# double vapor barrier in green roof



## Sparticle (Nov 1, 2004)

We're having trouble deciding on faced or unfaced insulation. Our green roof is built with 2x12's on 12" centers, then plywood, then 1/2" foam board, then EPDM, then Enkadrain drainage mat, then growing medium. We want to put R-30 insulation in between the 2x12's. Should we use faced or unfaced? Concerned that using faced would make a double vapor barrier since the EPDM is on the roof too. We're using pine car siding as the ceiling material. Thanks.


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## Mrrsteelers (Sep 21, 2008)

I don't know roofing, but I do drywall. All I can say is you never want 2 vapor barriers. It actually traps moisture between them and it will mold. I would think a roof would be worse because of the heat rising. 

Faced insulation doesn't seal well enough to be considered a moisture barrier though.

After the insulation is in, normally a layer of plastic goes over that. Then we drywall, or in your case box car side it. If you do it that way, I'd be worried.


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## Bat Farm (Apr 21, 2010)

Unfaced. It would be enough of a moisture barrier to cause problems. I hope we get to see pictures of the roof when it is growing


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## Ross (May 9, 2002)

Is the cavity between the roof and insulation ventilated? All roofs are moisture barriers, you want the faced insulation(or better super 6 mil plastic) between your living space and insulation, and the space above the insulation ventilated. If you can't ventilate you'll have to go with closed cell foam as insulation so it doesn't get soaked with moisture from inside. There might be a solution with good humidity control through an HRV and a steam humidifier if you get especially dry. Tough call.


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## PlicketyCat (Jul 14, 2010)

If you have vents, get faced and/or a proper vapor barrier on the warm side. If you don't have vents and don't want/can't install them, stick with unfaced so any moisture can "dry-in". A normal roof with tarpaper or Tyvek will "dry-out" since they are not perfect vapor barriers, just weather barriers; but EPDM and Ice Shield membranes are perfect barriers so you need to make sure that 1) no moisture ever gets in your insulation -- which is impossible; or 2) any moisture that does get in can get out.


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## wy_white_wolf (Oct 14, 2004)

Ross has it nailed. If you don't vent the area above the insulation you will end up with problems from both face or unfaced. Only a closed cell foam will work.

WWW


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## unioncreek (Jun 18, 2002)

You don't want a vapor barrier on the ceiling. You will also want to vent the roof area some how. The roof is where all the moisture exits the building at and if you put a vapor barrier inside the room on the ceiling you will be sealing the roof area up completely.

Bobg


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## fishhead (Jul 19, 2006)

I would look for a way to put ventilation under the barrier. They make plastic trays with ridges for some applications.


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## Sparticle (Nov 1, 2004)

Thanks for all the replies. I see lots of conflicting advice. Some say faced and some say unfaced. The roof is not vented at all. The entire north wall is bermed, no place for a vent there. I could put vents on the south wall overhang, wasn't planning to though. Can't afford the spray in stuff. We'll have an exhaust fan in the bathroom and kitchen. I'm thinking that Plicketycat makes the most sense.


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## Ross (May 9, 2002)

When unprotected insulation is exposed to warm moist air the moisture penetrates the insulation where it hits the dew point of cold outside temps and condenses, soaking the insulation. and even freezing in the insulation. That moves the dew point further in where the cycle repeats until you hit the point where the condensate just starts to fall onto your ceiling causing water damage and mold. The wild card is your roof, I don't know what thats going to do to the dew point. Move it closer to the insulation or further away.


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## fishhead (Jul 19, 2006)

You could probably fasten those plastic ventilation trays on the underside of the plywood holding the membrane. They're lightweight and only about 3/4" thick. They don't look very expensive either.

That would allow the moisture to migrate out instead of condensing on the underside of the plywood.

Here's what some look like. I've also seen simpler ones.

http://www.fascias.com/contents/en-uk/d745_Corovent_Roof_Products.html


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## PlicketyCat (Jul 14, 2010)

Another option is insulation batts that are fully enclosed in a plastic blanket. That at least keeps the insulation from becoming saturated if any vapor does happen to condense. If you have no other vapor barrier, and you don't get a ton of heavy condensation on the interior of your roof panel, then it will dry-in to the building if the season is long enough.

You could also opt for foam panels with/without a radiant barrier.

Faced or unfaced batts don't make a huge difference without a real 6-mil+ vapor barrier installed anyway since the wooden rafters are still "exposed"... they still wick vapor and still provide a thermal bridge that vapor can condense on. Unless you can keep all parts of an assembly behind an impervious vapor barrier, don't use one.


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## mossywyatt (Mar 24, 2009)

The only "attic" insulation that I have seen work unvented is foam. You could spray foam or you could take to roof off and put two layers of two inch ridged foam with taped and off set seams. I know you said that you can not afford spray foam but If you put in fiber glass go with a vapor barrier not retarder and plan on redoing it right within 5 years or you could have a mold/rot problem. You could also try and keep the moisture level in the house below the level of condensation of your coldest time in winter
wyatt


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## Sparticle (Nov 1, 2004)

Very confusing.

Thinking about this: vents in the south side soffit and a space between the insulation and the plywood. Will that work? Will vapor from the north wall travel up and over the ridge and exit the south wall vents?

Also, if I put plastic above the car siding and below the insulation, won't moisture form on the plastic and soak the car siding?


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## PlicketyCat (Jul 14, 2010)

Do you even have an unheated attic, or is it heated and/or cathedral ceiling?

If you don't have an unheated attic space, you don't really need to worry about venting because vapor will dry-in to the heated space. 

Considering that you're partially subterranean, I would be exceptionally careful venting your roof as this opens up your rafter cavities to bulk water and critters. Venting near grade level may end up doing you more harm than good.

If your attic space is unheated, then you could install a mushroom turbine at the peak with soffit vents on the open south side that would draw through the rafter cavities and out the ridge... but you would need to ensure there is adequate vent chambers in each of the rafter cavities on the north side so that the south-&-up suction would pull it out from there as well.

Might just be simpler to heat the attic or leave it a cathedral ceiling and use unfaced insulation with no vapor barrier... and make sure you get paperless drywall just in case you do end up with some condensation so it won't feed the mold.

Now - if you do have an unheated attic but you can't vent, you CAN use unfaced insulation and _install your vapor barrier on the floor/ceiling of the main space instead_, thereby drastically reducing the vapor infiltration into the attic, period.


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## Ross (May 9, 2002)

Moisture condensing on the cold side of the insulation is coming from inside the house, cooking, showers, even persperation. You could with an HRV drive down the relative humidity to near arid conditions and not get condensation problems. I would go with foam, either SM boards cut and sealed with gun foam or spray foam. Or I would get a profesional ventilation desgin and (betting) that uses a stack and power venter fan. YMMV


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## texican (Oct 4, 2003)

PlicketyCat said:


> Do you even have an unheated attic, or is it heated and/or cathedral ceiling?
> 
> If you don't have an unheated attic space, you don't really need to worry about venting because vapor will dry-in to the heated space.
> 
> ...


Plickety, I believe they're building basically an unconventional home, with a living roof... so seriously doubt there's any attic, heated or otherwise. Sparticle has a link on her posts that'll carry you to their homepage, that has pics of their home.


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## shebeen (Oct 21, 2011)

New here so I hope I'm not breaking any rules by posting a link, but these are the folks you want to ask. Extremely good forums and knowlegable members about everything 'Green' buildingwise.

http://www.greenbuildingadvisor.com/new-videos-greenbuildingadvisorcom


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## shebeen (Oct 21, 2011)

Sorry if I'm double posting here....new and just getting the hang of this forum.

Here's a site about everything 'Green Building'. Very knowlegable and LARGE group of members willing to help.

http://www.greenbuildingadvisor.com/new-videos-greenbuildingadvisorcom


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## Sparticle (Nov 1, 2004)

Yep, no attic is right. Still wondering if I put vents in the south wall will the air/vapor travel up and over the ridge(Very shallow roof pitch) and down the southern side of the roof and out. That would not be too difficult to do.


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## PlicketyCat (Jul 14, 2010)

I can't tell from the drawings on your PhotoBucket whether your rafters run N-S or E-W, or if you have a gable or shed roof... but if the rafters run N-S, then you can put a soffit vent in the bays on the south face and install a mushroom turbine on the ridge or back, or a ridge vent on the south side of the E-W ridge (if it's gable). But if it's gable and your ridge runs N-S with your rafters E-W, then you'd have to drill through your rafters (and possibly run a perforated vent tube) from the rake vent on the south face to a turbine on the north ridge so that you will draw air in, through all the rafter bays, then up and out.

Does that make sense?

You may want to consider, if you haven't already, using a simple shed roof with the N-S rafters, with the low side on the north in the hill and the high side on the open south. That way, you can install soffit vents on the south side, and install a turbine on an extended tube on the north side to draw out any moisture in your roof.

But really, if you have an open/cathedral ceiling, you could do without a vapor barrier between your finishceiling material and your unfaced insulation, and do without any venting because any vapor that enters the rafter cavities during the winter heating season will dry-in to the house in the summer unless you live somewhere really humid or run the A/C all summer.

In 99% of the cases, a _heated roof_ , as opposed to an unheated attic, *does not* need a vapor barrier or venting.


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## Ross (May 9, 2002)

> In 99% of the cases, a heated roof , as opposed to an unheated attic, does not need a vapor barrier or venting.


 Hmm I hate these statements, cause I know 100% of hot roofs in Ontario need a vapour barrier. Your complication is the green roof being a vapour barrier and having insulation on the warm side of it. Hey maybe Missouri is a dry place, the building code is different as you travel around though. Have you checked the local building code? It can be quite helpful.


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