Bill writes from Rochester, Michigan: "I have a half basement and half crawl space. When I bought the home the crawl walls were insulated with batts and the floor is covered with plastic. The house above the crawl still gets very cold in the fall and summer. We have hardwood floors, so it gets really chilly. Can I insulate the ceiling/underside of the floors? If so what should I use? I heard I may want to seal it off by stapling plastic to the floor joists after putting insulation under the floor? Is that recommended?"
Answer: Yes, you can insulate the underside of floors under a crawl space. I recommend using unfaced PINK FIBERGLAS® insulation to fill the entire cavity between the floor joists. You should not use plastic film after the fiberglass has been installed because the plastic would create a double vapor barrier. Also, plastic is never recommended in below-grade applications. Plastic is too strong of a vapor barrier in high moisture areas such as crawl spaces and basements.
Jay writes from Mesa, Arizona: "I have 2X6 exterior walls with a 2X10 flat roof on my addition. What would be the appropriate insulation to install? I want the best insulation value for the space."
Answer: The best you can do in this situation is to install R-21 fiberglass insulation in the 2 x 6 walls and R-38C in the 2 X 10 joist.
If you have an open attic space above the 2 x 10 joist, I recommend using blown in insulation. You can get more R-value and it will be more affordable.
Adam writes from Lake Zurich, Illinois: "When insulating an unventilated space, should I put the batts flush to the sheathing, or should I still leave 1" space for air flow?"
Answer: You should leave a 1" space for air movement.
Steve writes from Villa Park, Illinois: "I am hoping you can assist me with a few quick questions on insulating an unfinished attic/crawl space. I own a home that was built in 1923. While re-wiring the front half of the house, I discovered the insulation in the floor of my unfinished attic was quite old and in poor shape. This explains the high electric & gas bills during my 1st year of living in this home. I am about to re-insulate the floor and an inside wall of the 10' by 30' space. The floor has typical joists (I think 2 x 16 material) and is above my living room. The inner wall is 2 x 4 construction and divides the attic from an upstairs hallway and bedrooms. The roof is uninsulated and has two vents in it (one on either end). Your website indicates that I need an R-value of 49 for my area (Zip Code 60181), however you only sell insulation that goes up to an R-factor of 38. I would assume this means I need to put down two layers of insulation. My question is can I put two layers (one on top of the other) between the floor joists? I do not want to do a criss-cross pattern since I plan on decking over the insulation/joists and use the space for storage. If I can do this, would I use unfaced insulation on the bottom, and faced insulation on the top? Also, what are the advantages/differences to foil vs. paper facing? Also, is there any advantage of insulating the roof joists, even though it is a vented crawl space? Or, is this just a waste of money? Finally, should my roof vents be closed/sealed for the winter?"
Answer: For the attic floor, fill the cavity with unfaced insulation without compressing the existing insulation. Compressing fiberglass insulation will cause it to lose R value.
For the interior wall, if the studs are 2" by 4" you can install high density fiberglass insulation with an R-value of 15.
Don't be concerned about the facing. With the floor, you are putting insulation on top of existing insulation and you don't want a facing between or on top of the insulation. For the wall, an interior surface is already in place and will prevent you from attaching a facing properly. You shouldn't have any moisture problems with proper ventilation. Keep the attic vents open in winter and summer; ventilation is important to help moisture and excessive heat escape from enclosed areas below and at the top of a home. Ventilation helps prevent damage to building materials and structural components such as the roof.
There is no reason for insulating the attic rafters unless you are planning to finish the attic for living space. If not, it would be a waste of money.
Brad writes from New York: "I own a house that is approximately 90 years old. The walls are plaster with some sort of wood slatting behind it. i am uncertain if there is any insulation behind the walls but i assume not. How does one check the space behind the walls without drilling a bit hole in it? Is there some type of device to look into the wall to check the amount of space? Thanks."
Answer: You may be able to find evidence of insulation around electrical outlets or light switches after taking the face plates off the wall.
Otherwise, we can suggest working with an insulation contractor because adding blown-in insulation to existing walls is not a good do-it-yourself project. The work could damage interior walls if not done properly. You state that your home may not have any insulation in the walls but if there is even a small amount the existing insulation will likely block the new material from getting everywhere it needs to be. Bottom line: I recommend that you contact a local insulation professional.
Mike writes: "I live in Chicago and I am remodeling my basement. The studs are 2x2 so I am wondering if FOAMULAR® foam board would be better than compressing Kraft-faced fiberglass insulation."
Answer:
I like your choice of PINK FOAMULAR insulation. That is a better choice than compressing fiberglass insulation batts because compressing fiberglass will cause it to lose insulating value, or R-value.
I need to remind you, however, that you need to cover the FOAMULAR insulation with a 15-minute thermal barrier such a drywall. It is a code violation to leave foam insulation exposed.
Also, if you use an adhesive to adhere the boards to the basement walls, it must be free of petroleum.
Tony writes from Monroe City, Missouri: "I have an older 2 story house about 900 sq ft Attic we have suspended ceilings at about 8 foot downstairs would it be better to put insulation above the ceilings on the first floor or more in the attic? Bedrooms are on the second floor."
Answer:
Put the insulation between the warm and cold air. In your case, that means in the attic and not above the ceiling on the first floor.
Todd writes from Dallas: "Would using two layers of R-13 fiberglass insulation in a 2x4 stud wall, and thereby decreasing the airspace between the fibers, be more or less effective that one layer of R-13?"
Compressing insulation will reduce the R-value. If you want more R-value in a 3 1/2" cavity, I suggest using 3 1/2" high density insulation that has an R-value of 15.
William writes from Ossining, New York: "I have a home built around 1955. My attic has some fiberglass insulation and blue Styrofoam boards covering the fiberglass on the attic floor. I don't plan on using this space. Can I roll r-30 unfaced insulation over the top of these boards or do I need to pull the boards out and roll the insulation on top of the old insulation? Should I use raft-r-mates if this is the case, and will 2" soffit vent circles provide enough airflow?"
I recommend that the polystyrene foam be removed since it should not be left exposed. That is a code violation.
Roll the new insulation directly over the existing fiberglass insulation.
Use RAFT-R-MATE® attic rafter vents only at the eaves where the insulation meets the roof deck.
You will need to do some math to calculate your ventilation requirement. The minimum ratio is 1/300. That means you need one square foot of ventilation for every 300 square feet of attic space. We recommend a ratio of 1/150.
