For especially high performance systems (or walls that are highly sensitive to moisture damage), a more conservative value can be chosen, such as the coldest month, 10☏/6 ☌ less than the monthly average or 9 ☌/15☏ above the 99% design temperature. The average winter (average of the coldest three months) temperature is considered a reasonably safe value (and is readily available). For materials with some resistance to moisture (e.g., glass-mat faced exterior-grade gypsum sheathing is quite resistant to moisture), and/or with some capacity to safely store moisture (e.g., plywood and OSB sheathing), a much less stringent design criteria is justified than for materials with no storage (e.g., foil-faced insulation) or high moisture sensitivity (paper-faced gypsum). It is difficult to choose the exterior temperature to design for, since any level of condensation protection can be chosen by the analyst, from none to total. Although data for outdoor air temperatures is readily available, even north-facing walls will have some exposure to diffuse solar radiation, which will warm the cladding (and hence the wall), above the outdoor air temperature for many hours of cold winter months. Hence, the choice of conditions for the analysis is very important. The occurrence of interstitial condensation is in itself not usually an indicator of a design flaw: if air leakage condensation occurs only under extreme conditions (e.g., the 99% design condition listed in the ASHRAE Handbook of Fundamentals or other sources), air leakage during the many hours following this rare event will actually dry the wall when sheathing temperatures rise above the interior dewpoint. If an assembly is shown by calculation to be safe against air leakage condensation (using the method described below), then diffusion condensation cannot occur, even if absolutely no vapor resistance is provided inside of the sheathing (i.e., no vapor barrier or other control layer), and even if the sheathing is a vapor barrier (such as foil-faced insulations). In walls with sufficient exterior insulation, the dewpoint temperature of the interior air will be below the temperature of the back of sheathing: therefore condensation due to air leakage cannot occur within the studspace. This condensation can accumulate as frost in cold weather, and subsequently cause “leaks” when the frost thaws and liquid water drains down, or cause rot if the moisture does not dry quickly upon the return of warmer and sunnier weather. To control damaging condensation inside enclosure walls and roofs, air barriers are used to stop airflow and vapor control layers (vapor diffusion retarders or barriers) are used to limit diffusion flow.Īir leaking outward through the enclosure wall in cold weather will contact the back of the sheathing in framed walls. Diffusion usually does not move sufficient quantities of water vapor fast enough to generate a problem. Cold-weather condensation is primarily the result of outward air leakage.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |