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Vapor barrier

I have recently purchased a three family home in Massachusetts. The house was built in the 1860's and has no insulation in the walls that I'm aware of. When I purchased the home the exterior had recently been vinyl sided and insulated with 3/4 inch insulation according to the seller.The house has a walk in attic that is not insulated at all, but the second floor apartment has had some insulation put into the ceilings. You can see the insulation in the ceiling from the attic.
I am planning on doing some renovations to the interior with stripping the old horse hair plaster out and replacing it with R19 fiberglass and new sheet rock. I am totally confused as to use a vapor barrier or not. After reading some of your articles I'm inclined not to use a vapor barrier because the outside has a vapor barrier in the form of the insulation added. If I'm correct on understanding your article. If I use a vapor barrier on the inside walls I will be trapping any existing moisture in the walls with no where to go. Am I correct in this assumption?
On another note that is happening within the house. My moisture levels in the second floor apartment are running right around 50% or higher in the middle of December with radiator heat. I currently have an issue with high moisture readings in one of the first floor apartments of 73%. In this apartment the previous owner had sheet rocked over the existing plaster. I cannot figure out why the moisture is so high in the first floor apartment. I have purchased a de-humidifier for the tenants to use, but this is just a band aide for something else going on that I do not know.
This is why I'm so concerned about whether to use a vapor barrier or not. I know this is a lot of information, but I had to give you the total picture of what's going on here. If you could direct me to someone locally in Mass that I could talk to it would be great help. I'm planing on renovating in the spring. Please help or advise.

Asked by Ronald Benoit Jr
Posted Mon, 12/16/2013 - 02:05
Edited Mon, 12/16/2013 - 02:11


2 Answers

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First of all, the building code does not require an interior vapor barrier, and an interior vapor barrier is not recommended. If you want to install a vapor retarder -- a less stringent layer than a vapor barrier -- you can either use the kraft facing on the fiberglass batts, or you can install vapor-retarder paint. (Some building inspectors insist on a vapor retarder, even though it isn't necessary.)

For more information on this issue, see:

Vapor Retarders and Vapor Barriers

Forget Vapor Diffusion — Stop the Air Leaks!

Do I Need a Vapor Retarder?

The only way to determine why these apartments have high indoor relative humidity (RH) is to do a little detective work. The first culprit to suspect, especially in an older building, is a damp basement or crawl space. If you suspect that you have a wet basement, you might want to read this article: Fixing a Wet Basement.

It's also possible that these apartments need better control of moisture that is generated in the bathrooms and kitchen. You might want to verify that the units have operating bath exhaust fans and range hoods with exhaust fans that are ducted to the exterior -- and that the tenants are using the fans properly.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/16/2013 - 09:50
Edited Mon, 12/16/2013 - 09:51.

Helpful? 0

Stud framing spacing & depths in 19th century homes are nowhere near the finished-lumber framing standards of mid to late 20th century homes, which makes ANY batt solution difficult, and third-rate at best (let alone R19 fiberglass, arguably the worst insulation legally sold in the US.) Blown/sprayed insulation (fiber or foam) is almost always going to work better, for both fit and air-retardency reasons. Open cell foam or damp-sprayed cellulose will likely be your best bang/buck.

If batts are your only option, carefully sculpted & fitted rock wool is a far superior solution followed by high density "cathedal ceiling" fiberglass. An R19 batt has the same weight per square foot as R13s designed for 2x4 construction- it's a "fluffed" R13, and barely more than an air-filter for your infiltrating air. Higher density batts are far more air retardent, and if fitted perfectly with no gaps or compressions, will perform similarly to damp sprayed cellulose.

Moisture problems can be from any number of sources, but before insulating any antique like this you have to assess whether moisture is getting in around the window trim, since it probably has no flashing whatsoever. With empty framing the drying rates are high and minor bulk water leaks well tolerated, but if you insulate the exterior wood runs colder, and air flow through the cavity is impeded, slowing the drying rates by an order of magnitude or more.

Answered by Dana Dorsett
Posted Mon, 12/16/2013 - 13:17

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