50% shade cloth over the vents will reduce air flow by about 50%. If you can fashion the shade cloth to avoid going over the vents, you would have most of the shade without reduced ventilation, as shown in the photo.
Also, I have a few small solar panels installed that power several inexpensive computer fans, which work well for horizontal air flow, without the need for 120v electricity.

What percentage is your shade cloth?

My greenhouse is located in the foothills of central California, so in the summer I need a 47% shade, and in the late spring and early fall I use 30%. With bungee cords on each grommet and screws to secure them to the greenhouse, it is an easy change, and seems to work well. I made some 3 ft X 4 ft. independent panels of shade cloth to fill in the end and side shade pieces. The shade cloth has so far lasted over 10 years, so was a good investment.

Thank you Mudhouse....I recognized your name from your blog on steps to put the HFGH together. I swear I read the entire thing so many times I had it memorized! We are in the southwestern part of Montana (Bozeman). Our normal winters are cold with quite a bit of snow....except this year it's been more like spring. I think the elevation at our house is around 5000 ft. I just ordered 4 univent automatic openers for the vents on top. I am thinking of getting the 50% shade cloth and running it from the bottom of one side, up and over the crest, down to the bottom of the other side and perhaps cutting out the squares over the vents and securing it at the bottom of both the sides. My husband is not into the GH stuff so I have to figure all this out but he WILL do the labor of putting the new vent closures and the shade cloth up. My plan for the GH is to plant veggies in big pots and bins 6" deep, 8"deep and 24" deep according to how deep the plants need to be. This is all a crap shoot since I've never done anything like this!!! Any help you have is greatly appreciated,

I've read good things about the Univent automatic openers here. I really should have tried those before giving up on my roof vents (without some kind of automated opener, mine would not stay down in the wind, even in the "locked down" position.)

I hesitate to give advice about shadecloth density to anyone in locations other than my own, but your approach seems reasonable to me.

In addition to figuring out my shadecloth (what density, and where to use it) I learned that the interior of the greenhouse has very different light zones...for me. So then I learned which of my plants wanted the sunnier spots (bench tops, south and west sides) and which wanted the shadier spots (under benches, north and east sides.) For me it was just a gradual learning and tweaking process, I'm sure you'll have no problem figuring out what works for you.

I eventually made a curtain out of the Aluminet shadecloth to hang in the doorway; that way I can keep the doors open during the day, so breezes can enter, but I get fewer confused birds stuck in there! (Hummingbirds and silly doves.) I keep the curtain clipped to the sides of the door opening with little clamps (like clothespins) so it doesn't flap around. Just another example of a tweak I added, after seeing how things worked.

When you get ready to do veggie planting, if you have questions, I'd start a new thread and just specify "veggies" "flowers" (etc) in the title (no need to specify Harbor Freight.) I'm no help in that department but many folks here are absolutely terrific with growing advice.

Sounds like you are well on your way. I understand the "crap shoot" feeling. That's one reason I was happy to start with the HFGH, in spite of the well-known weaknesses of the kit. After doing the necessary modifications to prevent panel loss in winds, I could give greenhouses a try without the scarier prospect of investing thousands of dollars. I didn't even know anyone with a greenhouse, so I often felt like I had no idea what I was doing. The nice folks here at Gardenweb saved the day for me. Thank goodness for the internet!

I will post back later with a few threads about how other folks have added bracing to the HFGH for snow. That may be a concern for you in your location. Too much snow weight up there can cause roof failure in the HFGH pretty easily, and even if you don't keep plants in your unheated GH during the winter, we'd hate for it to be damaged while you are waiting for spring.

Thanks for the comments! Will keep in touch!

Back again, here's my list of threads that have to do with snow load and the HFGH. Maybe reading through these will give you some ideas.

Starting off with a roof failure thread from 2008 that first called our attention to this weakness of the HFGH kit. Note that this GH owner said the weak points were the centers of the roof studs:
Roof Failure (HFGH 10x12, Need Roof Studs)

Polcat has a very handsome solution for snow load bracing:

Polcat's snow load bracing photos

Web4deb has a wonderful blog about building and
modifying the HFGH in amazing ways. This part of the blog mentions
modifications for snow load:
web4deb's snow load modifications

Rosepedal's two threads about snow load bracing:
Snow Load Brace...

Midwest Snow Storm

And a few other threads on the topic of snow load and the HFGH:

Snow, how did you make out?

Harbor Freight Aluminum Greenhouse, Need Advice for Michigan

In addition to roof support, some HFGH owners in snowy climates
find that the sliding doors can freeze shut, especially in wet icy conditions.
This has only happened to me once in 8 years, but I live in a warm climate. Ice
accumulated in the bottom door track and nothing moved for 18 hours (I was
pretty sure wrenching the doors free would damage the soft aluminum frame.) I
have never caulked my panels in place, so I always know in a worst case
scenario I can remove a side wall panel and enter the greenhouse that way, in
an emergency.

Some people have engineered ways to
replace the sliding HF doors with hinged storm doors, if the frozen sliding
doors continued to be a frequent problem. It doesn't look like a super easy
adaptation, but anything is possible with these kits, with a little time and
determination (and a few extra dollars.) Here are a few links to photos:

Yorkie89's door replacement photos are in this thread:

Ripping out the doors...HFGH

Web4deb also made the kit sliding doors work as hinged
doors; scroll down to the part of the page that says:

New Doors on the Greenhouse

I hope this information helps a bit!

Sometimes there are inexpensive, simple ways to achieve a goal. I need shade in late afternoon on the west facing end. White plastic party tablecloths from the dollar store, secured with binder clips does the job quite well. It takes a second round of tablecloths mid-season because they are not sun proof and tend to disintegrate in 2-3 months.

Tootsie, of Tootsie Time blog uses window screen instead of more expensive shade cloth in zone 4 or 5. Do what works.

Air movement is important. If you can't install a ventilating fan, any fan is better than none.

for making your HF greenhouse more stable check you tube there are many videos on doing it

Thank you all, I ordered a 50% and I think my plan is to cut around the vent openings so that they can open yet keep the rest taute so it won't flap in the breeze.

Hello Mudhouse, how are things, we haven't talked in awhile. You my friend got me through a greenhouse build 10x12 HFGH. You also helped me with many links to you on snow support. We had a mild winter initally but then all hell broke lose like the year before. Snowed everyday or every other day, 5 - 10 in. at a time. I think in the month of Feburary alone we received at least 5 ft in that month plus the other months. Snow in some parking lots went away in Apr. monster piles. Anyway, I used EMT tubing and ran it from side to side and front to back and accross. Then I ran tubing up to the roof and connect it from the EMT running accross. On the Harbor Freight I also went to Lowes and got another support and ran it from front to back in the middle of the roof for more support. From the ground I used 2 inch schelude 40 PVC pipe and connected it to the EMT tubing. I also made all my benches out of the same PVC pipe. It may sound confusing but it resembles some of the pics I saw people sent you for snow support. My doors worked fine all winter. I had pieces of metal that I placed on the tracks when the doors were close so snow would not get on them. When I wanted to get in the greenhouse I just picked them up and slide open the doors. I mainly used one door all the time. I also put a fairly heavy multi-purpose grease on the door tracks and they are still sliding good today. I made some new supports to keep the doors from moving off the track and put adjustable rods on the outside of the doors with a adjustable turnbuckle in the middle. Just like a barn door or gate adjustment to make tension on the door. Now during the summer I am going to make a little roof over my door tracks to keep off any snow. The gutter sucked and held back all the snow trying to slide off the roof. I purchased a big brush with a long handle and purchased foam insulation that goes on plumbing pipes and put it around the brush and taped it on. Now I can use the handle and put it up on the greenhouse roof and it fits between the panels and I pull off the snow when I get 5 inches or so. Very easy and the foam slides on the roof and doesn't hurt the roof panels. Anyway, just wanted to say thanks and I should send some pics. Sorry, I don't have any now.

Hello mighty_turf, thanks for posting an update. I'm happy to hear that you and your greenhouse are doing well!

I know you folks had a really rough winter/spring. I have seen the photos online, and my hat is off to everyone in your part of the country. It sounds like you stepped up to the challenge, and you should pat yourself on the back for getting your HFGH through the season.

I think you came up with some great ideas for dealing with the snow; I'm glad some of the photos in past GW threads dealing with improvising supports for snow load were helpful to you. Covering the door tracks to keep them free of accumulated snow and ice makes very good sense (and I agree, really you only need to open one most of the time.) I really liked your idea for using pipe insulation over a brush to make a long-handled snow pusher.

So often it's hard for folks (pre-build) to envision how solutions can be found, once they have the structure in front of them, in their own climate, so they can truly see what's needed to make things work for them. I think this is where the HFGH works well; for folks who are comfortable with a little creative problem-solving, there are ways to deal with the shortcomings of the kit, and adapt it to our needs. It's clearly not designed for heavy snow load, but there are ways to deal with that weakness, with some time, work, and a little (not too much) money.

I have always loved the gardening saying: The best fertilizer is the gardener's shadow.
It's so true. I think most gardening success comes from simply being involved, paying attention, thinking ahead, figuring out what is needed, and doing the work.

I doubt your HFGH would have made it through last winter without your good stewardship and extra efforts. Good for you! I hope you are enjoying the season now, and enjoying your greenhouse! It sounds like you're having fun, and that's what it's all about.

Mudhouse, I need to ask you about your exhaust fan. I know I have seen you install it in your greenhouse pics. Can you refresh my memory on was that a 12 inch fan with louvers? Also, how well does it work for you and where did you purchase it. I have researched fans to death and there are so many. I was thinking of a 16 inch. with a thermostat and possibly a plug already but if not I will wire it up. Any suggestions, Ken

Ken, the fan purchase was somewhat of a leap of faith, as I had no one to advise me locally, and like you, found the research to be a little overwhelming, for a beginner.

We bought a 16 inch three speed exhaust fan with louvers from ACF Greenhouses. The three speeds allow 2950,2050, and 1200 CFM. I thought I'd use all three speeds depending on the time of year, but in our hot climate I always run it at the highest of the three speeds, and almost never use the lower ones (for what it's worth.) Looks like the fan we bought is currently priced at $175. It has always run perfectly, with no problems, and I'd be in a bit of a panic without it, this time of year. It came with a plug so all we had to do was have the outlet nearby, and I plug it into a thermostat so it operates automatically.

I used the greenhouse fan CFM calculator on the same website to help determine the size I needed. The 12 inch size would not have been enough. (Cooling is our biggest challenge here.)

I bought three of these pre-wired thermostats from kkontrols.com and they've worked great also. One for each of my two electric heaters in the winter, and one for the exhaust fan.

I think the conventional thing is to install louvers on the opposite end of the greenhouse to introduce air, but my more crude method is to just leave at least one door open, or partially open, with an aluminet shadecloth curtain clipped across the opening (to keep those pesky doves out.) I also have screens in place on my south wall all summer, so even if I close the greenhouse doors, there's still plenty of air coming in.

I'm sure there are lots of good fans and thermostats out there, but these are the ones we bought, and so far, so good.

Here is a link to the part of my blog that shows the photos you remembered, with links to the products too. Just scroll down to the sections that say "Adding an exhaust fan" and "Adding thermostats."

HFGH Part Seven, Greenhouse Enhancements

Sheri

Thanks very much, I will look again at your pics. Everything I just read is what I have been thinking of. I just was not sure which fan to purchase. I have installed louvers on the front and sides for air flow when I install and use the exhaust fan. I also need to look at the shadecloth. I am using landscaping netted fabric for a shadecloth now, it's ok. The fan seems very reasonable, nice that I don"t have to wire the fan and then I will look at the thermostats. Again much appreciated, you have helped me make my decisions easier. Ken