To vent or not to vent - small 288 sq ft shed


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Old 06-19-10, 04:12 AM
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Question To vent or not to vent - small 288 sq ft shed



The outside shed diagrammed above is the topic. I have gone to great length to convert it into a semi-living recreational & work space. It is almost 100% airtight and has electric in it. It will be a home craft workshop and home office.

I will be sheetrocking the walls and ceiling. Above the flat top part in the diagram is where the thought of "to vent or not to vent" comes into question. The ceiling will be the shape of, you could say, 3 sides of an Octagon (as seen in the graphic).

Basically above the ceiling would leave a triangular air space completely sealed off from anything else (approximately -maybe- 104 sq feet of dead air space), as again I've gone to great length to seal everything.

There is radiant barrier on the underside of the roof and I plan to insulate it with that foamular 150 or 250 stuff from owenscorning.com - which can be found at home depot.

I am in South, LA.... unfortunately near the oil spill.

I will have air conditioning in the shed running pretty much all year except for when the room has cooled enough or during winter.

Basically, do I need to worry about venting this small space above the ceiling?

If I don't vent it could something buckle open under hot air pressure? or would pressure hold and the structure just expand/contract slightly over the years?


I really don't like the idea of cutting large holes in my shed, after I've gone to great length to ensure it is sealed from outside air.

How hot do you think it could get up there in that small triangular 104 sq ft space during hot 95 - 100 degree summer days? Is ventilation really going to be worth it for such a small space after I've sealed everything else so well?

We have a lot of bugs out here too. I really didn't want to open anything up for them to enter in.
 
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Old 06-19-10, 07:01 AM
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As for how hot, 190 is possible, enough to cook some shingles. Very light colored shingles will do better.

I doubt if the air sealing is tight enough to hold much pressure under heat expansion. In an air conditioning enviroment, the cold condensing surface will be on the inside and the air barrier and vapor barrier need to be on the outside. I'm not sure where you have accomplished the air sealing, but that warm moist LA air should not come in contact with any cool/cold surfaces.

There is radiant barrier on the underside of the roof and I plan to insulate it with that foamular 150 or 250 stuff from owens corning I'm not sure where you are going to put this insulation?

Bud
 
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Old 06-19-10, 06:17 PM
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The shingles are a lighter grayish color.

So are you saying you wouldn't vent it with all the humidity we have here? Its thick humidity too everyday in the summer. Windows drip like they've been rained on in the main house.

In this shed, the area I'm talking about "not venting" would be like a miniature attic... very miniature. Everything above the sheetrock ceiling would be sealed off from the interior/living section below. This "miniature attic" would not have an entry point, not used for storage and would hold dry ambient air if I don't vent it.

I've sealed the stud holes for the wiring with fire block foam and behind all outlet boxes/lighting boxes and once sheet rock goes up I plan to seal around all edges. Every single edges outside the shed has been sealed with silicone or that aluminum tape stuff. All stud/board edges inside sealed with spray foam or silicone. Its so well sealed its hard to open the door when walking in cause of vacuum.

The foamular insulation I was going to put up would be placed over the radiant barrier on the roof underside interior and on all the walls interior and then I was gonna spray foam all the edges of the foamular put in place to make a good seal between the wall studs and roof truss'. This would leave no insulation immediately above "touching" the ceiling sheetrock, but just about a foot or less upwards the insulation would be on the radiant barrier on the roof underside.

So in the end (my idea) you'd have foamular insulated interior walls and over the radiant barrier roof too, with an air space between the roof and ceiling sheetrock.

So if I was an ant on top side of the ceiling sheetrock, I'd be on sheetrock looking up at foamular with the radiant barrier just above the foamular..... with the foamular and radiant barrier touching each other on the underside of the roof.

The cold air from the a/c would never reach the "mini attic" area, but would not having insulation immediately atop the ceiling sheetrock (and only above against the radiant barrier) cause moisture to build on the top side of the ceiling sheetrock?
 
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Old 06-19-10, 07:02 PM
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If you don't have moisture generating activities like showers, cooking, plants or lots of people, your risks are minimal. Your sheetrock is forming a second air barrier, but as long as you don't have two vapor barriers to trap moisture you should be ok.

If the insulation below the roof is in contact with the radiant barrier, RB, then that side is no longer a radiant barrier, If the other side of the RB is the plywood or in contact with the plywood, then there is no RB at all. A RB needs an air gap to do anything.

All materials have a permeance, or ability to pass water vapor. Some are effectively zero, but sheetrock, plywood, and even foam insulation are considered reasonably permeable.
Energy Savers: Vapor Barriers or Vapor Diffusion Retarders
In the construction, care must be taken to be sure moisture can dry in at least one direction. Your efforts to super seal everything need to be reviewed to be sure you aren't making things too tight. The big however is, this is a small limited use space. If you were building an entire house, I would be more concerned.

Bud
 
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Old 06-19-10, 07:05 PM
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I made a rough sketch to put it into visual perspective.



Is that a bad insulation plan? Do I have to vent that? Should I also put insulation right over the sheetrock?
 
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Old 06-21-10, 02:21 AM
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Bud or anyone else... my graphic in post #5 look ok? Its a future plan, except for the radiant barrier and roof that already exist in place.

I think I've been missing something. I'm learning what all these materials are and how they work in an interrelated fashion.
water vapor barrier
insulation
spray foam
rigid foam insulation
radiant barrier
etc..

I have this: LP Building Products | LP TechShield Radiant Barrier
It is perforated radiant barrier laminated to the roof sheathing (plywood) underside. Since I already have this, should I NOT put spray foam or rigid foam insulation (foamular) in contact with it, such that it would render the laminated radiant barrier useless? It would appear that this form of radiant barrier, manufactured adhered to the plywood sheathing, does not require space from the plywood.

There is no plumbing or wet sources in this space and there will not ever be.

So, would I be better putting the foamular or other insulation sheet 4" below the radiant barrier (which is pasted to the plywood roof) so that there is a 4" space between the radiant barrier and the insulation?

...or should I put the insulation atop the sheetrock or should I not put insulation at all since I've got the space so well sealed?

At this time as I understand it, I have no "true" water vapor barriers in place unless you count roof shingles, the black covering under the roof shingles and the plywood roofing with the radiant barrier attached to it.

Basically, I guess what its sounding like, is that I should insulate right atop, in contact with, the sheetrock ceiling OR not put insulation at all.... since there will be no ventilation in this small attic space and maybe any humidity that seeps into the mini attic space would be pulled through the ceiling sheetrock into the a/c and out the a/c exhaust?
 
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Old 06-21-10, 05:58 AM
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I've made a new graphic of what I'm proposing to do now. Think this would work and I wouldn't have moisture concerns if everything is super well sealed? IE: Everything outside is siliconed 100% at all edges, including the drip lines at the shingles/roof edges.



I was thinking too, that I could put 2 small vents in the top of th sheetrock ceiling and thus share this mini attic space with the a/c below the ceiling line, since I don't want to make vents for the mini attic. Wouldn't that idea help a lot to control moisture build up in the mini attic? and should I worry about insulating the sheetrock ceiling if I do this?

Sorry I'm getting so complicated. I just want to be sure, because my family and I will be spending a lot of time in this thing working or retreating from the front house.
 
 

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