Roxul over foam board help!
user-7619439
| Posted in Energy, Heating & Insulation on
Hello all from Pittsburgh, PA.
I remodeled my Great Room with vaulted ceilings and wanted maximum R-value. The exterior side of my walls are vinyl siding, Tyvek then 1/2″ OSB sheathing. The 2×4 stud cavity is 1″ Extruded Polystyrene foam board against the sheathing then r-15 Roxul. During the cold winter months, I was getting bad condensation between the foam board and Roxul where it was exposed to the indoor air(no drywall). I removed the Rock wool and didn’t put it in the stud cavities until right before I hung my 1/2″ sheetrock. My question is, is this going to be ok for long term as far as moisture in the wall cavity? Is the sheetrock going to prevent condensation from building up between the foam board and the Rock wool?
Replies
sounds like the sealing and thickness of your foam board is not enough to keep the interior surface above the dew point.
this is not likely to change with a layer of drywall.
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The way I read your discription is the foam board has been cut to fit between the studs, on the inside surface of the wall, then the nominal 3.5" thick Roxul.
If that is the case, then the foam board is a vapor barrier preventing any moisture from drying to the outside, and the foam surface is cold enough to trigger the condensation. Also, the Roxul will have to be comporessed to fit within the studs, reducing its effective R value. Adding a vapor barrier ( poly film or sealed drywall ) on the inside would just trap any moisture in the wall cavity, which will be a major mold and rot problem.
I would pull the drywall and insulation. Install the Roxul normally in the stud cavity, it should be snug against the sheating and studs, and be even with the inside stud faces. The foam board can be installed as a continuous layer across the inside face of the studs, providing the desired extra insulation and act as the vapor barrier. Then drywall.
You will need to tape all seams, add vapor barriers and seal around any electrical outlets in the wall, and tie in the vapor barrier to the window flashing.
From what I previously read, the stud cavity had to be able to dry to either the interior or the exterior and that this setup would dry to the interior side.
The problem is that the inside surface of your foam board is below the dew point, so that any water in the air comes out at the cold interface layer..
your approach would work IF you had a layer of XPS thick enough and sealed well enough to keep the temperature at the surface above the dew point.
adding drywall and paint will reduce the amount of air that enters the insulation, but will also retard the same moisture from escaping back into the interior.
At this point, is all the drywall up? is is finished and painted?
You can wait and see how it goes, but the danger is when you notice a problem, you will have to replace studs and sheathing in addition to replacing the drywall and insulation. You might also end up with a nasty mold problem to deal with.
How did you secure the foam in the cavity? Is it marked R5?
Thanks for your input. Most of room has drywall and 2nd coat of mud. Some of it just has the foam board glued and sealed with canned foam. It would be a very stressful and costly set back for me to tear it all out but I would do it if I had to. I am thinking about purchasing a moisture meter and checking the wall cavities.
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One thing to do is look for air leakage around your foam board. turn on all the exhaust fans in the house and run the dryer in air fluff mode. then feel closely around the edges of your foam. on a nice cold day, you might find some air leaking you can seal up.
Is the sloped ceiling treated the same way?
and where, exactly was the moisture problem you saw when the roxul was in place?
My cathedral ceiling is just R-49 fiberglass batts against the 1/2" sheetrock.
I found condensation between the Roxul and Foam board before I had the drywall hung. I removed it all and and didn't put the batt insulation back in until right before I hung the drywall.
I mean where on the wall was the wet roxul. was it uniformly wet, or just in one or two places?
Dry to the inside works when there is enough insulation on the outside of the the air /vapor barrier to prevent condensation in the wall cavity.
Your wall assembly has the air barrier at the foam board / sheathing. You can make that work by pulling the siding and adding insulation to the outside of the wall.
If adding insulation to the outside is not practical, then the air/vapor barrier needs to be moved to the inside surface of the wall.
What are you heating the room with now and is it closed off from the rest of the house during your remodel.
And, did you get frost, ice or moisture along any glass in the room?
Heating with a combination of electric heat pump and a wood stove. It was never closed off to the rest of the hosue. No moisture on any glass.
This is pretty straight forward. You need thicker foam in your climate. One inch doesn't have enough R-value to prevent the inside surface from reaching the dew point. See this article for a deeper understanding. https://www.finehomebuilding.com/2018/01/11/choosing-right-thickness-exterior-foam
Electric Heat pump and a wood stove heat the room. It's not closed off to the rest of the house. No moisture on any windows