Vapor/moisture barrier on underside of OSB roofing?

[Diggin It]

For reasons too involved to get into typing them out, I'm planning on building a shed roof porch covering type thing on the side of the house. Simple reason is to channel rain away from the house, but there's more to it. It'll basically be a big awning along one side of the house.

Current plan is posts 8-10 feet from house wall, 2x6 rafters (about 12' run but span should only be 10' or less, rise somewhere between 2/12 and 3/12), OSB and felt under metal roofing. I could go straight metal, but this will be directly outside a bedroom window so I don't really want the noise from rain on metal only. The ridge will attach to the house and will need serious water/weatherproofing.

Ground is turf/gravel so there will be at least some moisture evaporation and there is a window A/C unit it will cover.

Question relates to the underside of the OSB on top of the rafters. We get hot and humid and I don't want that able to affect the OSB. I also have issues with Carpenter bees and a few different types of wasps that like to build nests in higher darker places. But I also don't want to trap moisture in the OSB between upper and lower layers of moisture barrier.

Plan below shows layers; rafters, barrier, sheathing, barrier, felt, roofing. I know I don't need both barrier and felt on top, but again, I want to kill as much noise as possible.

Or should I be using something other than OSB? I've never used ZIP board, so I'm not familiar with cost, workability and weight.

 

[Diggin It]

Doing a little reading on ZIP panels. Cost per panel is 2-3 times that of OSB, but no separate barrier is required, so that cost is saved. What I can't find is the weight per panel.

Annoyed that the company issued rebate of $5/panel is not available to individuals.

 

[EddieWalker]

Where I live, OSB is $11 right now and the same sized ZIP System is $23. You have to add the tape to the ZIP System, but you don't have house wrap. The math isn't perfect, but I consider ZIP to be twice the cost of OSB. I use it a lot because it's a better quality OSB. It's tighter, there are no voids, it doesn't peal or separate like regular OSB does. Then there is the added protection of the polymer green coating that replaces House Wrap. Besides being a better quality OSB, it allows you to leave it exposed to the elements for years instead of rushing to cover it and deal with house wrap coming off, or sealing it to the existing building. For my house, I use it because I like the quality of it, but I'm doing my house siding one side at a time, so that makes it very easy to seal everything up and work on in when I have the time and the cash.

For your application, I do not think that I would use it. Going with regular OSB, felt paper and metal will work great at dealing with moisture from above.

For the underside, I would buy a good quality paint or if its really a concern, find a polymer coating, and paint it on.

You do not want to trap air or have any gaps between house wrap or plastic, under your wood. Moisture will get in there from condensation and build up over time. That moisture will create mold, and mold creates rot.

 

[s219]

The whole key to managing moisture on the underside of a roof is ventilation. Modern roof design assumes airflow under the roof decking (except for a few variations like spray foamed roof).

Humidity by itself won't hurt OSB. I have built several OSB roofs on barns that have no issues. Also a couple carports. And OSB is stored outdoors all the time at lumberyards.

Liquid moisture is the problem, so you want to avoid condensation under the roof. That is where ventilation comes in. If it's a closed structure that means some combination of eave, ridge, or gable vents. If it's an open structure then I would make sure there is at least a path for air to enter the eaves and the rest will work out through natural flow.

If you can put down a vapor barrier on the ground underneath this structure, it will really help a lot. Could be plastic that is under gravel, for example.

I would not sandwich the OSB with any non permeable barrier. I would put roofing felt on the top but not put any barrier on the bottom. That is asking for trouble. There is already moisture inside the OSB and it will change with temperature/pressure/weather. The worst thing you could do is trap that in the OSB and prevent it from breathing. It would rot out over time.

Also pay attention to the labels on the OSB. There is a specific side that goes up when used for roof sheathing, or out when used for wall sheathing.

If you need to protect the underside of the roof from insects, consider vented vinyl soffit material, vented bead board, etc, nailed to the underside of the rafters. But make sure there is a good air gap to the underside of the OSB within the rafter cavities.

 

[Diggin It]

Yeah, I wasn't really wild about the sandwich idea.


It'll be fully open on two sides, partially enclosed on one and fully enclosed against the house. I suppose I could put small vents at a couple of points near the ridge along the house, but don't want to create bug places.

I'm planning on screening the whole thing eventually, maybe along the underside of the rafters. I could do traditional screen walls, but then I get into screen doors and whatever else.

Maybe paint the underside of the OSB before installation?

 

[herm0016]

paint would be great. sandwiching it between plastic is a terrible idea. we don't call it vertical mulch for nothing. use some decent osb and paint it. use the peel and stick ice and water if you want to really reduce noise on the metal and you will have a roof that will last longer than the rest of the house.

 

[War Eagle 2]

Don't exactly know what part of the country you are in but in hot climates it can get extremely hot under metal roofing. So hot that some felt
will breakdown due to the heat. There is an underlayment that is made specifically for this (one product name is "Fire and Ice") that can handle heat temps above 130 degrees F.

I built a low pitch patio cover that is nearly the same size of your proposed project (36l x 12w)using 2x6 rafters, 1/2 in OSB covered with Fire and Ice underlayment and metal roofing.

This will meet what ever old man weather throws at it and when it rains heavy or hails out we really can't hear any noise that is any different than what we hear on the house roof.