fan or vent or both??

kate_rose(7a TX panhandle)December 23, 2007


I am in the middle of converting an old hottub room a lean2 against the house into a sunroom/greenhouse. It has no glazing on the top (just a normal roof) but quite a bit on the south and west sides (just a bit on the east). I am going to put at least a vent on one of the gables for summer venting. I am not sure whether I need to put a fan in that location instead. The room gets quite warm in the winter but not unbearably so. Any advice would be really appreciated.

The room is about 10' X 9' X 25'



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stressbaby(z6 MO)

"Quite warm" in the winter could be blazing hot in the summer, even without any glazing on the roof. I would guess that a fan would be needed.

    Bookmark   December 23, 2007 at 12:35PM
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I think you`ll need more than one vent otherwise you won`t get much airflow even with a fan.
Ideally the exhaust vent wants to be high up, so the gable plan sounds good. Fit several low level vents to draw in fresh air.
You may not need to run the fan all the time as the height difference between intake and exhaust will drive the airflow passively.

    Bookmark   December 23, 2007 at 12:52PM
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If the roof is wood framed, you could install powered roof fans; the kind with mushroom caps. They are fairly cheap and would provide far more hot air release than any passive GH roof vent.

But you will also need to bring cool air in and for that, powered intake vents set as low as possible in a shaded location, hard wired to the roof fans through a single thermostat would likely make a large difference in how cool you will be able to keep your GH in high summer.

    Bookmark   December 23, 2007 at 1:05PM
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kate_rose(7a TX panhandle)

The thing is that I will have an overhang + big trees so that I won't get direct sun on the south facing windows in the summer. Given how warm it is in the winter & spring I do think you are right that a fan is in order.

I was planning to put some low down vents on the other side of the room so that should work with the fan. The thing is it is so dry here in the summer. I will loose a lot of humidity when the fan goes on. Maybe I need to get a swamp cooler eventually or devise a way for the incoming air to go through earth tubes or through one of my big water tanks that hold rainwater. So many options. I want to try them all!! Must stay focused on the fan first though. Any suggestions for a reliable brand? Or should I just go a searching on the web?

Thanks for the quick response. It is great to have folks to bounce ideas/questions off of. My husband is really hopeless about that bit.


    Bookmark   December 23, 2007 at 1:10PM
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kate_rose(7a TX panhandle)


I had considered the mushroom type exhaust fans but was quite worried that it would let a lot of hot air out in the winter. I know that most people wrap them up in the winter but can you really get them airtight enough to prevent a lot of heat bleed?

    Bookmark   December 23, 2007 at 1:13PM
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I like innovative ideas.
Personally i try to avoid using power whenever possible and if that means installing a few more vents then its all good plus it reduces running costs.

The dry air situation probably means using a misting system or a swamp cooler.
Routing the intake air through cold earth or water tanks in summer to add humidity wont work too well as the earth/water will be much cooler than the air.
It may even dehumidify it when it reaches dewpoint temperature along the way ;)
It should be cooler though with the added bonus your water/earth will store some btu`s for later use.

    Bookmark   December 23, 2007 at 2:12PM
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kate_rose(7a TX panhandle)

Hey Hex,

Thanks for the encouragement. I love alternative energy & low/no power consumption systems. This is my first to build so I'm pretty excited about playing with it.

I am not sure I really want to store BTU's anyway in the summer as it will just make the next batch of air coming through hotter. It can't really be helped though. In the fall & winter I plan to use solar heat to warm the water in the tanks and heat the greenhouse.

I know that you are right that as the air cools dewpoint can be reached but I don't think our relative humidity outdoors is high enough for this to happen. Now this is bugging me & I have to go look. . .

OK after some searching here is what I found

assuming 90F & 40% humidity of outside air
and assuming that the air cools about 15 degrees at it passes over water (this is about right for a swamp cooler anyway)So we are at 75 F.

At 90F & 40% the air would contain 1.08% water.
At 75F the air can hold about 1.9% water. So if the 75F air is saturated after going through the tanks it would hold more water than the incoming air had. (course this is just based on some rough estimations from a humidity X temperature graph).

I know all this math stuff makes me a fuddy duddy but I am big into water conservation because we get so little rainfall so I didn't want to be loosing my precious humidity.


    Bookmark   December 23, 2007 at 3:03PM
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ole_dawg(7 UpCountry SC)

I am trying to see in my mind your set up. From all I have read the INTAKEs should be mounted low and the EXHAUST should be as high as possible. I plan, if I get enough money, to use FOUNDATION VENTS. They open and close via a bi-metal coil. They have a screen to keep out the bugs and they are EXACTLY the same size of a concrete block. I will then use a power/shutter fan on one gable end. The heat is what you are going to have to worry about in your area. If you want, email me and I will give your the sque number from Lowe's.

1eyedJack and the Dawg

    Bookmark   December 23, 2007 at 5:02PM
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kate_rose(7a TX panhandle)

Hey Dawg,

I was trying not to overwhelm folks with details but since you asked ;) here are a couple of quick pics. Please forgive the mess but we are still under construction.

This is a pic of the inside of the room facing east. On the left you see the sliding glass door which leads into the house and the temporary door I have constructed which leads outside is on the right.

This is the end we are talking about for the fan & it faces west. The brick wall to the left is the outside wall of the house. The other 2 walls in view are grey on the bottom 1/2 because they are cement. Actually they are the walls of the water tanks/cisterns that I just put in. You can see that one is inside the walls (at the end of the room)and you can just make out the one outside the glazing to the left. The water will come from roof runoff. The two are internally connected so that one can flow into the other if I choose.
I have designed the system so that the cisterns will be covered (good area to raise seedlings) and air can be blown across the surface of the water. I am planning to construct a trickle through solar panel to heat the water in the winter. I probably won't get to it this winter though because there is too much finishing to be done.

So there you have it too much detail.

Hope this helps & thanks for any advice you can offer.

I am sold on the fan but am now debating whether I should but it in the roof (one of those mushroom looking things) or in the gable at the west end.


    Bookmark   December 23, 2007 at 7:22PM
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Hi Kate
It looks like a great project.
Running the air over the water surface is a good idea.

I`d be tempted to put a seperate fan on the cisterns to recirculate the greenhouse air constantly to help raise the humidity. Use the exhaust fan/vents solely to regulate temperature as needed.
If the exhaust fan controls the airflow through the cisterns you`ll likely be venting your humidity outside as fast as you generate it :)

    Bookmark   December 23, 2007 at 8:49PM
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stressbaby(z6 MO)

Kate, which way do your prevailing winds blow? A fan situated in the west wall might work against your prevailing winds if they are generally W -> E.

Hard to tell what temp effect those cisterns will have, particularly the one outside. Do you have an idea about that? If you won't gain any heat from them, you might put foamboard insulation along those walls.

    Bookmark   December 23, 2007 at 8:57PM
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greenhouser(Middle TN Zone 6)


Your plants may enjoy the warm months outdoors in the natural environment as mine do. My GHs are empty in summer.

In summer the roof would protect your plant-room/GH from excessive sun since the sun would then be overhead. Lightweight white curtains should help on the south and west walls/windows. But of course auto vents would be best.

Don't forget good fans for air circulation when plants are in your GH-sunroom.

    Bookmark   December 24, 2007 at 12:52AM
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kate_rose(7a TX panhandle)

There will be a fan to circulate air across the water in the winter time but I was thinking that as the incoming air moved across the water it would be cooled and the fans thermostat would be kicked off.

I am not sure which way my prevailing winds blow. I suspect it is highly variable because we get weather systems from both the south & north. Also the west side is always warmer because it is not as shaded so I think most of the incoming vents should be on the shady side.

I am not sure how the cisterns will work in terms of temp. I am hoping to be able to heat them both with solar so they will radiate the heat back into the room. I also have installed pipes to blow air across a radiator so I can transfer heat or cool from the water into the room more quickly.

The outside cistern has the foundation completely insulated with foamboard. The inside one was poured on an existing slab. I think I will have to experiment before I decide whether to insulate the walls from the inside. My idea is to use the water in the cisterns as a heat sink so that they will keep it cool in the summer and warm in the winter with the solar heating. Basically it should even out the temperature fluctuations.

Like I said this is my first shot at playing with such a system so it will take a while for me to figure it all out. So far I have noticed that the room is noticably cooler than last winter but the cisterns don't have any water in them yet, let alone warm water.

    Bookmark   December 24, 2007 at 1:11AM
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Totally off topic, and I apologize...

What an awsome set up for growing and selling pond plants!

You may want to think about that.

O.K. back to the thread.


    Bookmark   December 24, 2007 at 1:51AM
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greenhouser(Middle TN Zone 6)


I raised pond plants for years. They need more sun than what she'll have in her sun-room. They do best outdoors in full sun. She'll only be able to keep plants around those windows. The rest of the room will be too dark.

    Bookmark   December 24, 2007 at 2:48AM
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She has a tank outside!

    Bookmark   December 24, 2007 at 2:58AM
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orchiddude(+7b ALabama)

The one thing thats probably the most important thing to remember when creating these growing spaces, is to make sure you know what you will be growing and make the structure work for you. When I built my greenhouse, it had several purposes. Therefore, I had to create it like I have it now. Besure to think about all the possibilities that you might run into or the types of plants you may want to grow in the future. You might want to put some clear panels in the ceiling, this would open up more possibilities for growing things, unless this structure is more for storage.

    Bookmark   December 24, 2007 at 8:23AM
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Kate: You expressed concern about sealing up a mushroom cap roof fan for winter.

As they require 2 x 4 framing, the frame can be used to hold foam insulation. We have a large one on the roof of our house and my husband shoves a block of foam in the frame for winter. It fits snugly and seems quite effective at sealing against air intrusion, because when I'm under it in the attic in winter, I don't feel any drafts.

If you want to assure good interior air circulation, mount a ceiling fan at each long end of the space and run them in opposite directions. It really does work, and like the mushroom capped roof fans, they are fairly cheap and available at any large home center.

The outdoor ceiling fans in my GH run continually. The one at the end above the floor mounted heater pulls air up and the one by the door pushes it down, so it circulates top to bottom, end to end. It gives me even temps throughout my GH and the plants love it.

    Bookmark   December 24, 2007 at 8:53AM
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kate_rose(7a TX panhandle)


The dry air here is sort of shocking to most of my plants so currently I only take my lemon outside & it is starting to get to big for that. I am hoping I will be able to use this set up to make a sort of jungle indoors. I am sort of strict about using natives in my yard but I can have whatever I like inside.

I think I could probably do pond plants better in the cistern outside but thats not really what I had planned. I also have a pond so I can grow stuff in there. But I might be able to get some cool tropical lilies that would be OK in the inside cistern for the winter . . . good idea.

You are right its not ideal for a GH but much better than I had before. I designed it so that it would extend the living space of our house a bit so I am planning to have a table & chairs in there at least for the winter months. It is so dry here that it will be a great relief to have a more humid haven. I will put seed starting benches along the front wall & I bought a hydrangea vine for the back brick wall. The rest will fill up with many of my tropical houseplants + more. We have also talked about putting some plant lights in the back.

We are both into herpetology so we are thinking that we might get some chameleons to live in the plants & they would need full spectrum lights to bask in.

I also have some secret plans to build an actual GH out in the yard but don't tell my DH. That will only happen if I overflow the sunroom which is actually fairly likely :)

Thanks for the reply it seems like the mushroom type fans will work great as long as they don't get too rusty from the humidity. I think my ceiling might be too low for ceiling fans but even when I had the hottub in there and only a tiny space I had a fan to make sure there was plenty of air circulation. I will have to figure out how that will work with the new setup.

    Bookmark   December 24, 2007 at 6:22PM
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stressbaby(z6 MO)

Kate, a couple of thoughts...first, any benefit from having the exhaust fan on the shady side could easily be undone if a steady wind cuts your fan efficiency 10-20%. The difference in air movement across my GH, depending on wind direction, is impressive. Second, ceiling fans for air circulation in the greenhouse is nontraditional and unstudied. BW and I have debated this before and we just have to agree to disagree on this. It may work well, I don't know, but it isn't horizontal, and horizontal air flow (HAF) is what is proven to provide the necessary thigmomorphogenetic benefits to the plants. It mimics natures fan, wind.

It is exciting to see your ideas take shape.


    Bookmark   December 24, 2007 at 9:20PM
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Kate: As Stress noted, we have debated ceiling fans, but with a pair, mounted at each end of a space, turning in opposite directions, the air flow IS horizontal. Actually, within the closed space of a heated, winter GH, it's circular. In summer, with both set to pull, they force the cooler air from the floor up, and out of typical GH roof vents. But if your ceiling is too low to allow any, the issue is moot for you in any case.

Powered roof fans are designed to survive more humidity than you might think, although eventually the motors do give out. The one on our small poultry house ran pretty continually for over 14 summers before it finally seized up, but was probably as affected by feather dust as moisture and the ducks made plenty of it, splashing about. My DH found it both cheap and easiest to just buy a new unit at Menards and take the motor from it. The old frame was still fine, so he just switched the innards.

Stress did make a vital point about the location of exhaust fans and there we do concur. If it's facing prevailing winds, they will reduce the effectivenes of a wall mounted exhaust fan, so coupled with the fact of hot air rising, logic would dictate the roof to be the best location of all.

    Bookmark   December 25, 2007 at 8:56AM
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kate_rose(7a TX panhandle)

Stress & Bird,

Thanks for getting back to the original topic. I am convinced about doing the roof fans. Now I just have to figure out exactly how given that there is a tiny attic space above the room. I guess I will just frame them in. I am sure there is a good description of how exactly to do that on the web.


    Bookmark   December 25, 2007 at 12:21PM
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Kate: framing the rough openings for the fans is pretty basic, but first determine the centers on the exterior roof rafters. They are most likely 16", but check to be sure. Then buy the fans. The RO requirement dims. will be on the installation instructions, or even on the box.

Just a thought but- if you have an attic space above that low ceiling, it's apparently not structual, so have you thought about taking it out, and giving yourself a higher ceiling?

If you insulated it to the depth of the roof rafters, you would still have a very well insulated ceiling, but more light and air possibilities, and the higher the ceiling, the easier it is to cool the space at floor and plant bench level.

    Bookmark   December 25, 2007 at 4:19PM
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stressbaby(z6 MO)

I think BW has a good idea there.

    Bookmark   December 25, 2007 at 4:27PM
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kate_rose(7a TX panhandle)

Hmm . . .

It would be nice to have a bit higher ceiling. I will think about what it would entail in terms of construction. I think for now I am ready to have this room done for a while but we discovered terrible dry-rot from bad vapor-barrier design when the room was a hot tub room & there is still one wall to tear down & rebuild. Maybe if we do that wall next fall we could do the ceiling too.

Thanks so much for all the input you folks are great!

Oh one more question. BW you said you thought two of the mushroom fans would be best. My DH is balking at putting one at the end of the room with the cistern because of humidity loss. What do all of you think??


    Bookmark   December 26, 2007 at 5:34PM
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I suggested two because of the length of the room and the presumtion that you might not be too eager to need to frame headers for a really large, single one, because I honestly don't believe one small fan would be enough.

That's on another presumtion of course- that the roof rafters are on 16 inch centers, and any fan that would fit between them would be pretty small.

    Bookmark   December 26, 2007 at 6:15PM
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Since you have windows in the room, why not buy 2 box fans and put one in a window on each side of the room. One set to exhaust hot air and another to draw in air from outside. This would give good cros ventilation. If you are worried about low humidity, keep some 5 gallon buckets filled with water in the room. They will evaporate and add humidity.

    Bookmark   December 26, 2007 at 7:19PM
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To be honest i don`t think humidity will be an issue once the gh is filled with plants, they add a lot of moisture to the air via transpiration.

If you need more, route the water from the internal cistern through shallow gravel beds or trays which will increase the water/air contact area.
Control the small pump via a timer or ideally a humidity stat.
As the gravel heats up, the water evaporates to generate effect a low temperature sauna :)
The water will also transport some of the gravels heat back to the cistern.

    Bookmark   December 26, 2007 at 10:08PM
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kate_rose(7a TX panhandle)

I appreciate all the advice. I will try to post some more pics when I have made some progress.


    Bookmark   December 28, 2007 at 1:20AM
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