02-11-2007, 06:45 AM
Thanks David for the suggestion. I've looked into the Protec SIPS. It's nice that the wall construction is simple using them but I like my way better. I see that they use XPS (extruded polystyrene), probably 4 inches thick. XPS has a permeability of about 1.1 perms/inch which would make the permeance for 4 inches about 0.275 perms. That's not very breathable and it only counts the insulation; it doesn't count the cladding.
I plan to use medium density EPS (expanded polystyrene) which breaths better and use only an inch. One inch of medium density EPS has a permeance of about 3.5 perms. I did some calculations and expect the total permeance of my wall assembly to be just over 1 perm which is probably about 3 times more breathable than using the SIPS.
Also my assembly has more barriers to prevent water infiltration from the outside. And there's a scenario I see happening with the SIPs that I don't like (this is a wellknown problem that happens in wall assemblies):
* It rains
* The walls get wet and the concrete cladding soaks up some of the water
* The sun comes out and heats up the walls and produce water vapor
* Since the walls are warmer than the inside of the house, water vapor migrates into the walls and into the house. Much of it would be stopped by the XPS though, since it doesn't breath very well, but some of it will make it through and make the house more humid inside and some of it might condense inside the walls.
The same thing can happen with my assembly also but using back-ventilation (rain screen) and waterproofing the cladding helps prevent it from happening. It might look like I'm overdoing things but the average rainfall in Mountain View is around 200 inches/year; I don't want to have problems.
I plan to use medium density EPS (expanded polystyrene) which breaths better and use only an inch. One inch of medium density EPS has a permeance of about 3.5 perms. I did some calculations and expect the total permeance of my wall assembly to be just over 1 perm which is probably about 3 times more breathable than using the SIPS.
Also my assembly has more barriers to prevent water infiltration from the outside. And there's a scenario I see happening with the SIPs that I don't like (this is a wellknown problem that happens in wall assemblies):
* It rains
* The walls get wet and the concrete cladding soaks up some of the water
* The sun comes out and heats up the walls and produce water vapor
* Since the walls are warmer than the inside of the house, water vapor migrates into the walls and into the house. Much of it would be stopped by the XPS though, since it doesn't breath very well, but some of it will make it through and make the house more humid inside and some of it might condense inside the walls.
The same thing can happen with my assembly also but using back-ventilation (rain screen) and waterproofing the cladding helps prevent it from happening. It might look like I'm overdoing things but the average rainfall in Mountain View is around 200 inches/year; I don't want to have problems.