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Community and Q&A

Insulating an old house with no vapor barrier

Wr8 | Posted in General Questions on

Greetings –

My house was built in 1896. The construction method is different than any other old house I’ve ever owned. There’s no exterior sheathing – it’s all inside.

So you have 3/8″ plaster (drywall in some remodeled areas), 3/4″ tongue and groove wood that also acts as the key for the plaster, stud bays, wood clapboards.

There is no insulation in the walls. We want to change that. I’ve gotten all sorts of different advice:

1: blown insulation (cellulose) won’t do any good, just seal exterior and interior air gaps.

2: blown cellulose will help, but moisture coming from the interior will condense on the backside of the clapboards and cause the paint to fail.

3: blown cellulose will slow any air leaking out and it probably won’t make it far enough at a warm enough temperature to cause any problems.

I don’t know who to believe or what to do.

The only air leaks out of the plaster are a few electric boxes and maybe a nail hole here and there.

Attic is sealed and are R-55. Working on improving sealing the rim joist and insulating there. Fixing original windows and adding storms are also in the works.

We just want a comfortable house that doesn’t cost an arm and a leg to heat. Trying to make an old Dame as green as possible!

Some extra details – 2 story house, northern Indiana, walls are 2×4, house is not balloon framed.

Thanks for any insight.

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  1. Expert Member
    BILL WICHERS | | #1

    What is the exterior siding? Will you be replacing it in the near future? It’s important to know what the exterior of the wall is as well as the interior when trying to determine how to insulate without causing problems.

    It isn’t difficult to seal the electrical boxes. If you only have access from the front, id recommend using red silicone fire caulk (the high temperature type, not intumescent) to seal any screw holes or knockouts. Just use a little where you need it, try not to have big globs inside the box. You want a good seal around the perimeter of the box to to seal to the plaster wall.


  2. Peter Yost | | #2

    Working on older homes can be daunting. First thing to do is make sure you manage bulk water well, then air seal the building, starting with the attic, then your basement rim joists, then any chases running bottom-t0-top of your home.

    Check out this GBA resource on building assessments:

    And then look for a BPI-certified technician on assessing your home's performance (

  3. GBA Editor
    Martin Holladay | | #3

    First of all, can you tell us your name? (I'm Martin.)

    The headline of your question is "Insulating an old house with no vapor barrier." Perhaps the first thing you need to know is that the presence or absence of an interior vapor barrier is irrelevant. (If you want a vapor retarder on the interior side of your walls, you can always apply vapor-retarder paint. But that's not usually necessary.)

    What matters is air sealing. For more information on vapor diffusion, see these articles:

    "Do I Need a Vapor Retarder?"

    "Vapor Retarders and Vapor Barriers"

    Right now, you are considering whether it makes sense to install dense-packed cellulose between your studs. The answer depends on what type of siding you have.

    Bill is correct that the best approach is to remove the existing siding; insulate the walls; install sheathing; and install a new WRB and new siding.

  4. Wr8 | | #4

    My name is Michael - sorry I forgot that!

    The siding is wood clapboards. I'm not sure of the species of the wood other than it's not cedar. Removing it would be a nightmare. Although it is a possibility in the future to do so.

    But yes, basically I'm asking if it would make sense to dense pack cellulose now, or if I'm just going to cause problems and throw away money.

  5. 730d | | #5

    I live in the Twin cities in central Minnesota. I observed in the 70's Many vintage old 70 and 80 year old buildings, now over 100 years old that where in very good condition. These buildings where insulated, walls and ceilings. After a few winters the plaster started to deteriorate severely. Before insulating they where drafty and dried fast from wetting events like bulk water or condensation. They where likely insulated with fiberglass. One possibility is to insulate with blown in cellulose or expanding foam. Insulate just a section and observe it for a year or two. If it does not give you problems you are good to go on the rest of the house.

    Agreed the better answer is to remove your siding , insulate and sheath the walls install a WRB
    and siding.

    These old building that where ruined from these insulating efforts in the 70's collected large amount of frost in the now insulated areas. Than a warming event occurred and it poured liquid water in everywhere.

    There are people here that no better than me. If you are determined to insulate without removing the siding I would do a test wall first.

    I sincerely wish you good results in whatever you decide.

  6. Expert Member
    Dana Dorsett | | #6

    Michael- what climate zone? (It makes a difference!)

    With no exterior sheathing dense-packing the wall cavities will result in failure of the exterior paint, and possibly the interior finish wall in almost any climate. But if a second layer of siding is added built as a rainscreen with an air-gap between the clapboards & new siding, dense packing would still cause the old paint to fail but the clapboards will stay dry enough to survive and the interior finish wall would make it too, since the moisture drive from the exterior will have been relieved.

    If the interior walls have had 5 coats of paint on them over the past 122 years they are probably already very vapor retardent- a single layer of leaded paint would be pretty much a true vapor barrier. But if you're not sure and are in climate zone 6 or higher, painting it with "vapor barrier latex" primer would guarantee at least Class-II vapor retardency, which is good enough in a wall with rainscreened siding. Air sealing the interior walls would be key to making it all work.

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