Flat Roof Venting

[kurt]

This was also posted over @ the JLC forum, but the paucity of responses was lamentable. Anyone got an opinion?

Solid load bearing masonry structure, brand new, low slope mod bit roof system w/a roof deck.

No vents. No air barrier, fiberglass batts between roof truss joists (I think they're 22"?), no vapor retarder beyond the kraft paper, no apparent attempt to seal bypasses, no vents. Basic block box, studded on the interior, kraft faced glass, drywall walls & ceilings.

Owner is asking my opinion, I told him he should have vents (basic mushromm types) because I've seen these run w/condensation in our winters. Builder, of course, "learned building from his Daddy", and never vents his flat roofs. I tried getting into the fundamentals, never heard of air barriers, VR's, etc. The guy wouldn't hear of it.

I've seen these things go to mushroom farms in the attic/overhead space without vents. The builder's dodging every which way to avoid putting in vents. I told my guy to dig in his heels for vents.

Any argument w/that? Approx. 1500sf of roof area; any good ideas on approx. SF of vent space?

[hausdok]

Hi,

These might help a little:

http://www.buildingscience.com/document ... main_topic

http://www.buildingscience.com/document ... l-climates

OT - OF!!!

M.

[hausdok]

These might help too

Click here for #1

Click here for #2

Click here for #3

Click here for #4

Click here for #5

[kurt]

Wow. This should keep me going for a while. I already have the Canadian stuff, and have read most of the buildingscience.com stuff, but I'm going over all of it again.

This place has all the ducts in the attic, no vents, lots of bypasses in the form of can light openings, speakers, heat register openings, etc.

Couple that w/the total lack of air barrier, no vents, impermeable membrane roof, & the usual moisture creating components like humidifiers, and I just get the feeling I'd like to see vents on this sucker.

Any personal gut opinion on the conditions I described? Because, even after reading up on this stuff for the last 12 hours, I've still yet to come up w/the lock down conclusion. Anyone want to hazard an opinion, I'd be up for listening.

[Brandon Chew]

Opinion - you can design and build an unvented or a vented attic space and either can work effectively if done properly. What you have

described is not proper for either type of system.

The unvented roof assembly needs to have the thermal barrier and the air barrier close to the roof deck.

The placement of the thermal and air barriers you've described are those for a vented space -- at the attic/ceiling interface. It's improper because it is missing a key detail -- the vents.

This place has all the ducts in the attic, no vents, lots of bypasses in the form of can light openings, speakers, heat register openings, etc.

Those conditions are going make it very difficult to create an effective air barrier at the ceiling/attic interface.

The fix is to either leave the insulation and (lack of) air barrier the way it is and vent the attic space (at a higher cost in lost energy over time), or to move the thermal and air barriers to the underside of the roof deck and turn the rest of the attic space into conditioned space.

Model codes specify a minimum net free vent area of 1/150 of the area of the space ventilated. It can be reduced to 1/300 if you can properly install a vapor barrier with a perm not exeeding 1. Due to the construction details you've mentioned I doubt you could seal it effectively to make the 1/300 work.

[Jim Katen]

Originally posted by kurt

. . . Any personal gut opinion on the conditions I described? Because, even after reading up on this stuff for the last 12 hours, I've still yet to come up w/the lock down conclusion. Anyone want to hazard an opinion, I'd be up for listening.

It needs to be vented. The ductwork alone will leak enough to cause trouble.

However, without the stack effect of a pitched roof, I believe that breather vents are of limited value here. How's the air going to know it's supposed to go out through the vents?

I'd be exploring mechanical ventilation through the sidewalls.

- Jim Katen, Oregon

[hausdok]

Originally posted by Jim Katen

without the stack effect of a pitched roof, I believe that breather vents are of limited value here. How's the air going to know it's supposed to go out through the vents?

I'd be exploring mechanical ventilation through the sidewalls.

- Jim Katen, Oregon

I used to think that too, until I'd inspected a couple of dozen flat-roofed multi-family housing structures with flat roofs and vented attics and didn't encounter any mold, rot or anything like that.

Most of those were vented with nothing more than a fascia vent around the perimeter and didn't have any mushroom vents at all; yet, they were venting fine. Now I just try to figure out where the barrier is placed. If it's at the roof plane, then the area needs to be unvented and turned into conditioned space. If it's at the ceiling and it's going to be a cold roof there needs to be ventilation or they've created a petri dish.

ONE TEAM - ONE FIGHT!!!

Mike

[kurt]

Two (oops, make that 3) very thoughtful opinions that I agree with. All the observations are the same as my own.

Brandon's comments describe the conundrum. Jim's comment about "how will the air know to go through the vents(?)" might be answered by the basic stack effect of the house. These are all tight lot, urban structures, w/overall design being vertically oriented, i.e., small footprint, and 2-3 stories.

All the developers of the boom put up these masonry boxes that are nothing but stack effect; the concept of air barrier is completely unknown to all of them. I've been on the roofs of these things in winter, and the snow melts around the breather vents; that, by itself, tells me there must be some air moving, but I'm open to any option @ this point; I'm looking for ideas.

Mechanical venting thru the walls might be accomplished how?

[Jim Katen]

Originally posted by kurt

. . . Jim's comment about "how will the air know to go through the vents(?)" might be answered by the basic stack effect of the house. These are all tight lot, urban structures, w/overall design being vertically oriented, i.e., small footprint, and 2-3 stories.

That'll drive air from the house up through the breathers. That's not the same as effective attic ventilation. Air movement isn't enough. Sometimes moving large quantities of warm, moist air through a cold attic space just makes things worse. You need to seal the ducts and ceiling as well as possible. Then you need a source of outside air and a way to drive it throughout the entire underside of the roof deck. Otherwise you get short-circuiting and the still areas experience condensation from the air movement through imperfections in your barrier.

All the developers of the boom put up these masonry boxes that are nothing but stack effect; the concept of air barrier is completely unknown to all of them. I've been on the roofs of these things in winter, and the snow melts around the breather vents; that, by itself, tells me there must be some air moving, but I'm open to any option @ this point; I'm looking for ideas.

Sure, you're seeing warm air coming out of those vents. That's bad, not good.

Mechanical venting thru the walls might be accomplished how?

Knock some holes in the masonry walls. Put fans in one side and louvers in the other. Push the air in, don't pull it out.

- Jim Katen, Oregon

[kurt]

Understood.

At this point, there's no such thing as installing air barriers, or making any other improvements to the existing; the place is built, and doing it the right way would mean taking it all apart; not gonna happen. I have no idea what's going on w/the ducts, whether or not they are sealed adequately, or anything else; it's all shooting blind right now.

I like the idea of power sidewall venting. I agree, the breather vents just aren't going to do what I want them to do for all the reasons you stated. That's what started all this; I know these structures are nothing but stack effect, and figuring out the best angle & approach is problematic and confusing. It's why I wish more folks would hire me before they get under contract w/these monsters.

You think pressurizing the plenum and letting the air find it's way through would be better than exhausting air. I agree. What would be a simple way to describe why this is better?

[Jim Katen]

Originally posted by kurt

Understood.

At this point, there's no such thing as installing air barriers, or making any other improvements to the existing; the place is built, and doing it the right way would mean taking it all apart; not gonna happen. I have no idea what's going on w/the ducts, whether or not they are sealed adequately, or anything else; it's all shooting blind right now.

I like the idea of power sidewall venting. I agree, the breather vents just aren't going to do what I want them to do for all the reasons you stated. That's what started all this; I know these structures are nothing but stack effect, and figuring out the best angle & approach is problematic and confusing. It's why I wish more folks would hire me before they get under contract w/these monsters.

You think pressurizing the plenum and letting the air find it's way through would be better than exhausting air. I agree. What would be a simple way to describe why this is better?

If you suck air out, you depressurize the attic. This will draw warm, moist indoor air into the attic and drive the condensation engine.

If you blow cold, dry outdoor air into the attic, you don't drive the condensation engine. You will probably push cold, dry outdoor air down into the living space -- it won't harm a thing.

- Jim Katen, Oregon

[vilson]

Hi,

Yes. you are right. He should have had the roof vent. If you have owned a flat roof for several years, it is still a good idea to inspect your roof and know how to maintain it. Undiscovered leaks can be devastating and costly, especially in areas with irreplaceable paperwork or expensive equipment. Flat roofs have layers that may direct the water to another area in the building, often many feet away. The damage could also be from a plumbing leak or another source other than your roof. It is also possible the leak causing the stain was already fixed, and the ceiling was not. To be safe, it is best to hire a roofing contractor to locate the leak.