Simple Infiltration Barrier Question
We wrap houses to keep the wind out but let the water vapor escape – right? But then we put vents into the eaves and vents out on the attic roof. Presumably, this keeps the vented attic space in pressure equilibrium with the outside world. Why don’t we lay Tyvek over the ceiling insulation and seal around all of the openings just like we do the walls; or alternatively run it up the bottom side of the rafters to the roof vents creating a sealed attic space? Isn’t it the pressure differential that drives air infiltration? Doesn’t gusting wind create the same kind of pressure differential in the attic between the vented space and the interior space as you get against an exterior wall?
Would closed-cell foamed ceiling insulation be measureably better relative to infiltration? Would appreciate pointers to the right direction. TIA.
Replies
Go here; http://www.buildingscience.org and learn the science for yor particular area of the country.
Enjoy.
Dave
Great questions, and good thinking. Dave's suggestion is very good. A few other sources for info include:
http://www.affordablecomfort.org
http://www.homeenergy.org
http://www.eeba.org
http://www.nesea.org
http://www.pathnet.org
Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Edited 8/2/2006 12:14 pm ET by Andy_Engel
Did you mean for that last site to be on your list? Looks like a medical site on my puter.
Other sites are great. I bookmarked them.
Thanks,
Dave
OOPs, that should have been http://www.pathnet.org.
I'll edit the original.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Presumably, this keeps the vented attic space in pressure equilibrium with the outside world.
It allows air to flow thru, carrying the heated air out.
Nothing simple about that question.
Venting is just commonly accepted (and generally required by law) but the science behind the practice is questionable.
In a cold weather situations;
If there are air leaks into the attic space, venting will accelerate their velocity by increasing the temperature differential and through wind pressures (as you suggested). Also, the vented space is going to hit the dew point much faster and more frequently.
If there are no air leaks into the attic space, the venting is not really needed, and will increase heatloss to some extent.
If you have fiberglass insulation, wind washes through it and diminishes its performance. Even absent wind, convective currents can go through the fiberglass, such that perfomance diminishes as it gets colder outside.
I do not think the answer is to add house wrap. I think the answer is to not use fiberglass in the first palce and to completely seal the attic plane. Then venting or none, you still get good thermal performance.
Thanks for the input.