Okay. I assumed the link would be to where the informatin was. They fooled me with the sidebar layout - putting the info where the ads for other stuff typically appears.

I'm afraid I'm not impressed with their "technical description." It appears to have been written by an ad department (or a bricklayer) with only a vague concept of physical phenomena, and uses pseudo-tech-speak to impress without actually saying anything (correct) technical.

The kind(s) of construction for which this appears suited are not too often seen in my parts of the US, but porous stone in contact with any source of moisture can absorb and hold rather large amounts of moisture. In the structure shown, where there are two separate "stone walls," ventilating the space between them will help to keep excess moisture from being held in the inner wall, where small changes in temperature can cause the absorbed moisture to be released and drip on either the interior side – into the room – or on the outside – into the cavity between walls.

Too much ventilation would destroy any insulating effect of the outer wall. Too little ventilation would fail to remove sufficient moisture. Based on a few installations, an appropriate compromise should be fairly easy to reach, and it's possible that the spacing of these vent devices has been "tried" enough to be a useful approximation to an "adequate installation" for typical walls.

With two holes connecting to a single cavity, wind blowing across both holes will usually produce very little circulation through the cavity. The main effect is to "pump up" the cavity without much through-movement. The baffle between the two holes helps to produce a pressure drop, if the flow is restricted slightly where it goes over the end of the baffle. That probably contributes to more air actually moving through the cavity. The flow cross section through the restriction (at the end of the baffle) must be a bit smaller than either of the two external holes, so the air velocity there will be higher than at either the inlet or the outlet hole, and will in fact be slightly cooled.

The amount of cooling to be reasonably expected is miniscule, and in fact would be counter-productive if it were to be significant. Cold air simply holds less moisture, and hence can pick up less moisture when it flows over a damp surface.

The higher velocity where the air flows past the baffle does probably increase the amount of moisture that can be entrained, simply because it reduces the thickness of the boundary layer of stagnant air that's always present adjacent to a surface with air flowing over it. The faster the air flows, the thinner the boundary layer gets. A more effective "drying" effect would in fact be obtained if the air could be heated, rather than cooled; so the bit of babble about cooling the air so it picks up more moisture is bassakward to what really helps.

The bottom line is that ventilating the airspace adjacent to a water retaining inner wall in this way will help to remove some water locally where the air contacts the inner wall. Moisture from other parts of the wall should "wick" toward the dryer areas fairly efficiently, so if these vents work at all, the whole wall should (eventually) be somewhat drier. (And the ability of stone to act as a wick to transfer water from inside to outside answers TIA's question - the water goes through the wall.)

These vents appear to incorporate a suitable baffle to help with pumping some air through the embedded chambers. Note that with wind from the other direction, the air flow will go the other way through the chambers, so the assymetrical structure – insulated on one side and not on the other – would only work half the time; but that's almost irrelevant if, as I suspect, that part of it doesn't really do much of anything.

The "technical notes" are, so far as I can tell, pretty much BS. I don't get the impression they actually understand how it works; although it probably does provide some ventilation.

The "tests" are too vaguely described to really tell whether they did something. They did measure RH at a few places, and the results indicate a benefit from the ventilation. There is no comparison with other methods of ventilating the space between the two walls, so no conclusion can be made about whether this is better or worse than other options. The only comparison is between walls with these devices and walls with NOTHING.

The "references" cited:

Various advisory groups recommend using the Schrijver Systeem®. This is due to comments by satisfied customers and good reviews in the media. In Holland and Belgium the system has been shown on television and various newspapers have published articles on the Schrijver Systeem®. You can write for a (translated) copy of the articles and references.

Anectdotal customer BS, and some media printed some of them? Shown on TV (BFD). Newspapers have published articles. ???? Great sources of factual and technically accurate information. (Don't miss the sarcasm.)

Again: They're wall vents. Wall vents work. They typically install a lot of wall vents. Lots of wall vents is good, if you don't loose too much heat through them.

They do sound a bit expensive, but its a fair assumption is that retrofitting into existing walls is going to incur some cost.

Have you talked to the guys at the local building supply *dealership, to ask how they'd do it?

* Not the ones who sell this item.

John