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Old 01-17-2009, 10:43 PM   #1
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Moisture on Inside of Sheathing


I just finished framing and siding an addition on the back of my house, but now with the insulation installed, the inside of the OSB sheathing is wet to the touch and has a layer of frost on it. It is framed with 2x6 studs with R-19 fiberglass.

Soon after adding the insulation I ran a flex duct to this area for heat since it shares an uninsulated wall with the main part of the house. Not thinking, I didn't open the return and found the moist sheathing a week later. I opened the return and ran a dehumidifier and I thought I had the sheathing completely dry, but tonight, which is over a month later, I noticed the sheathing was wet again and had the frost layer.

Any ideas why I have so much moisture in the walls? The sheathing is obviously cold, but I have faced R-19 in all the cavities. There isn't any drywall up yet as I haven't finished electric and plumbing. Would drywall act as a vapor barrier that I'm missing?

Any help would be great, thanks.

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Old 01-18-2009, 07:10 AM   #2
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Moisture on Inside of Sheathing


Why would the insulation be up before the plumbing and electrical work? You will need a vapor barrier 2 mil plastic. then the drywall. Drywall will over no air or vapor or moisture barrier

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Old 01-18-2009, 07:50 AM   #3
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Moisture on Inside of Sheathing


are you using a propane heater for space heating while your working?
They can put out alot of moisture
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Old 01-18-2009, 08:01 AM   #4
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Moisture on Inside of Sheathing


I'm told that maintaining too high a humidity level in the house during winter will overwhelm the normally built insulated wall and cause moisture accumulation on the inside of the sheathing. Unless you went over and above the call of duty and installed a nearly hermetically sealed vapor barrier such as a continuous poly' sheet as opposed to faced batts that have vapor barrier joints every 16 inches.

Any moisture in the wall must diffuse through the siding to the outside faster than more gets through from the interior.

Last edited by AllanJ; 01-18-2009 at 08:03 AM.
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Old 01-18-2009, 08:44 AM   #5
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Moisture on Inside of Sheathing


You really need to understand two parts to why there is condensation. Everyone mentioned that moisture is getting to your OSB. What you also need to take into consideration is the fact that the inside of your OSB is below the dew point. You don't have enough insulation in your wall to allow the temp of the framing on the outside to be below your dewpoint. Even if you are prohibiting the migration of moisture from the inside out, you might still have frost due to moisture from the outside. Northern areas of the country add 1 to 2 inches of polystyrene now to deal with this. It keeps the temp on the outside of framing from reaching the dew point thus preventing frost from building up. Most older houses have frost inside the sheathing all the time, people just don't know it. Generally, this moisture just evaporates when it warms up outside but sometimes it can be a problem. If your OSB is warm enough you are going to get condensation. It is a combination of moisture and temp.

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Old 01-18-2009, 09:14 AM   #6
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Moisture on Inside of Sheathing


is the addition built on a new slab? concrete takes awile to dry
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Old 01-18-2009, 12:53 PM   #7
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Moisture on Inside of Sheathing


Thanks for all the responses.

I found out over a year ago that a small addition on the back of the house, 8x26', was rotten on one end and improperly built on the other. I hand dug a crawl space and rebuilt everything except the shed roof. Since it is attached to the back wall of the house, I needed to keep it warm in the winter or risk freezing myself out. Therefore, I ran the single flex off my air handler to provide heat.

I haven't finished the plumbing and electrical because I was more concerned with keeping it warm and finishing the space as time permitted. I guess my problem is from putting the cart before the horse and adding heat before the walls were finished.

The problem is, at what point in a construction do you add heat and not have the condensation buildup on the inside of the sheathing? Does condensation always build up on the sheathing as stated above? Does the drywall act as enough of a vapor barrier to control the warm air from reaching the sheathing? Or is the only choice to insulate the exterior of wall as also suggested above.

I already finished replacing the siding so I'd rather not add the foam board to the exterior. If I have to drywall immediately then I'll do that. Since I now have 1/4" of ice on the inside of the sheathing, what's the best way to dry it out? It's freezing or just above freezing outside (Northern MD) and approximately 65 degrees inside the addition so if I remove the insulation it would just condensate more, correct? Should I wait until spring to attempt to dry it completely?

Thanks again.
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Old 01-18-2009, 02:38 PM   #8
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Moisture on Inside of Sheathing


your insulation has a vapor barrier facing the inside right? primed and painted drywall will help to keep warm moist air off the sheating
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Old 01-18-2009, 03:55 PM   #9
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Are you sure that you don't have any air leaks? Through the plates, cracks in the sheathing, penetrations of any kind?
You could try putting up poly over your walls and see if this ends your condensation. A roll of poly to do your room should be fairly cheap and it will only take a few minutes to put it up. What kind of temperatures outside are you experiencing that creates the icing up? What is the humidity level in your house? Is it reasonable for your region? Dry wall does not act as a vapor barrier, although most paints do, however in practice most carpenters and inspectors want polyethylene. Everyone who has posted so far seems to think that excessive interior moisture is causing your buildup. If that is true, the poly will take care of your problem. The only other reason is not enough insulation which I suggested. If the temp of the exterior has not dropped enough because of a lack of insulation, the warm air on the interior will cause ice to form on the sheathing. No matter what you do, this will continue to happen, vapor barrier or not. You might be able to slow the process down with a vapor barrier but you will not fix the problem of the wall temp being within the dew point. Warm air hitting a cold surface leads to frost.

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Old 01-18-2009, 07:34 PM   #10
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Moisture on Inside of Sheathing


The thermostat inside the old part of the house within six or so feet of the addition shows 30% humidity and the outside temperature was approximately 20 degrees or less for the past few days. The temp went up into the high 30's today, the ice melted and now the sheathing is just damp again.

I placed a dehumidifier in the room set at its lowest setting 35%, but it's not running since it's so dry already. I'll give the poly a try, but I just can't understand what is so different in my application to cause so much condensation when houses are built the same way everyday (well not so many anymore) and they don't have this issue. I insulated with R-19 in all of the cavities and I don't have any penetrations, cracks, or holes that I can see.

To clarify, it's common practice to place a sheet of poly on the wall side of the sheetrock? The old part of the house has tar paper on the inside of the board siding, but I wasn't aware that a plastic sheet was commonly used in building exterior walls. If so, I assume I would need to tear the paper off of the insulation to prevent having two vapor barriers?

Anyway, thanks for your time and the advice.
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Old 01-18-2009, 08:47 PM   #11
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dont use the poly you dont need it if you have a vapor barrier attatched insulation
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Old 01-18-2009, 08:54 PM   #12
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It is common practice and required in many states under inspection to have a vapor barrier on the inside under the sheetrock, usually polyethylene. Some require the ceilings to have a vapor barrier as well. Most paints that are applied on drywall have a very low perm rating as well, but building inspectors don't accept that. Yes, most older houses are built this same way. The reason that we've escaped so far is that most houses aren't OSB, are generally very leaky, and after walls are painted the moisture available to condense is much less available.
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Old 01-18-2009, 09:32 PM   #13
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you might be confusing him, he said his insulation aready had a vapor barrier
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Old 01-19-2009, 12:32 PM   #14
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Hi. I'm new to this site. If I was you, I'd slash the paper facing on the insulation, and then put a continuous poly vapor barrier over it.
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Old 01-19-2009, 01:03 PM   #15
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hmmmm....

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