Ventilating small air tight building


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Old 07-06-16, 07:28 PM
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Ventilating small air tight building

The home I recently bought has an outbuilding that was made to be a "man cave" kind of thing.
It's 12' x 14' x 8' ceiling, with full sheetrock and paint interior.
It's a wood frame structure with an attic and includes a drop down access to the attic and three windows (one window has a small A/C unit in it).
However, when the windows and walk-in door are all closed, that sucker is AIR TIGHT. No air circulation at all inside.
Being just a "hobby shack", I of course don't want to run the A/C all the time but I do want to keep it aired out when I'm not in it.

It often has a mildew (mold) smell because any moisture and condensation that happens, stays inside and just sits. (I can't find any visible evidence of mold, but you all know that "stuffy" smell in rooms that are closed off for a period of time).
On 100 degree days, it is absolutely stifling when I first walk in the door.

There are screened vents on each end of the attic, so there is some cross ventilation there for some cooling (and drying) of the attic, but nothing at all to vent the room below to anywhere outside.
The neighbor told me that the previous owner had it built by a "really good handyman", and I'll accept that because the construction and finishing of the building is top notch. Still, what I see is excellent work by a guy who can build things, but he sure wasn't schooled in the other engineering aspects of building construction (like proper ventilation).

So I'm wondering what the pros think about how to fix this and put some air movement in the room while still allowing a small air conditioner to be used when I want to be in the building.
I had a thought of installing a solar fan in the ceiling that would vent to the attic, and then an intake vent near the floor. I was thinking along the lines of a 100cfm fan just to keep the room dry and to keep it from getting stale.

I would appreciate any thoughts on that, of if I should go a different way completely.
 
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Old 07-06-16, 07:41 PM
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You say you have windows, so crack them open an inch when you're not conditioning it. I would probably install a 6" duct in the ceiling (round grille) and out the roof just like a chimney. Put an inline booster fan in the duct and put it on a switch. Kind of like this one. I doubt you need to introduce additional ventilation. Enough make up air will come through the AC unit.

If it smells moldy it might be mold on the back side of the drywall from air conditioning a shed with so much exterior wall sq ft. Cold drywall will act just like a dehumidifier coil when it's hot and humid on the other side of the drywall. Paper gets moldy if there is no vapor barrier... and insulation gets moldy if there is a vb. Seen it many times.
 
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Old 07-07-16, 05:47 AM
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Any soffit vents for make up air for the gable vents?
Got a picture of the outside of this building?
Any vapor barrier under it?
 
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Old 08-04-16, 06:28 AM
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OK, sorry I took so much time off, but I had to go through some surgery and recovery, but I'm at least back on my feet.

I can get a pic a little later, but suffice to say for now, there are no soffit vents at all. I think the building was built a bit tighter than it should have, probably due to inexperience of the builder in matters of ventilation.

There are open vents on both ends of the attic that are louvred (permanently open) and screened to keep bugs out. I'm pretty sure that the builder thought that there would be a constant crossflow across the attic, but it sure feels stuffy and hot when I go up there.

I don't really want to leave the windows open because of rain getting in if I forget to close them, so what I would like to do is to have constant circulating air going into the attic which would (should?) force the hot air of the attic out through the existing vents so that both the room below and the attic would be kept cooler and drier.
I would want to be able to close off the exhaust vent when I'm in the building and want to turn on the A/C.
Does that sound like a good plan?

One thing I've found is that the smallest booster fans as linked earlier run about 1000cfm or more, and that's way more noise, volume, and cost of running than I think I need. All I really need would be something like 100cfm or so to keep things cool and dry (I think).

Ironically, I have a similar situation in my 3-car garage where there are no soffit vents, but there are 2 roof vents (unpowered).
I don't have all that much moisture and heat problems in the garage, but only because I am in and out of there often enough to let air exchange.
However, I am in the process of finishing off the interior walls and ceiling with insulation and sheetrock, so that will tend to close off the garage like the workshed is that I originally asked about.
In the garage, my thought was to install a mini-split A/C unit, but the more I think about it, the more I think that if I vent the garage similar to how I might vent the workshop, I might be better off with the additional ventilation, and I could just run a portable A/C unit in the garage when I want to drop the temp a few degrees to make it "workable".

My thought is that a lot of other people run into the same kind of situation (especially here in the South), so I'm hoping for a magic solution that someone has found.
 
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Old 08-04-16, 09:11 AM
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The last 6" inline fan I bought from a box store is so quiet you can barely hear it running when standing right next to it.

No matter what you do, it will be stuffy and hot in an attic. But a fan would probably drop the temp a few degrees.
 
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Old 08-05-16, 07:15 AM
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Well, I think you are probably suggesting a direction I think I want to go.
However, instead of venting out the roof, I think I'd be better off venting just into the attic and letting that air be forced out the open vents on each end of the attic.
That should serve to cool and dry the lower room the same either way, but I think that would add the benefit of cooling and drying (some) the attic as well.

As far as an earlier response about what's in the walls, I really can't say for sure because the building was built several years before I bought the home.
I can't see really well down the walls from the attic, but I will make it a point to do so with a good light (and I even have an inspection scope I can use for the purpose).
I do want to see if there is a vapor barrier or not, and I am assuming that it was at least insulated when it was built.
 
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Old 08-05-16, 10:21 AM
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I think I'd be better off venting just into the attic and letting that air be forced out the open vents on each end of the attic.
Not a bad idea since you already have the vents up there.
 
 

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