Sealing rafter deck underside


#1

would paint sealing the rafter deck underside (7/16 osb) be a good thing to do? Would it help protect the wood from occasional condensation? Batts or rigid would be placed against the deck.


#2

The answer is no and no…
If this is an attic…Your insulation should be between the floor joists above the heated room space below not between the roof rafters…


#3

no attic here. its a flat roof - and no venting either. Probably should have said that upfront.
This a tough problem - everyones got stumped by it. I’m getting experimental. Want to block the vapor from hitting a cold deck underside. First I’d paint the deck underside and cracks with a thick paint-like glue sealer. Then fit in strips of XPS board. Seal all cracks all around. You could call this a “poor mans” foam compared to sprayfoam. Since its sealed and in direct contact with the decking, moisture would have no place to hide or get in there. Sprayfoam is waterproof anyway so what else would the sealer paint do to the deck? Can it work or is it just a hack? thanks


#4

icynene.com/InsulationSystem.aspx


#5

Is this an interior heated living space or is this an occasionally heated outbuilding?


#6

its an interior heated space below this flat roof.

I just pulled back the batts in some areas and
there is already extensive mold growth on the 4 month old new OSB decking. Even areas that had an intact plastic vapor barrier on the warm side are showing mold growth. This roof is a disaster. Can’t believe I paid $40K for a POS roof like this. If something isnt done soon, this deck will be toast in 2-3 years. I know this is pretty hopeless, most everybody is stumped, and nobody wants to hand me a “wet blanket.”


#7

[quote]no attic here. its a flat roof - and no venting either. Probably should have said that upfront.
This a tough problem - everyones got stumped by it. I’m getting experimental. Want to block the vapor from hitting a cold deck underside. First I’d paint the deck underside and cracks with a thick paint-like glue sealer. Then fit in strips of XPS board. Seal all cracks all around. You could call this a “poor mans” foam compared to sprayfoam. Since its sealed and in direct contact with the decking, moisture would have no place to hide or get in there. Sprayfoam is waterproof anyway so what else would the sealer paint do to the deck? Can it work or is it just a hack? thanks[/quote]

If you don’t want to go with a product specifically for this problem, your above described solution may actually work
To paint the underside of the deck I would recommend Kilz, or similar product.
You may want check with a heating and cooling guy and make sure your mechanicals are not causing/contributing to this problem.


#8

there’s plenty of humidity here.
I could take measures to reduce it like better bath fans and more plastic on the crawlspace dirt floor but I’m not expecting miracles. Alot of people live here - lotsa showers and lotsa cooking. They probably like the high humidity and would be reluctant to use the bath fan anyway even if I insisted on it. I need a roof that can withstand humidity.


#9

I did a home a while back that had an indoor pool.
In order to deal with the high humidity, all of the walls and ceiling were covered with Ice & water shield.
This creates a very effective vapor barrier.
This does nothing to solve the problem of interior humidity levels.
There was also a cold roof over this portion of the home.
Special air exchangers were installed to deal with the interior humidity.

The commercial roofers here would be your best bet for a solution to your problem.
Icynene will solve the problem of the roof deck being destroyed by humidity, but this doesn’t address the root cause.
Instead of the roof deck rotting, it will be your ceiling.

Your home is a mechanical system, all aspects of it should be designed to work together.
Your home is obviously not working as such.
Have you consulted with an Architect or Engineer?
It doesn’t sound like a roof problem to me.


#10

You have answered your own dilemma…Your living space is loaded with excessive moisture from cooking, showers etc. and you must take steps beyond most inorder to eliminate it…
1.You must get a better stove/range exhaust fan(not one of those typical home units that they sell in Home Depot but one preferably with a deep hood & wide enough so as to fully cover over the range area and one which is powerful having at least a 9" round flue exhausting out through the roof [you probably will need to pull this exhaust with an additional inline fan or perhaps the better alternative (which will keep the noise down) a powerful and efficient sound insulated rooftop unit]…
2. You must get a bigger &/or additional exhaust fan(s) for the bathroom and set it up with a trip switch that automatically turns it on and off…this way your roomies behavior doesn’t matter(fan will automatically be on when they close the door and will shut off when the door is open)…
3. You should also consider doing these things A.S.A.P. as well as possibly installing additional thru-wall type exhaust fan units if needed otherwise there will always be mold and rain in the house

P.S. What is the R value of the ceiling batts?


#11

A roof consultant has seen this roof. He said that sprayfoam under the deck or a PMR roof above the membrane were the only proper options to correct a defective design.
Can moisture source control with bigger fans make up for defective design? Yes, and having nobody living there would help even more. All those fans will make the heat bills higher too defeating the purpose of the insulation that I’m trying to keep dry. If I tossed all the r-13 batts and kept the rafter bays empty, the mold problem is solved. As proof of that, I just looked at a bathroom right below some empty rafters, not a speck of mold there. 10 feet away, in an area with well fitted kraft faced batts, there’s mold. The walls, windows and doors are loose enough that moisture would easily go through them so they are not as vulnerable as the ceiling/roof - moisture goes up more easily than sideways. I have a defective roof deck design. The moisture source control methods is good common sense but its only going to make the inevitable take longer.


#12

…GET ON BOARD & RIDE THE MONEY TRAIN…
THE CONSULTANT APPARENTLY HAS TICKETS TO SELL