Hey you moisture experts-
I’m building a house out on Cape Cod right now–a home energy rater that I’ve been talking to was putting down Icynene as too expensive a product and said I could get the same effect by simply using fiberglass insulation and covering the interior walls with plastic sheeting before the sheetrock goes on. I always thought that was a moisture trap. I’m doing standard 2×6 construction with Advantech sheathing, 30 lb felt, and cedar shingles. Can I do this without growing mold all over the place?
thanks for your help.
Replies
The poly barrier installed as you describe doesn't trap moisture, but prevents moisture from migrating from living space into the wall. In a heating climate, that is the proper way to place it.
Now, if you had a second layer of poly somewhere else inside the wall, that would be a moisture trap, and that would be bad. Housewraps like Tyvek and Typar are not vapor barriers, and breathe to allow water vapor to escape from the wall in order to prvent the type of problem you describe. So does the 30# felt.
Edited 10/28/2004 8:24 pm ET by BarryO
This guy doesn't sound like he knows much (or you didn't list some of the other things you talked about). To make an energy efficient home you need both a vapor barrier and an air barrier. The poly sheeting makes a great vapor barrier, but not a good air barrier. If you don't seal up air infiltration to your walls, you might as well not insulate and keep your windows open. Air movement kills fiberglass insulation and small air leaks bring in a lot more moisture than would naturally migrate through the wall.
MERC.
Here in Illinois, most homes are built just as he described, insulation, plastic barrier and drywall.
You won't have a problem with moisture build up if you use only one vapor barrier. But I am suspicious of the Advantech sheeting. My gut feeling is that it also is a moisture barrier and may trap moister inside the wall.
You get out of life what you put into it......minus taxes.
Marv
check out , think it's energy conservation resources in FHB they sell an 8 mil poly, Teno vapor barrier, that is oxygenated so it doesn't fall apart in the wall like standard poly stock use it under basement slab too handles nicer than thin mil poly, crete guys like working w it
"I'm building a house out on Cape Cod right now--a home energy rater that I've been talking to was putting down Icynene as too expensive a product and said I could get the same effect by simply using fiberglass insulation and covering the interior walls with plastic sheeting before the sheetrock goes on."
Fiberglass is a miserable material. Not at all equiv to foam.
Look into densepack cellulose if you want to save money on materials and energy.
Look into "airtight drywall approach" over poly films.
Folks are very slowly coming around on this stuff, and catching up to latest building science. Myth still prevails in the marketplace.
Same construction as BarryO described is used in Colorado. You might want to research "infiltration". As far as I know (having installed thousands of insulating windows) the movement of air is from heated (inside your house) to unheated (outside) if we are talking winter. Infiltration gives you the notion it is the other way around. Cold air will "infiltrate" (sounds like a military maneuver) the exterior walls of a home if penetrations (an oversized hole for say a dryer vent, window, etc.) are not foamed tight and a housewrap not used. Tar paper on the sheathing behind siding or brick really acts like a water resistant backup to the veneer used.
Brick/Siding is not 100% waterproof so the tar paper prevents moisture from affecting the sheathing and directs it down to the weep holes and out. Wrap is generally considered to produce a "tighter" house. It does the same a tar paper for moisture from outside but with fewer seams, and adds protection from winds that otherwise would work their way past the brick/siding, tar paper, and through joints in sheathing to the space holding insulation. It has two distinctive sides. Exterior and interior.
Remember Gortex fabric? It shed water and acted as a windbreaker on the outside but would allow vapor to escape from the inside. Same principal.
I don't think you get as good an insulative barrier with the fiberglass/poly combo compared to Icynene foamed in place. But you can get to work in a Mercedes or a Ford Focus. Sometimes good is good-enough. Tyr
Tyr said " As far as I know (having installed thousands of insulating windows) the movement of air is from heated (inside your house) to unheated (outside) if we are talking winter. Infiltration gives you the notion it is the other way around. Cold air will "infiltrate" (sounds like a military maneuver) the exterior walls of a home if penetrations (an oversized hole for say a dryer vent, window, etc.) are not foamed tight and a housewrap not used."
Without help, heat will go to cold. Cold will not "go" to heat"
But, under the usual "drafty house" conditions, cold air will infiltrate into the warmer house thru penetrations in a wall. It happens because the warm air is escaping upwards from somewhere else in the house. So it's a convective air movement. Warm air finds a way up and outside thru air leaks in the attic plane (wiring holes in wall plates, chimney chases, around vent stacks etc etc) going to the colder attic. The heated air that leaves creates a "suction" of air so to speak drawing air from anywhere of least resistance. Maybe from around electric outlets which draws air from outside thru the outside wall penetrations/leaks.
A very tight ceiling/attic plane can actually make older "leaky" windows less drafty (drafty = letting cold air IN). There's nothing to draw cold air in. You could open a window in a tight house and not much cold air would enter as opposed to a "leaky house".
I'd still replace old drafty windows with new insulated and well sealed windows. Along with and after air-sealing the attic plane.