For every 18% increase in relative humidity,...

the vapor pressure is ___________ (decreased, increased, doubled, tripled, or halved), and the life of wooden building materials is _____________ (decreased or increased) by ________%.

It’s a Building Science question.




For every 18% increase in relative humidity, the vapor pressure is doubled and the life of wooden building materials is decreased by 50%.

Inspectors should understand how buildings in humid geographical areas age, deteriorate, and break down - naturally.

And not just in humid climates.

Around here, in the winter, everyone thinks that they should crank up their humidifiers to 50 - 60% because the want to keep theri hardwood floors from cracking or they want their piano to stay in tune or they just want to keep their noses from drying up.

But, as the outside temp decreases you also want to decrease the hunidifier. In the winter, with low double digit or single digit temps, you want to keep the inside humidity at about 20 - 25% RH. The extra humidity will affect ALL the building materials AND the increased partial air pressure will drive the humidity through the drywall, the Kraft paper facing of the insulation (even if it properly installed, which is rare) and it will condense on the interior of the sheathing, causing mold.

Hope this helps;


I still find most Americans stuck in the “vapour barrier” mode of explaining condensation in walls/attics…Air leakage…air leakage…air leakage…air leakage. Air leakage is the prime mover of moisture into walls/attics, estimated to be as high as 99%!!! From a 1975 Canadian NRC, Division of Building Research (DBR) “Building Digest”* :

"Air leakage is now considered to be the prime cause of most condensation problems in walls and roof spaces. If, therefore, a building can be made tight against air leakage it may not need a vapour barrier, as defined. On the other hand, if there are openings that permit air to leak from the warm side to the cold side of the insulation, adding a vapour barrier (even of zero permeance) that does not seal off the openings will be useless."

from an online article:
"[FONT=Sabon-Roman][FONT=Sabon-Roman]One such study compared potential moisture contributed by air leakage vs. vapor diffusion over a period of one year. The results: Infiltration/exfiltration proved to be a significant potential source of moisture accumulation in walls and/or high relative humidity levels in interior spaces. Water vapor diffusion, on the other hand, was shown to be a much less significant potential source of moisture than once thought. In this study, air leakage was 228 gallons, which equals 45.6 five gallon buckets vs. less than 6 ounces of vapor diffusion.[/FONT][/FONT]*
[FONT=Sabon-Roman][FONT=Sabon-Roman]Stop air leakage…air leakage…air leakage…air leakage!!!

  • Joe Lstiburek learned a lot from these and the old guard that wrote them. Matter of fact, when I first met Joe at a 1985 building conference (Avoiding Failure; Learning from Experience), he was with Gus Handegord who wrote in the last two lines of a September 1960 Digest on vapour barriers:
    "The most important general principle to be followed in both design and installation is to* reduce to a minimum the number of openings** in the barrier. Where such openings are necessary, special care should be taken to seal the barrier so as to approach complete continuity."*

BTW, Joe was in Halifax, NS last Thurs/Fri. In a taped interview played locally today, he tore apart the LEED system with a simple statement, “they don’t work!!” which he then had to defend. (which he did simply!!)

I totally agree, Brian. I was only, in my post, responding to the question. But you are so right. Air leakage, due to unsealed exterior openings (pipes, sillcocks, unsealed and untaped house wrap, no caulking on flange windows, unsealed rim joists, etc) are a BIG cause, as I documented last week on this board. Why do toilet supply pipes, at least 17’ from all exterior walls freeze? Air leakage at the rim joist which travels between the floor joists.

An LEEDS is a joke, as are most “standards” that are being proposed by the agenda driven “activists” as opposed to actual building science professionals.

If you see my video, you would understand.

If you don’t, send me a copy of the invoice and I will pay you back, personally.

I put my money (and my rep) where my mouth is.


What show was Joe’s interview on? I’d like to try to find it online.

The CBC Noon show. He was interviewed by Costes Halobresas (sp?)

Yes. Where there are air leaks, there are vapor leaks.

The idea of “air barriers” or “air flow retarders” involves the continuous sealing of air leaks. The barrier (the sealing of air leaks) goes around the entire house and can be the combination of many different things, from sealant/caulk, to rigid foam board, to a gasket between the sill plate and the top of the poured concrete foundation wall, to interior gypsum drywall, to exterior plywood sheathing, spray foam in holes though which conduit/pipe/lines run. Any break, any hole (big or small) in the continuity is a major open conduit through which vapor can move. The concept of “air barriers” “air flow retarders” and “air sealing” is a whole house system approach.

Air sealing practices also helps homeowners with (not just vapor intrusion), but indoor air quality, sound, vermin, and insects, etc.

Air movement accounts for more than 98% of all water vapor movement in building cavities.

Being into education and training, I’m interested in…
When building (home) inspectors point out to their clients that there is (for example) an open gap in the siding through which the electrical line goes from the meter to the electrical panel, the inspector should be able to speak about the larger issues that go far beyond the observed hole. A home inspector should be able to engage the client in a conversation about “air sealing” “air movement” “water vapor” “air sealing” “building science.”

For those interested in Building Science check out >>>How to Inspect for Moisture>>>]( You can’t understand one without the other.
And Will Decker in >>>Building Science and Thermal Imaging>>>]( video.

I’ve invited Joe Lstiburek to NACHI.TV. He has not yet responded.

I heard Joe Lstiburek speak at an energy conference last week, and got to have a 10 min chat with him and a few other guys later on after the conference. He is truly a remarkable speaker, and has a unique way of getting his point across. If anyone ever gets the chance to hear him speak, its worth the trip.


Well, maybe not you, Brian, living in Arizona and all.

Maybe you would be interested in finding out about proper building science in dry climates.

Wanna get together and discuss it?

Be careful Brian… I think he heard you were single… If you recall he claimed to being a pro on using IR on human private parts in another life…:twisted::twisted:

There are many reasons to study Building Science.

Surprisingly - One is post-inspection liability protection.

After a home inspection, my client understands that his/her home (building/structure) is a “living” “breathing” thing, a system of over 10,000 independent parts, and every part has an effect on the others.

And as time goes on… as with any living, breathing thing, the parts break down, fail, and leak. And those future conditions/events are not the responsibility of a home inspector.

Setting my client’s expectations about the my duty and responsibility as a home inspector includes communicating how a building can “naturally” age, break down, and deteriorate over time (a main theme in building science).

Understanding Building Science = **Post-inspection liability protection

*** …(Taking Will Decker’s course on Building Science is one best things an inspector can do. IMbiasedO)*

Dan, I heard you may be coming out of the closet soon. You can do it here and we will accept you. Your love of sausage between your buns would not be a surprise revelation to us.:p:p:p

Despite this silliness (and slimyness ;-)), my only reason for the request is to learn. I have very little experience in houses in dry climate areas.

I thought that an exchange of experience and knowledge whold be a good thing, benificial to our fellow HACHI members.

Sorry that some people don’t see it this way.

Is it possible to perform energy audits with an IR camera, or moisture intrusion inspections with an IR camera, without taking a training course that incorporates the subject of Building Science with Thermography?

**No. **Put that camera down.

My belief is that one should not pick up an infrared camera without first attaining adequate knowledge and understanding of building science.

For example, your IR camera detects possible moisture in the corner of the finished basement. Your building science knowledge recalls the illustration (below). How do the two relate (IR camera and the illustration)?

I know Will. :smiley:

Here the with little to no moisture in the air the biggest problem is SUN. With that being said people here do not paint often enough and many times leave unpainted wood exposed to sunlight.

irandbldgsci-1.jpg How does the illustration (recalled from your building science study) and Thermography relate?

Assume you IR camera detects some cool surfaces at the bottom corner of the finished basement. Nothing visible to the “naked” eye. Moisture meter confirms “dampness.”

You could stop there and collect some money for your service. But I’d rather have the ability to go further in my assessment/evaluation of the moisture intrusion.

Using the illustration as reference, describe 10 things that could cause moisture intrusion that was detected by your IR camera.


  1. Sump pit is filled with water to the rim, high water table, water wicking from under the slab.
  2. Sump discharged improperly, surface water running back to foundation wall.
  3. Vapor barrier on inside of basement finish wall trapping moisture.
  4. Crack in poured foundation (not a structural defect) has water coming through it.
  5. Basement slab poured directly onto sand, moisture migrating up through concrete
  6. Missing kick-out flashing, moisture penetration behind stucco, down stud wall cavity, down band/rim joist, down basement wall.
  7. First floor window, rotten window sill, moisture intrusion into wall cavity below window, moisture down band/rim joist, down basement wall.
  8. Downspout not diverting water away from house, moisture coming back into basement foundation.
  9. Backfill settlement, caused low-lying area next to foundation, puddle formation, water penetration.
  10. Dog pee