Concerns about drill-and-fill cellulose insulation in walls without a vapor barrier

So my Crew Chief asks me the other day about installing dense pack cellulose insulation into walls lacking a vapor barrier.  The majority of the homes we are serving are in the Berkeley/San Francisco Bay area and are vintage (1920's, 1930's and 1940's) with wood exteriors and no vapor barrier between the studs and the siding.  We are concerned about long-term moisture issues.  Any thoughts?

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Let's think about moisture movement.  How will it get into walls?  When talking about "vapor barriers" the typical culprit was thought to be vapor drive.  We now know in reality it is airflow.  


The problem arises when warm moist air comes in contact with sub-dew point surface temperatures.  The more airflow, the greater the problem (if the surfaces remain cold).  


One point of dense pack is to slow air flow through these cavities.  Slow the supply of moisture and the building components may be able to naturally deal with it.  I think you'll see this idea of "vapor barrier" going away because they too often are causing moisture problems, not preventing them.  


In your climate where would you put the vapor barrier?  Inside wall? What about cooling season!  Outside wall?  What about heating season!  Both walls?  Now you've really done it.  Once moisture gets in it'll NEVER get out!  


Here's a good article you might find helpful from one of the fathers of modern building science: 

Vapor retarder like Tyvek would be good, but a vapor barrier like plastic would be bad (slight hint)


I have taken apart a few jobs like that where there were actually no issues- i.e. no issues with bulk water intrusion & the paint acted like the barrier / retarder preventing the dreaded moisture transfer from the outside - in


Would I do a job like that here - nope, as I can't guarntee that the issue isn't there, nor will be in the future


Might you be ok, possibly, but I would lookat it on a case by case basis - as you would have to judge it based not only on the overall climate, but the climate & conditions around the house - trees, shrubs, sprinklers, water flowing towards the house, mossy areas, water splashing back at the house, etc... next to the wall would all be bad signs

  A couple of things:  you mention no "vapor barrier" between the studs and the siding.  I'm just wondering if you actually are talking about the rain barrier, typically tar paper under the siding, which is the part of the assembly that is shedding the rain off the building.  If indeed there is no tar paper under the siding, that is cause for concern, and is something I would normally have to have a serious talk with the owner/contractor about.  Usually, I will recommend against retrofit dense pack in a house like this.  Lots of problems are possible with no tarpaper rain barrier,  although the mild climate here in the San Francisco Bay Area can cover up a multitude of sins. Vapor barriers are not required for the most part by building codes in this climate zone.  Tahoe or Minnesota:  that's a different story.  As regards possible problems with dense packing walls, I think that one possible culprit, and this is all too common, is that the room(s) are inadequately heated, therefore not drying the wall assembly to the inside. Or walls that never get any heat gain year around. What was mentioned previously about air transport being the main transport vehicle for moisture has been pretty well established.

  Overall,  I've seen no problems with dense packing walls over my 20+ year insulation career in Marin County.

I've also done some dense pack in ceiling cavities with no adverse effects (knock on wood).


Charles Bennett

Mr Insulation

San Rafael, Ca


I bought my house in 1981 and filled the walls and ceiling with cellulose, and was concerned for years about the vapor barrior issue which flip-flopped a couple of times.  Long story short, I'm glad that I refrained from using anything other than paint and polyurathane, as a moisture retardant on the interior finished surfaces.  The exterior siding is over asphault paper, the rain sheild.   I believe that any moisture migration in the cellulose gets an opportunity to dry out and, with my 30 years of adventures with my saws-all, have never seen any evidense of mold or anything else to be concerned about.  Cellulose is exceptionally forgivable.

That's my coastal Maine experience, and it's been very good to me.

i agree with the other comments, if you are talking about an interior vapor barrier, the need for it does not exist, a vapor retarder like two coats latex paint, or a 1 perm vapor retarder primer followed by paint. This allows the wall assembly to dry out without overwetting, functioning alot like a paper faced batt insulation would. The great thing about using Cellulose is that the materilial dries out unlike fiberglass because it is a natural fiber, so again adding more flexibilty to allow drying should wetting occur. One thing i would suggest is that you look for sources of wetting, making sure there is proper flashing on the exterior and water management, also moisture management in the inside, do they have bath fans, are there humidifiers running, do they use the fans, how many people in the home etc. Case in point i just visited a site that had mold in the attic, the homeowner had the attic insulated two years ago and installed an attic fan. now there is mold at the plate about thee feet up the sheating in the top level attic. the homeowner had called the famous restoration experts, OH BOY here we go, now they are going to suck out the insulation, fix the mold and give a clean bill of health, WHAT THEY ARE NOT DOING IS FIXING THE PROBLEM. the Insulatin contractor is being blamed because the baffles are slightly covered up by the added insulation and they want him to pay for the fix. AGAIN no one is looking for the problem, covered up baffles did not create this, could have escalated it, but not created it. So realaitive humidity is 58% in the attic, 43% in the home, a quick exterior inspection revealed downspouts next to foundation vents, elevation to low so vents partialy underground. so next questions whats in the crawl space ( i already know its probaby wet ) and this is Idaho so its a dry climate. guess what the homeowner does not want me to look at the crawl, or blower door test the home, they really did not want to find the problem just cover it up to sell the home. i would suggest you add Home assesments to your services to limit liability, and increase sales. what you are doing is good, but it or any improvment has the potential to cause problems if the home is not looked at as a system. good luck.
My on-the-ground experience is not as current as most of you who have given an answer to this question (I hung up my tool-belt in 1985), but I did wrestle with this question thirty years ago when PG&E had their zero interest program (ZIP) for homeowners.  PG&E paid contractors (e.g., me) to install six primary measures, any of several other measures the homeowner wanted, and then got repaid by the homeowner through on-bill charges.  One of the most popular "other" measures was blown-in wall insulation.  We declined to do that measure and so passed on several jobs because that was the main thing those homeowners wanted.  HUD had at that time just published a report warning of the danger of rotted sill plates from condensate inside the walls.  [and before you ask, NO, i do not have a copy of that 30-year old report]  To my memory, it appeared to us that the primary issue had more to do with occupant behavior and how that resulted in interior humidity levels.  For a households with teenagers (i.e., 45 minute hot showers), a taste for beans or other foods that essentially needed to boil-off a gallon or two of water most days, and inadequate mechanical ventilation, the moisture load could be too high for the wall materials to deal with naturally.  I would love to see newer research into how much each of those causes actually contributes to this problem.


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