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Old 06-19-2013, 01:07 PM   #1
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Spec for a IB PVC roof in San Francisco


I've been investigating roofing options for a flat roof, 1200 sq. ft. home in san francisco. I have a 2 foot crawl space with minimal blown in insulation resting on the bottom (top of the ceiling.) The roof deck is just boards.

I'm interested in the IB PVC system but am not thrilled with the white roof due to the fact that we don't have Air Conditioning and prefer a warmer house (SF is generally cool to mild).

I've also been concerned with condensation as my crawl space is not vented at all. We have venting skylights in the home (kitchen and bathroom).

My thought is this:
1. Go with the brown PVC color vs. white. This will raise my roof temp and possibly lower heating costs. Perhaps there is a UV degradation issue with the brown color but IB will give me a lifetime material warranty. This could also help (in conjunction with the vents below) to raise the roof temperature to evaporate any condensate that is under the membrane.

2. Add 2 (or more?) membrane vents to the membrane.

Maybe these two ideas are enough.

However, I could add a thin layer of insulation to the mix as well, underneath the membrane (creating a hot roof). This could keep condensate off of the roof deck and better allow it to evaporate thru the membrane vents.



Does anyone have any thoughts about this?

Thank you!

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Old 06-19-2013, 07:57 PM   #2
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Spec for a IB PVC roof in San Francisco


Are you referring to attic space when you reference "crawl space"?

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Old 06-19-2013, 08:42 PM   #3
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Spec for a IB PVC roof in San Francisco


Its basically an attic, but completely unusable because of its size. Can't put stuff in there, can't stand up in there, etc.

Maybe you can get your head up there if you can snake your body up to the little hatch!
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Old 06-19-2013, 09:33 PM   #4
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Spec for a IB PVC roof in San Francisco


Can you post up a picture of the home? Unless the area is treated as a hot/insulated roof deck, proper insulation, air sealing, and most definitely ventilation are a must.
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Old 06-20-2013, 12:28 AM   #5
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Spec for a IB PVC roof in San Francisco


Thanks Windows on Wash. I posted some pics of my roof and inside the crawlspace in this thread: Help evaluating bids for a flat roof in San Francisco

This has been a long odyssey for me.

Secondly, I found that the dark green PVC roof membrane offered by IB systems is even more absorptive (reflectivity green 0.073 vs brown 0.079) so I'll go with green.

Like I said, I want warmer indoor temps, and the heating of the roof membrane coupled with venting the membrane will evap off any trapped condensation.

Also, the more I read, the less of an issue I think there is with condensation in the bay area.... Probably overthinking as usual.
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Old 06-20-2013, 11:56 AM   #6
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Spec for a IB PVC roof in San Francisco


I dont think you understand how condensation takes place. As far as expecting the radiant heat from the roof to warm the inside of your home, I don't think so. With regard to any type of roofing, Heat kills. You want the roof to stay as cool as possible. You also want to ventilate your attic space, so the internal temperature is as close as possible to the external temperature. Thats how you eliminate the possibility of condensation. A glass of water at room temperature does not sweat. A glass of Ice water in a hot room, well you get the point.

I know nothing about IBC membrane, but I do know that you need a good design which includes proper detailing, which must include covering up and over those parapet walls and a metal coping over to prevent water coming in behind the flashings.

The best Thermoplastic Single ply membranes with the longest history of performance are Sarnafil, and Fibertite. IBC must have a good sales team in CA. It might be a good sheet, I could not say, but they use Sarnafil in Saudi Arabia, as it is the only sheet that holds up. SF is pretty easy on a roof by comparison.

You could also use EPDM. Whatever you use, go fully adhered over a good cover board.
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Last edited by jagans; 06-20-2013 at 12:44 PM.
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Old 06-20-2013, 12:52 PM   #7
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Spec for a IB PVC roof in San Francisco


Thanks Jagans,
I am always open for correction. Here's my thinking about condensation and ventilation:

Water vapor (moist air) develops inside the house from breath, showers, boiling water, etc. That moisture comes out of the air (changes state from vapor to liquid) when the temperature of the air is no longer high enough to support that level of saturation of vapor. This temp is called dew point. Moisture will condense on a cold glass of ice water, a cold window or if it rises (as warm air and warm moist air does) on a cool roof membrane which is impermeable. So let's delve into that...

Let's assume a white PVC roof that is generally cool with no ventilation. Moist air rises thru drywall, fiberglass and cellulose insulation and roof boards and comes to butt against the PVC membrane. If the temperature of the membrane is at dew point or below the moisture will condense on the membrane and will be trapped. It can be absorbed by the roof deck, but I doubt there will ever be enough heat on that roof or deck with a white PVC roof to again vaporize that moisture in the boards and even if that happened, it would only be trapped and eventually condense on the membrane because its not vented. This would be worst case. So I believe that even if this cool roof was vented, you may not get the roof to be hot enough to vaporize the condensed water during a day's heat cycle. This is obviously dependent upon local ambient conditions. I can see this as reasonable for SF bay area.

Let's evaluate the same scenario with a warmer and ventilated membrane. Again, moist air rises thru drywall, fiberglass and cellulose insulation and roof boards and comes to butt against the PVC membrane. Let's assume a green colored PVC roof that is generally warm, above dew point at some point during each day. Moist air gets trapped by the impermeable membrane, but does not condense due to the fact that the membrane will be above dew point (we assume) and is vented thru the membrane vents OR if the membrane is below dew point at night and vapor condenses on it, we assume that during the next day the membrane heats up again and vaporizes and exhausts the vapor thru the vents.

Do I have it wrong?
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Old 06-20-2013, 01:17 PM   #8
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Spec for a IB PVC roof in San Francisco


You have a wrong from the stand point that you cannot expect the moisture to dry to the inside and through the decking if it has condensed on the underside of the IB membrane.

In an unvented but unconditioned attic you will get condensation across the framing and across the decking which will create mold mildew and rock initiating.

You cannot straddle one side of the fence or another in this case. You need to either ventilate the attic door insulate the roof deck and treat the Attic as conditioned space.
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Old 06-20-2013, 03:22 PM   #9
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Spec for a IB PVC roof in San Francisco


I think that your thought process in thinking that the membrane has to heat up to vaporize water that has formed under the membrane is wrong.

I am sure you have seen vapor rising up off a swimming pool or a lake in the early morning when the air is cool. Basically you do not need radiant heat to have vapor pressure, and most building systems and components cycle all the time. The key is to have the drying time longer than the wetting time. That is why with regard to vapor retarders the rule of thumb is "When in doubt, leave it out"

If I was you, I would ventilate that attic space, and use a white membrane on the roof, or if you use EPDM coat it white. If you do a recover, make sure you have more R-Value in the new roof system than you do in the old, and do not go over a wet roof.

Good Luck to you
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Old 06-20-2013, 03:24 PM   #10
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Spec for a IB PVC roof in San Francisco


Quote:
Originally Posted by Windows on Wash View Post
You have a wrong from the stand point that you cannot expect the moisture to dry to the inside and through the decking if it has condensed on the underside of the IB membrane.

In an unvented but unconditioned attic you will get condensation across the framing and across the decking which will create mold mildew and rock initiating.

You cannot straddle one side of the fence or another in this case. You need to either ventilate the attic door insulate the roof deck and treat the Attic as conditioned space.
WOW, Why would he want to ventilate the attic door? Just Kidding. Liquid Lunch?
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Old 06-20-2013, 03:31 PM   #11
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Spec for a IB PVC roof in San Francisco


For both conditions I described, cool (white) membrane and warm (non-white) membrane I don't assume the moisture will dry thru the decking.

I assume with a cool, white roof there will never be the temperature on or below the membrane to evaporate the condensed water so it will be trapped there. This may not be true in practice (why else would IB systems spec the vents for the white roof), but I don't need or want a cool (white) roof and therefore I don't feel the need to get deeper into the details of this scenario.

For the warmer membrane (non-white), I assume that the moisture trapped under the membrane will exit thru the membrane vents and due to the color, there will be enough temperature under / at the membrane to evap or keep it vaporized and then it will eventually exit thru the membrane vents, not down thru the decking.
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Old 06-20-2013, 04:49 PM   #12
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Spec for a IB PVC roof in San Francisco


I think what Eric and I are trying to tell you is that if you ventilate the attic (What you call the crawl spece) the moisture you describe will not get into the roof system in the first place. If you are venting your bathrooms or your dryer into this space, that is wrong, you must take those vents up through and out the roof. I am not sure what IB is trying to do by installing roof vents, unless they are attempting to dry out an existing roof in a recover situation. This does not work. Never has. The Army Corps of Engineers ran extensive tests on this. Where you are, in SF you want no vapor barrier, Thermal Insulation over your deck mechanically fastened, designed so the dew point falls in the middle of the thermal insulation under most circumstances, an adhered cover board if you can afford it, and a fully adhered roof, with a ventilated "Crawl Space"

The "Nail one, mop one" method has, is, and always will be the best way to install both single ply and multi ply roof systems. It kills thermal bridging, and it buries the fastener heads.

Thats my story, and Im stickin to it, now do what you want.
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Last edited by jagans; 06-20-2013 at 04:52 PM.
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Old 06-21-2013, 08:02 AM   #13
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Spec for a IB PVC roof in San Francisco


Misprint via voice dictating it.

Ventilate the attic "or" insulate the roof deck.

Jagans and I are on the same page. You have to ventilate that roof.

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