I am installing insullation in a shell home I had built. Most books about installing
insullation also talk about installing a mositure barrier of 6 mil plastic sheet. Most
drywall contractors where I live say it isn’t usually done. I live in Central Texas
where winter is mild to sometimes cold. Is this a practice that is usually done in
climates where the weather is usually colder. The insullation even mentions putting
it in so the moisture barrier on the insullation faces outside if you live in Florida
or the Gulf Coast. If anyone has some input on this subject please respond.
Thanks
Replies
Dear J,
Congrats on your new house.
There isn't much valid information in print on effective design for moisture barriers. Search amazon.com for Joe Lstiburek and Gene Leger books. If you have FHb Aug99, you can read my deep thoughts on damp, moldy houses.
The truth is, poly belongs in the ground to block moisture diffusion through the foundation. This is the source for most all condensation problems. If you get that right, you can do all the other internal moisture barriers "wrong" and have a healthy, durable house.
Balancing and sealing your ductwork is far more important than some trashy plastic in the walls.
Regards, Fred
[email protected]
Thanks Fred,
The house sits on concrete and it was properly sealed with black Poly before
They poured it. What you say makes since and is a lot less work. Their is enough
left to do without doing unnessary steps. Jim
J.
Being that your idea of winter cold differs from mine (12" of snow in the past 2 days) it's hard to appreciate the other's concerns.
If you put a light bulb inside an inverted 5 gallon tin can in sub zero weather the inside of the can will quickly heat up toasty warm. Thermal resistance of a tin can is close to 0 but by containing the air as it's being heated by the bulb raises the temperature inside to cooking levels. But if you try to touch the tin can, you will be burned as well. Put on some insulated gloves and you will be able to handle the can because your gloves have resistance to quick temperature changes.
Two things are required to ensure comfortable use of your home. Sealing is first and foremost. If you contain the air inside the home you have better control on it's makeup. You can cool it, clean it, exchange it, heat it, add moisture to it and remove moisture from it.
Insulation is second. With insulation you reduce the energy cost of controling that same air.
I would argue the installation of the VB anywhere other than on the inside portion of the wall system.
Working your way through the wall system from the inside out....
Paint, ( paint seals the drywall and in most cases can be classified as a vb) drywall, (again this can be classified as an air barrier) poly (vb and ab) insulation (fg or cels are neither vb or ab) ply sheathing ( air barrier but not a vapor barrier) felt paper lapped, (not a barrier of any kind, it only deflects and directs moisture downward ala gravity) and finally siding. Masonry, wood, vinyl etc is not classified as a vb or ab in a wall system.
So most changes to the humidity in the building components would be from the outside in.
Another reason I don't prescribe to the warm side vb installation theory is that here we would have to move the vb to the outside in the summer and inside the wall cavity in the winter. As cold as it gets in the winter, as hot as it gets in the summer.
Gabe
Thanks Gabe,
The perimeter walls were my main concern. I wasn't thinking of doing the ceiling.
Since the attic is properly vented with continous soffet vents and ridge vents I
feel any moisture making it up into attic should find its way out thru the vents.
My area does believe in a good outside wrap before putting on the siding but like
I said the person I selected to do the drywall said its rarely done on the inside.
Jim