Home>Discussions>INSULATION & HVAC>Vapor barrier question in Rhode Island
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Peter K.
Vapor barrier question in Rhode Island

Quick question: I live in Rhode Island, and I had a contractor install 6 mil clear poly over unfaced insulation prior to hanging blueboard 10 years ago during a renovation of my 1906 house. The renovation was second floor only, and involved only 2.5 of the 4 exterior walls (2 bedrooms and a bath). I am now worried that this type of vapor barrier is too impermeable and will lead to mold in my unfaced insulation. This could potentialy be a costly mistake. Should I rip out the 6 mil? Would it be more costly to tear down the drywall/plaster, or take off the clapboard and sheathing? Or should I just live with this potential mistake? I'm sure I'm not the only homeowner in New England with this problem. I purposely do not use air conditioning (ceiling fans only) as I'm concerned I will contribute to mold.

Re: Vapor barrier question in Rhode Island

Have you seen signs of mold?

Re: Vapor barrier question in Rhode Island

You do not have to worry, the vapor barrier is actually keeping your unfaced insulation drier so that mold has less chance of growing. You can safely use your AC as well. There is some misleading information going around the web about vapor barriers and it should be completely disregarded except for a very few areas of the country that experience very hot temperature and very high humidities. These are a located along the gulf coast and affect only a very tiny area.

It is unfortunate that one person can start a panic thanks to the internet. Tens of thousands of engineers support the use of a vapor barrier in all but those very few areas, but one person disagrees and a panic erupts.

Most vapor comes from inside the house. Every time the water is turned on at a faucet, whether hot or cold, vapor is released. Cooking releases a lot of vapor, flushing the toilet releases vapor, houseplants release vapor, but one of the biggest sources of vapor comes from the people living in the house in the form of perspiration. Most of this vapor leaves the house via infiltration around doors and windows, some leaves through the vent fans that may be installed over the stove and the bathrooms. Some gets out through open windows on nice days.

The vapor barrier keeps the vapor from escaping through the walls and the insulation where it would do damage. Typically, the outside air has less moisture in it, even if the relative humidity (RH) is higher. How can that be you ask? RH is a measurement of the amount of moisture in the air, called absolute humidity (AH) compared to the maximum amount of moisture it is capable of holding. AH is measured in grams of water per cubic meter of air (g/m3) where RH is a percentage of measured AH to maximum AH. If the outdoor temperature is lower than the interior temperature, the the outside air can have a higher RH but lower AH.

On cooler days, the vapor barrier is keeping the moist warm air from getting into your walls. The air inside the wall cavities warms up due to conduction of heat from inside the house, it becomes lighter and rises up and escapes through the siding. This draws in cooler outside air. Once inside the wall, the cooler outside air warms up. Its AH does not change, but because the temperature rises, its RH goes down. It will now attract any residual moisture out of the insulation and rise up and escape, drawing in more cool air and the cycle repeats. This cycle continues and keeps the insulation dry.

Now we come to warm days where the outside temperature is higher than the inside temperature. Now I am talking about running the AC here. The warm. moist outside air gets drawn into the wall and is cooled down, increasing its RH. But in order for condensation to occur, the RH has to go above 100%. The temperature where RH reaches 100% is called the dew point. Only a very few places in this country are capable of reaching a dew point that higher than room temperature. Yours is not one of them.

If you don't run the AC, chances are that your interior temperature is going to be higher than the outside temperature except maybe in the hottest part of the day. If your house is well insulated, then the interior temperature will me moderated from the outside temperature, that is the interior will stay warmer at night and cooler during the day. But it will certainly be higher than if you do run the AC.

If you keep your windows open so that you catch a breeze, your interior RH should not rise significantly. If you keep the house closed up and don't run the AC, the RH inside the house will rise significantly. If you didn't have the vapor barrier, this moisture would add to the outside moisture in the wall cavities and could possibly cause some condensation to occur, although I doubt that would happen in New England, it could happen in the deep south. Since you have the vapor barrier, it pretty much removes this possibility completely.

If you run the AC wit the house closed up, the AC will remove much of the interior moisture so it would never get into the walls in the first place, vapor barrier or no. For that reason, you may want to consider running the AC on the uncomfortably hot days. Open the house and save money on the nice days.

And stop worrying about the vapor barrier. Look forward to adding the vapor barrier and insulation to the other 1.5 walls of the house.

To look at a chart for AH vs RH and dew point, go to this link. It will help you to see what I have been rambling on about.

Re: Vapor barrier question in Rhode Island

Opps, forgot to add the link and the post went to moderation. Moderator, I would appreciate if you would add this link to my post and then delete this one, sorry to add to your workload.


Peter K.
Re: Vapor barrier question in Rhode Island

Keith and dj1,

Thank you for such a detailed response.

1. Have I seen signs of mold?: While I haven’t specifically checked, I have kept ~ 10’ of exterior wall in a storage area open (no drywall/blueboard yet), so the poly vapor barrier and insulation is “exposed”. I just haven’t gotten around to closing it up yet. There may be a trace amount of black mold (will inspect more closely) in one or two of the stud bays, but not much. Certainly no smell of mold. Should I leave this area unexposed to periodically "check"?
2. Misleading information on the web about vapor barriers: There certainly is conflicting information. Unless you live in Northern Canada or the Gulf Coast, there is some consensus that a clear poly vapor barrier should not be used. Check out this segment provided by Tom Silva: http://www.thisoldhouse.com/toh/video/0,,20464634,00.html
3. Ventilation: We have wooden clapboards (not brick or stucco, which I understand can be very problematic). Our windows are typically open throughout the summer months (particularly at night) with ceiling fans running. The roof was recently replaced with a good ventilation system installed.
I’m curious if either of you have seen the semi-permeable product Tom Silva refers to at the end of the video. I think for future construction, I’m inclined to use kraft-faced insulation to just take the worry factor out of this.

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