I have shade cloth, four roof vents with automatic openers, a fan contolled by a thermostat, and a screen door, but no cross vents. My experience is that this lowers the temperature by at least 15 degrees F.

You donÂt say you have anything that provides cooling. When I open my house to the outside air, and even though I have fans, the inside temperature will normalize to the outside air.

So I donÂt understand how you say that your inside greenhouse will be 15 degrees lower than outside air, with just fans.

Normally you are not going to see a difference with shade cloth. Shade cloth is really only good to block light because as you said above, the outside air is the same as the inside air except that in a greenhouse heat is trapped faster and therefore will heat up faster. I have shade cloth, fans, 4 windows open 100% of the time, the door open 100% and it will still be 100+ degrees or hotter when its 80 degrees outside. In a greenhouse heat is trapped and makes the structure hotter than outside unless you move the air at the rate of 2 or 3 times a minute. So yes, a greenhouse will get hot and stay hot with or without shade cloth.

In my case, I use a Aqua Fogger, and therefore I can keep my temperature below 80°F. I have seen 70°F and lower as well.

I don't think the important comparison is inside with outside air; it's inside air with and without a shade cloth. I have the small Harbor Freight and a 10" exhaust fan on a thermostat. On a sunny day in the low to mid 80's I can easily hit 110-115 in the greenhouse even with the door and roof vent open and the fan running. Generally, I can drop this 10-15 degrees with a 50% shade cloth.

Thank you all for sharing your thoughts. The greenhouse I built this time is a whole lot different than the one I did 20 years ago, and I had never used shade cloth before, I think I used whitewash. IÂve a friend who told me, IÂd "better get shade cloth". What I have done, I kind of come across by accident. My husband and I built a 10 x 12 greenhouse last Nov. using corrugated clear polycarbonate. I am having so much fun with it. Of course, there are things we still need to finish. I used a space heater through the winter, and thought by the time warm/hot weather came around, IÂd likely be putting in a swamp cooler. We didnÂt put in any ceiling vents, however, we did put in an attic fan, and those attic fans can pull lots of air through the greenhouse. I found out really soon, that on a day up into the 80Âs, it could pull air fast enough that it simply didnÂt get hotter in the greenhouse. And it is on a thermostat, so I donÂt have to watch it.

Additionally, we put a storm door on the front of the greenhouse, it has a large screen behind one window, which I open and leave open. I had this mister on a stand out in the barn (I used it to cool animals), so I put this mister right in front of the open door window. So guess what? It is working like a swamp cooler, with that attic fan pulling air in through the door! IÂve had days up to the 100Âs, and I have a minimum/maximum thermometer in the greenhouse, and it is NOT getting hotter than the normal outdoor air. If anything, I will simply add another mister or two in front of the door. I have sand in the greenhouse pathway, so any extra mist just falls on the sand, and the sand is really good to hold and release moister too.

So, so far I do not have plans to get a shade cloth. I want my plants to have all the sun they can get. But before I considered shade cloth, I wanted to know from you folks, what it really would do. I do appreciate having the feedback.

Just curious but if your fan maintains at no higher than ambient, what exactly were you thinking the shade cloth might do to improve on that?

I'm not sure. A true swamp cooler can lower the temperature, like it would for a house. Or maybe the water evaporation would work better with a shade cloth or partial shade cloth. I may experment with adding a few more misters.

It has to be at least a little bit above ambient, or it's not really a GH! ;-)

Cuestaroble showed me a study a while back that looked at the rate of air exchange and GH temps. Exchanging the air with an exhaust fan every 1/2 minute keeps the temp about 4F above the outside temperature. Exchanging the air every minute keeps you within 8F of outside temp. My memory is vague for the rest of the table, but I recall that exchange every 2 min = 15F above outside temp and every 4 min = 25F above outside temp.

I must disagree with my friend Orchiddude...I think that the shade cloth DOES lower the temps. The link below suggests a 7F drop in the temp with the addition of shade cloth. My anecdotal experience is similar to that.

Here is a link that might be useful: Cooling link

I would agree with Stressbaby.
I have a 10X12 GH and the experts recommend 1000 cfm fan but I use a 2350 cfm with a speed control to slow it down in cooler weather.
The shade cloth should lower the temp 5 degrees or more.
Since I don't have to combat extreme temps the shade cloth and fan Will keep me at outside temp or very near and that would be high 70's low 80's
After late April we put everything in the ground and the GH will be empty till late fall so I don't worry about the temp in July.

stressbaby, that link was very much appreciated.

Stressbaby,
Your recollection on the air exchange vs. GH temps is exactly correct. The figures from the link you provided concerning a 7 deg. temp. reduction with shade cloth are interesting. I suppose the amount of shade , ex. 30% vs. 80% would affect the temp reduction. Using the evaporative cooling size calculator on the same site, shade cloth reduces the size of the evap. cooler needed proportionately to the amount of shade. 50% shade cloth would reduce the cooler size by 50%.

Stressbaby....I think we are comparing apples to watermelons. We all know, I think that the less light equals less heat, only because light is made up of heat energy. So if you change one you change the other, but in the case of a greenhouse, where you are trapping heat unless you are moving it out faster than you are trapping, its going to stay or get hotter with shade cloth or not.

The way a "real" greenhouse works is that it uses the tall ceiling height to help move the heat upward. When you have a tall ceiling, you get the heat off the ground and put it above your head about 6 feet, this makes it seem cooler or the same as outside. Along with getting the heat up greenhouses need to move the total air volume at an exchange rate of 2 or 3 times a minute.

I think the problems we are seeing with small greenhouses is the fact that the heat is on our heads. The greenhouse makers of these little 6x8's dont tell you that you will burn up in them during the summer, unless you remove all the light, and put an air condition in it. LOL

You can do several experiements to see what works best for you, and yes, shade cloth will block some of the light/heat, its the build up of heat that I think people are not looking at. A little $12 fan at walmart will not remove that kind of build up, it takes a bigger fan.

In my case, I made a choice to move from a 16 foot ceiling, to a 9 foot ceiling, only because of the gas prices during winter. I made a great choice, but in doing so I was able to keep my heating bill to a max of $300 a year, but during the summer, I had to fight the heat that would build up. This is where my aqua fogger came in. Works wonders too. Its like an air condition for your greenhouse, yard, etc... With the fogger, I am using the heat at the top of the greenhouse (because its drier) plus I am moving the air at 2400 cfm (cubic feet per minute), then add the fog to that and you will need a coat, well maybe not a coat but its nice and cool.

I think when looking at heat removal, we have to look at several things and not just shade cloth. There is one thing that is true, what ever the temperature is outside, that will be your temperature inside, unless you change the water vapor in the air. There is no way to put shade cloth on a greenhouse and it be cooler than the outside temperature.

I know we are talking about shade cloth but I thought it would be neat to see what the power of water can do also.
Today it is 89°F where I live. Using my fogger in the greenhouse, my wireless temp tells me its: 76.8°F, that is excellant for the conditions I want.

{{gwi:298162}}

The power of water is amazing.

Orchiddude, we're on the same page again!

Note that your exchange rate is quite a bit higher than the usual rate quoted on many commercial/proprietary GH sites, that of 1 air exchange per minute.

The only (academic) exception I will take to your post is this: The way a "real" greenhouse works doesn't have anything to do with the height. A GH works by trapping the heat by using a glazing which has differential transmittance of PAR/light and thermal infrared.

I agree that height (and size and volume) are an issue and I agree that people with small greenhouses can run into problems as a result.

SB

:-) I am sorry for the confusion. Again, I am using the word "real" to mean a standard commerical greenhouse.

I remember when the 10X12 HFGH first came out some were complaining about how high the roof was and one poster said that he was going to make it lower.
I have a 10X12 HFGH and I did not take the advice on those sites that say change the air once a minute.
This was my first GH but common sense told me that a bigger fan would be better so I used a 2350 CFM and use a speed controller to slow it down in cooler weather and it works great.
People that are building their first GH should ignore what they read at those sites and buy a fan that can exchange the air at least twice per minute.

I expect that many of you will get tired of hearing this from me, but once again: If you want to see the effects of a fan sucking air up from the floor and discharging it out the roof vents of a GH so quickly it creates a real, cooling breeze so strong it will blow out a Bic, try a pair of 52", 5 blade ceiling fans.

My GH is 16" long, with 2 sets of roof vents, so we put in 2 fans, set directly below the vents. I would say at least one for every roof vent area.

Then in winter, they will move the warm air down, where you want and need it to be. When the GH is sealed for winter, set each to run in opposite directions, to create continual air circulation.

Hunter brand, 3-speed waterproof outdoor ceiling fans, complete with lights: Less than $100.00 at any big box home center.

They are also literally silent, even running at full speed, a nice little extra in the one place most of us want to go for peace- and quiet.

birdwidow,

Do you have any pictures of your (ceiling fan) set-up??
I would love to know more on this idea..
My 8x14 GH has two (2x4) vents & I have nothing other than a couple small circulation fans so-far.
Do you also use these (ceiling fans)in place of a wall mounted exhaust fan? Any side intakes?
Sorry for all this but your way is very interesting to me.

Thanks,
paul

Paul:

I have both powered intake shutters and an exhaust fan, all connected to a single thermostat, with a separate fan speed control.

The ceiling fans were an idea that struck me after we bought the GH and the shutters/fan. I was sitting out on our open sided, but covered porch late last summer, with the outdoor ceiling fan running at high speed, blowing down, cooling me and blowing away every flying insect that ventured in, and I knew I wanted one in my greenhouse, then thought about how much air it moved, and the purpose of roof vents on a greenhouse.

The roof vents will release hot air, but not enough, without help. That's what the ceiling fans do: pull air up and out with a lot of force; cheaply and silently.

The ceiling fans have proven so effective, that with only the screen opening on the north facing storm door of the GH, with the ceiling fans running full speed, the exhaust fan hardly runs at all, which is fine with me. I didn't realize how noisy it would be.

(Idiot me. All I had to do was to think of what a racket they make in the barn loft.)

The only trick with the ceiling fans was figuring out how to mount them, and for that, we used 3/4" aluminum angle, to create what I can best describe as ceiling joists, except that there is no ceiling, just the joists, in a grid formation, attached to the perlins every 2 ft. across the width of the GH, and crossed with two strips of the same angle bars, set about 4 ft. apart, down the entire length.

They are all connected with SS nuts & bolts and give total rigitity to the fan mounts: no vibration at all, and also serve as mounts for some shop lamps, for general work lighting.

To mount the fan motors, we added short pieces of the angle, to create the equivalant of the standard "H" bracket sold for fan mounts in homes.

However, you need a high enough roof to get the fans up enough as to not decapitate yourself when they are operating, and why my 11 ft. high Cross Country GH was set onto a short kneewall, to get the purlins (and "joists") up to 8 ft., which puts the bottom of the fan lights at 7 ft.; where they would be in a house with 8 ft. ceilings.

I hope that all made sense, because my digital camera crashed last month and I have yet to replace it.

Birdwidow, I think your ceiling fans is a great idea, and I certainly can see it working in a larger greenhouse. I can't see much of a way to do it in mine, I simply don't have the ceiling room for a large fan with the structural supports I have in my wood framed greenhouse. And for someone like me who is using every spare bit of room I can in my 10 x 12 greenhouse, I have a number of hanging plants which hang from the structural supports and I wouldn't want a fan chopping them up! But your idea is one I'll keep in my head for the day I may ever have a commercial greenhouse!

Kitty:

If you have a roof vent and can rig up some type of support for a box fan, if the fan were set face up, directly under the vent, it wouldn't pull as much as a big ceiling fan, but I would bet it would pull enough for you to feel a difference.

Box fans aren't very deep; maybe 6 inches, so one might just fit above most of your hanging plants. Really cheap too, so if the moisture got to it after a season or two, replacing it wouldn't be a budget buster.

Or- and with a wood framed structure, retrofitting is easy compared to aluminum: what about framing in for a powered roof vent?

BTW: My 12 X 16 Cross Country is hardly a "commercial" model. I'm just grateful I thought of the fans before we finished the foundation and as it was, I had to reorder the door drop, to accomodate the increased height resulting from the additional 12 inches for the kneewall.

I know the fans cost me a lot of hanging plant space, but in the end, that loss has proven well worth it, at least for me, in an area where it can get so blistering hot in summer, the nearby city of Chicago opens cooling stations for indigents, to prevent heat related deaths. On the other hand, it regularly drops below zero in winter.

It's those vast seasonal temperature differences that make a year round GH such a challenge in my area, so it's a case of coming up with workable solutions, or use them only in spring and fall.

Since you don't have a photo here is mine from a post that I made back in the spring of 2006. If you click on the link at the bottom of the first post and scroll down you will see a picture of my ceiling fan. GH was not done in fact the fan was installed before anything else was installed.
I remember when I said in an earlier post that I was going to use a ceiling fan some said it would not work but it works fine. Two years running and no problems with it so.
far.

Here is a link that might be useful: link to earlier post

Gardener:

I too got a lot of flack over the subject of ceiling fans in a GH, and couldn't understand why, because they aren't an exotic item, and their ability to pull a lot of air for very little cost in equipment and energy to operate is also well known.

Aren't we the smug ones now. LOL!

The point is that they aren't HAF fans.

For those who have a structure like the HFGH I used a flat piece of aluminum bent it to match the roof angle and used four T bolts to secure it to the roof

Here is a link that might be useful: {{gwi:298161}}

I think ceiling fans for winter use would be great. I think it depends on the set up as if they will work during the summer. In my case, I dont need them. I have thought about using them during the winter but they would rust over during the summer.

Orchiddude:

The Hunter fans I put in my GH are made to be used outdoors: Gasket sealed powder coated aluminum motor housings, and vinyl blades.

They are identical to the one that's been on the ceiling of our open sided, north facing porch for 6 years now, and it still looks and operates as it did the day we first installed it.

Check out the outdoor ceiling fans currently available at most big box home centers and you just might become a convert.

birdwidow and gardenerwantabe,

I am really thinking of trying this out.
A couple more questions please,

How large are your fans? 52" ?

I could probably fit a 52 but a smaller one could be mounted a little higher.

Would a 42" or even a 30" would do a fair job?

Good ideas guys,

Paul

Mine is a 52 inch. I do not use my greenhouse in the summer just in the spring to grow plants from seed. Jan. to May. I don't need a mister and I have no rust problems.
I think it is far Superior to haf fans for heating it don't allow the heat to collect at the ceiling.
I don't know how well a small one will work but my 52 is great. I got mine on sale at Lowe's for $39.95

I am happy to continue to play the role of resident ceiling fan skeptic!

The purpose of HAF is twofold. First, as GWB noted, it is to acheive a more uniform greenhouse temperature. I have little doubt that a 52" fan in a 10x12 GH will do that, although I think it is an overstatement to say that it is "far superior" to HAF. It is a testable question, but you would need a GH with both HAF fans and a ceiling fan to do it. I, personally, have some doubts as to whether it will be better than HAF at all, given the directional airflow and obstructions to this air flow by things like benches. But the only way to know that with certainty is to test the idea.

The second purpose of HAF is to improve plant health through disease reduction and thigmomorphogenic stimulus. I have seen no data that ceiling fans offer an equivalent effect in this regard. There is every intuitive reason to believe that ceiling fans would not be equivalent because they do not mimic the horizontal air movement of nature's fan, that is wind. But again, it's testable...show me the data and I will be convinced.

You have to know what you don't know.

Another issue to consider is the issue of economics. I may have said last year when we had this discussion that even if ceiling fans were inferior to HAF, the cost/benefit analysis might favor ceiling fans because of their lower cost.

I have at least three outdoor ceiling fans around my house, all 4-6 years old and all with some cosmetic rust but without any malfunctions. I don't have any question about the ability of newer ceiling fans to withstand the elements.

SB

SB you make a good argument against the ceiling fan.
If I was to read this and did not already know the difference I'm sure I would not buy a ceiling fan but since I have used the ceiling fan for two summers and one winter I know that your argument looks good on paper but in the real world it just don't work that way.
Pros for ceiling fan
Lower purchase price
Much lower operating cost
Much lower noise level
Last longer no brushes to wear out since it is magnetic drive.
Ceiling fan is better in winter to keep heat from collecting at the roof.
And YES it is better at providing a uniform air flow over the plants.
I have no moisture problems or fungus in fact if you were to put it on high I think it would break the stems on the young plants it will shake them really hard.
I have no scientific data so you will have to test it for yourself.
It makes no difference to me what fan anyone uses I don't own stock in any fan company but after using both types I will stick with the ceiling fan.
I have a exhaust fan mounted high in the end if needed and it uses 1500 watts my ceiling fan runs on 30 watts that alone is a good reason to consider a ceiling fan.

GWB, you make good points as well.

SB

The ceiling fans in my 12 x 16 X 12 ft. high GH are 52 inch, with 5 blades each, and as already noted, are outdoor models, and I have yet to see aluminum, or vinyl- rust.

However, I would also argue that in a smaller, lower height GH, 2 smaller fans would be superior to a single large one, and I'll explain why. It's a bit of a long tale, but bear with me, because what I learned came from actual experience.

It has been said that necessity is the mother of invention but in our case, it was more like desperation.

Our house is a fairly typical early 1950's ranch, with a shallow hip roof and 8 ft. ceilings. (It's only real attribute is the fact that we burnt the mortgage 25 years ago.)

It also has a combination forced air heating & AC system, with baseboard vents, which is about as inneficient a means to cool a structure as you can get, and until we replaced the old H/AC with a high effeciency system 2 years ago, cooling this place in our often blistering summers was often a decision between keeping cool, or gassing up the tractors.

The first ceiling fan was a small $19.99 cheapie installed over the stairwell to the basement, intended to pull some of the air in that lovely sink of cool- up.

It helped enough to encourage us to install a fan in our bedroom, which is long and narrow, with no cross ventillation, as the windows are all on the one exterior wall.

However, because of the length of the room, we ended up installing 2 of them, both 52", and due to the ceiling height, mounted close to the ceiling, which reduces their effectiveness. Our bed is directly under one of them, and the first time we ran them in winter with the blades set to blow down, the breeze was just too much, so we reversed just that one, and to our delight, discovered total air circulation.

The fans move the air continually, end to end, floor to ceiling and other than for cleaning, have not been turned off since.

Once the porch ceiling fan had convinced me that I wanted one in my GH, the experience in our bedroom and pure logic dictated that the same would apply in my GH; so 2 fans there also. All it took to prove it, was a handfull of down.

The fans are currently set to pull up, to draw air up and out of the roof vents. This winter, they will be set to run each in a different direction, to the same affect as in the bedroom.

In a closed space, multiple speed ceiling fans will move air gently, or with force, depending on the set speed, and that's another feature missing in HAF's.

I do not believe that ourdoor air moves in an even, horizontal direction, as our planet really is round, but will leave any further explanations to our resident expert on things scientific- Nathan.

wow...

t_d

All this information is wonderful. Does anyone have a HFGH 10x12 with a ceiling fan? if so, building it as is (no knee wall), is there enough room for it and what size blades?

ljl:

I don't have an HFGH, so am not sure of your headroom, but the bottom line is just that. Even the plastic blades on outdoor fans would be dangerous if you walked into them.

I think the answer would depend on the degree of slope of your roof. On an open ceiling, you need enough space on either side for the blades to clear the ceiling, but even the smallest, 24" ceiling fan is better than none at all, and most of the latest ones have 5 blades, and running at high speed, will still move a lot of air.

If your GH has a gravel or brick floor, wetting it down with cold water would help too. The cooler it is at ground level, the more effectivly a ceiling fan under a roof vent will cool the air in the middle.

BTW: the size of ceiling fans is determined on the total width, so a 24" fan will have fairly short blades. Measure from the center of the motor out, and that will tell you how far down from your roof peak you need to mount it, to prevent the blades from hitting the sloping ceiling.

If that still won't leave safe headroom, you could try hanging a lightweight box fan directly under the vent. Any amount of air you can pull up from the ground and out the roof, will make a difference.

Can anyone point me in the direction of either directions on fixing the shade closh to the HFGH 6x8, or better yet, how to make, and fix the shade cloth.

Also, I'd love to share pictures of my HFGH in my yard, but I don't know how to post pictures. Help on that would also be appreciated.

Cheers,

DG