Posted By greentree on 21 Sep 2010 08:48 PM
Dana,
What happens when perms get too high? At what point does a high perm rating become a disadvantage?
There are no simple answers to these questions, and the specific perm ratings at which problems might occur are both climate-specific and structure specific.
In a stick-built structure with air-leaks or high vapor permeability from the interior into the wall cavities in cold climates where the sheathing will be below the dew point of the interior air for weeks/months on end moisture from the interior is added to the sheathing & cold edge of the framing. The wintertime vapor pressure of cold dry outdoor air is low, and some of that moisture CAN dry to the exterior even when it's well below freezing but it's slow, and the lower the perm rating of the wrap, the slower that process is. The amount of ventilation between the siding/cladding & sheathing also affects the drying rate, particularly but not exclusively from wetting events (wind-driven leakage of the siding drips from roof leaks/ice-dams, etc.). As long as it can dry at the same or faster rate that moisture is getting in there (from whatever source), accumulation of moisture in the wood is limited, every thing is happy. The lower the perm rating of the wrap, the higher the risk. Rainscreen gaps and inherently back-ventilated siding with large air spaces such as vinyl siding increases the drying rate of structural sheathing dramatically, if unencumbered with a semi-vapor-retardent wrap.
Old school goods like #15 felt have variable permeability- they're in single digits when the humidity is extremely low, but rises above 50perms as the humidity rises- something of a "smart" system (but not perfect). Typar stays pretty much ~12 perms, doesn't change much with humidity & temp. It's not bad, unless there is bulk water leaks that get behind the wrap, or high rates of condensation from interior air leaks behind the wrap. But under rainscreened siding it should dry at least as well as a high-perm wrap behind siding that's tight-to-the wrap & sheathing. (Another important aspect of rainscreens is that it lowers the infiltration drive into the structure created by wind pressures by shunting some of that pressure around the sheathing envelope.)
SIPs are inherently more air tight than stick built when done right, and the center-foam is relatively low perm, restricting the amount of moisture traveling from the interior to the exterior via vapor diffusion. But unless the builder was religious about air sealing (including a post-assembly blower-door testing & air-leak remediation step), there will be guaranteed to be air leaks at joints edges, window/door openings, plumbing & electrical penetrations, etc., and simply slapping an air-tight housewrap over the exterior doesn't fix the wintertime water accumulation issues. Preserving a high capacity to dry to the exterior is always the safer bet.
In buildings with insulating foam sheathing between the structural sheathing and the siding, the exterior insulation raises the average temp of the structural timber, limiting the number of hours that the wood is below the dew point of the interior air and thus susceptible. The more exterior insulation there is (as a fraction of the total R), the warmer the wood stays, and the lower the overall risk. At some point (which varies with specific climate) the risk of accumulating enough water in the wood to cause problems drops to zero with increasing exterior R factor, at which point the drying capacity toward the exterior becomes irrelevant.
These are fundamentally cold climate zone issues- the warmer the climate, the less of a concern there is, since there are longer periods of reasonable drying conditions. If the average outdoor temp (not average daily low or high, but average overall) of your coldest winter month is above 35F you can pretty much forget about it, since even the outdoors will be above the dew point of the conditioned space air roughly half the time- any small amount of moisture that might condense on the wood overnight disappears like the morning dew as the day warms up. (The dew point of 68F 30% RH air is ~37F- if you keep the place warmer & more humid than that, adjust the average hazard temp upward.) Some water gets wicked into the wood during the colder spells, but there is sufficient drying time that it doesn't accumulate to damaging levels. In a two week cold snap the moisture accumulation is rarely more than the wood itself can buffer and release during warmer temps without damage. But with average winter temps well below 35F that moisture stick around in the wood, absorbing enough water that might not dry until May unless the wall stackup limits the amount of moisture reaching the colder wood (with air-tight & vapor retardent interiors) and/or the drying capacity toward the exterior is maximized (high perm wrap & rainscreen gaps.)
Drier is always better, and on an assembly that must dry toward the exterior, higher perm layers on the exterior is usually a better choice than lower perm.
And in any stackup, interior air-tightness is more important than perm rating of the wrap (or the interior!), since air transported moisture is far more likely to create moisture transport rates high enough to cause problems than vapor diffusion from the interior. SIPs are easy to make air tight, but SIP structures are not inherently hermetically sealed, nor can they be counted on to remain as air tight as on day 1 for the next 50 years. Designing & building to be more resiliant to faults is always a good idea, even when striving to build perfectly.
There are methods of modeling all of this (WUFI, etc) but the general trends are clear.