When I first started practicing architecture, primarily residential, back in the Stone Age in the early 1970’s, about 90 percent of all new homes in my region (Building Climate Zone 4) had heating-only mechanical systems, with only about 10 percent having Central Air Conditioning systems installed as original equipment. Today these percentages are exactly reversed, with even the “modest” homes built to Habitat for Humanity standards in our region out-fitted with Air Conditioning as part of their original mechanical equipment package. While some of this is due to the fact that when constructed to EnergyStar or comparable standards exceeding current code requirements tightly sealed buildings may develop mold or mildew issues, most are the result of ever-climbing cultural standards -- the same phenomenon that has made a second bathroom pretty much standard in a three bedroom home as small as 1200 square feet.
Older homes framed before April 1977 in New York State (when the State Code first incorporated an Energy Efficiency component) were pretty loosey-goosey; that is the Air Changes per Hour performance was in the double digits. Walls breathed. With a vengeance. The tightening of the construction of our homes that followed the famous Oil-Embargo Era of 1973-1974 caused us all to focus on wall construction including the specifying of vapor barriers. Best practice at the time always admonished us to place it on the “warm side” of the wall – simple enough to do in Florida or North Dakota, but more difficult to do in climate zones 3 or 4 where you were clearly in a place that required predominantly heating hours, but also called for significant hours of AC operation. Or built the other way around in a predominantly cooling region.
We understood that placing the vapor barrier on the “wrong side” of the wall instead of the “warm side” would most certainly be the cause of condensation within the wall, causing not only mold and mildew which were known health problems for the occupants, but cause rot - which could threaten the integrity of the structure to the point of failure.
Fast-forward out of the Stone Age to today and we find that in the Era of the Second Home – which might be a small ski-chalet shared with friends to a beach house somewhere in a warmer clime – properly engineered and designed wall and roof sections fail to perform correctly which is confirmation of a popular adage in our office that “User Patterns Trump Engineering.” What is occurring in these homes is that while a house might be located in a primarily heating zone, it goes largely unoccupied during that season and instead is largely occupied during the vacation season – in this case, the cooling season. So while “properly” constructed for the heating season with the vapor barrier installed on the inside which is, of course the warm side during the heating season, the home is utilized mostly during the cooling season with the AC running full blast. In many such cases the hours of operation during the “wrong” season may exceed the hours of operation during the design season as those months find the thermostat turned way down or the house turned off and the plumbing drained.
What we need are walls – and roofs as well – that work equally well whether during the heating season or cooling system when the hydrodynamics are reversed. Many designers have now abandoned vapor barriers altogether, being mainly concerned about the ability of the envelope to dry itself out through either side or preferably both sides. The best construction for this in cavity wall construction utilizes dense-pack cellulose which will not settle, seals all cracks and voids as well as any foam, but unlike foam – closed or open cell – has the ability to wick moisture really, really well. This approach minimizes the need to “remediate” stick construction with cavities, as it acknowledges that moisture will enter the cavity but it handles that moisture extremely effectively allowing it to dry out as the wicking dissipates the water quickly, before any bacteriological activity can take place, by dispersing it over a huge area for drying through to either side of the structure.
My favorite way of dealing with reversing hydrodynamics it to build with SIPs. They have no cavities, the entire space between the two structural skins is filled with solid EPS foam. EPS foam has a vapor transmission rating of less than 1 perm according to ASTM E96 testing. That’s for Type I foam with a density of about 1 lb/cu. ft. which is what the great majority of SIPs are made with, other SIP types with different types of foam cores rate even better. But the rating of less than 1 perm guaranties that there will be no vapor transpiration into the core and therefore absolutely no chance of condensation occurring anywhere between the two skins. None.
And, of course the SIP is constructed with central axis symmetry, so regardless of air temperature, humidity or pressure conditions on either side, the SIP will behave the same, resisting vapor transpiration and moisture condensation regardless of orientation. Let’s have a round of applause for Hydrodynamically and Thermally Symmetrical SIPs.
We understood that placing the vapor barrier on the “wrong side” of the wall instead of the “warm side” would most certainly be the cause of condensation within the wall, causing not only mold and mildew which were known health problems for the occupants, but cause rot - which could threaten the integrity of the structure to the point of failure.
Fast-forward out of the Stone Age to today and we find that in the Era of the Second Home – which might be a small ski-chalet shared with friends to a beach house somewhere in a warmer clime – properly engineered and designed wall and roof sections fail to perform correctly which is confirmation of a popular adage in our office that “User Patterns Trump Engineering.” What is occurring in these homes is that while a house might be located in a primarily heating zone, it goes largely unoccupied during that season and instead is largely occupied during the vacation season – in this case, the cooling season. So while “properly” constructed for the heating season with the vapor barrier installed on the inside which is, of course the warm side during the heating season, the home is utilized mostly during the cooling season with the AC running full blast. In many such cases the hours of operation during the “wrong” season may exceed the hours of operation during the design season as those months find the thermostat turned way down or the house turned off and the plumbing drained.
What we need are walls – and roofs as well – that work equally well whether during the heating season or cooling system when the hydrodynamics are reversed. Many designers have now abandoned vapor barriers altogether, being mainly concerned about the ability of the envelope to dry itself out through either side or preferably both sides. The best construction for this in cavity wall construction utilizes dense-pack cellulose which will not settle, seals all cracks and voids as well as any foam, but unlike foam – closed or open cell – has the ability to wick moisture really, really well. This approach minimizes the need to “remediate” stick construction with cavities, as it acknowledges that moisture will enter the cavity but it handles that moisture extremely effectively allowing it to dry out as the wicking dissipates the water quickly, before any bacteriological activity can take place, by dispersing it over a huge area for drying through to either side of the structure.
My favorite way of dealing with reversing hydrodynamics it to build with SIPs. They have no cavities, the entire space between the two structural skins is filled with solid EPS foam. EPS foam has a vapor transmission rating of less than 1 perm according to ASTM E96 testing. That’s for Type I foam with a density of about 1 lb/cu. ft. which is what the great majority of SIPs are made with, other SIP types with different types of foam cores rate even better. But the rating of less than 1 perm guaranties that there will be no vapor transpiration into the core and therefore absolutely no chance of condensation occurring anywhere between the two skins. None.
And, of course the SIP is constructed with central axis symmetry, so regardless of air temperature, humidity or pressure conditions on either side, the SIP will behave the same, resisting vapor transpiration and moisture condensation regardless of orientation. Let’s have a round of applause for Hydrodynamically and Thermally Symmetrical SIPs.
RSS Feed