Using the wrong insulation correctly

November 25, 2006 8:37 AM

[Klutz Construction Filter] Insulating a daylight basement with the wrong size of insulation, can I safely make it work?

The space between the studs in the daylight basement are 5.25 inches deep. I have large amount of rolled insulation that is 3.5" deep, kraft faced with vapor barrier. My gut feeling is that doubling up to fill the space will create a problem with the vapor barrier and encourage mold.

I cannot return the 3.5" rolls of insulation. Can I safely make it work by slashing the kraft paper on the inner piece of insulation and laying another piece on top of it with the kraft and vapor barrier to the interior of the room?

posted by KneeDeep to home & garden (6 comments total)

Just peel the paper off the first layer. No biggie. You want the kraft face (paper side) on the inside of the wall (unless you live in the tropics). Paper goes to the warm side of the wall.
posted by Aquaman at 9:04 AM on November 25, 2006

The 5 5/8 stud thickness on the outside of your house is solely for the reason of better insulation (if you live in a somewhat recently built house).

Are you finishing your basement?

I'm not sure what safety issues you are talking about. Your jaw might hit the floor if you have ever been on a construction site and are concerned with a little bit of insulation around a window. I have seen people tear up insulation by the fistful and shove it in the exact location you are talking about.

There should be a vapor barrier on the outside of your house that keeps the moisture out. The "vapor barrier" on the insulation is just supposed to keep moisture out of the insulation, which if moisture has already made it that far the insulation is the least of your worries.

So, to answer your question I see absolutely no problem with the solution you propose.
posted by comatose at 9:15 AM on November 25, 2006

After Googling around a little, I'd say that you can, if by "slashing" you mean "tearing most/all of the kraft paper off." It appears that the vapor barrier is to be installed closest to the interior of the house, so here's what you want to end up with:

          |                           |

outside-> | insulation X insulation K | < -drywallbr>
          |                           |

X = removed vapor barrier
K = intact vapor barrier
(I wasn't exactly sure if this is what you were thinking when you said "inner piece of insulation.")
Wear long sleeves, gloves and a dust mask. Fiberglass itchies suck.
posted by spacewrench at 9:16 AM on November 25, 2006

Wear long sleeves, gloves and a dust mask.

If possible, wear some clothes that you can thow away after. That stuff doesn't wash out well, and it will get all over your other clothes and inside the dryer.
posted by StickyCarpet at 10:13 AM on November 25, 2006

There should be a vapor barrier on the outside of your house that keeps the moisture out.

Actually, what you have on the exterior of your house is a moisture barrier. A moisture barrier (Tyvek, Typar, etc) are vapor-permeable, which allows water vapor to escape the wall assembly to the exterior.

on the warm side of the wall assembly (the inside in warm climates, the outside in warm, humid climates) is your vapor barrier. The Kraft paper facing on insulation acts as a vapor barrier if installed correctly (the paper is usually impregnated with some sort of petroleum product), and it keeps the warm, moist interior air from condensing in your insulation and causing water damage within the wall.

Your instincts sound right to me, but I like the marked best answer to completely remove the inner layer of kraft paper. If these basement walls were against earth, I would be a little worried about moisture in an overstuffed stud cavity, but with a daylighted basement wall, you should be fine.
posted by misterbrandt at 4:18 PM on November 25, 2006

Remove the vapor barrier. Having two vapor barriers will cause moisture/mold problems down the road. Misterbrandt is correct that your vapor barrier should be located on the warm side of the insulation. If in doubt as to where that is, omit it entirely.

If you have housewrap (Tyvek, etc.) it's actually an air barrier. It is somewhat vapor permeable, but its main function is to reduce air leaks.

I'm in the process of insulating and sealing an 80 year old brick rowhouse in Baltimore. The best reference I've found so far is Insulate and Weatherize: Expert Advice from Start to Finish, by the editors of Fine Homebuilding Magazine. One excellent point they make is that the first step is to seal air leaks before insulating. Fiberglass insulation does not stop airflow and will not be as effective than if you seal beforehand.
posted by electroboy at 7:28 AM on November 27, 2006