I am going to use a small (5000btu) used marine hydronic radiant heater that I picked up from work plumbed into the solar thermal store to heat the GH. I already have many of the materials on hand - scrap and salvage so I should be able to build this system for about $1000 which should provide most of the energy needed for the GH and our domestic hot water.

Thanks again for all your input, I will post pictures when done - probably early spring.

That sounds like a really interesting project. I do hope you will follow up with pictures!

If you're ever interested in doing an entire solar system for your house, let me know. We put an 18kv system in last year that allows us to have whole house power if the grid goes down, sell extra power back to the power company every year and have about 4 days of battery back up or more, if we conserve. If the SHTF, we will have power for ever.