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Attic and Basement FAQ
By: Danny Lipford 
Attics, basements, and crawlspaces are the least frequented areas of a home, which is one of the reasons they often cause the most problems. A regular inspection can catch potential problems—like leaks, condensation, termite activity, or the growth of mold—before they become a major headache.
Attics can become virtual ovens in the summer, so work in the early morning or wait until a cooler time of year. Inspect basements after a heavy rain to catch any leaks. And don’t neglect the crawlspace under your home if it’s built off the ground on piers. While it’s not the most inviting place to spend an afternoon, early detection might save you thousands of dollars in termite damage or rot that would otherwise go undetected.
Here are some of the most common questions concerning problems with attics and basements:
Attics
Q: Is it necessary to install a separate vapor barrier between the living area of a house and the attic or crawlspace?
A: No, a separate vapor barrier is usually not needed since any condensation should be able to escape from a properly vented attic or crawlspace. For more information, see our article on Vapor Barriers.
Q: Should the facing on insulation in the attic or under the floor go up or down?
A: Install insulation with the facing toward the heated living area of the house. In the attic the facing should go down against the ceiling while under the house it should be up next to the subfloor.
Q: Should you install a layer of faced insulation in the attic on top of the insulation that is already there?
A: No, always put unfaced insulation on top of existing attic insulation. Since the facing acts as a vapor barrier, it could trap moisture in the insulation. For more information, see our article on Attic Insulation.
Q: If I install a second layer of insulation in the attic on top of the existing insulation, which way should it run?
A: If the existing insulation is even with or higher than the top of the joists, the new layer should run perpendicular to the joists. If the existing insulation is lower than the joists, lay the new layer between the joists.
Q: How much venting does an attic require?
A: A typical house should have one square foot of vent for every 150 square feet of attic floor space. Ideally, half the vents should be located in the soffit at the bottom of the roof and half in gable or ridge vents near the top to create a flow of outside air into the attic. For more information on venting your attic, see our article on Adding Soffit Vents.
Q: What size should an attic vent fan be?
A: An attic vent fan should be large enough to completely replace the air in the attic every six minutes. To determine the size needed, calculate the volume of your attic in cubic feet and divide by 6. This will equal the rated capacity of the attic vent fan in cubic feet per minute (CFM). For more information, see our article on Attic Ventilation.
Q: Can you cover electrical wires and boxes in the attic with insulation?
A: While recessed ceiling fixtures should not be covered, wiring and ceiling boxes for surface mounted or hanging light fixtures can be covered by insulation.
Q: How can I prevent condensation in my attic?
A: Condensation can occur when heated air from inside the house condenses on cold surfaces in the attic. This can usually be prevented by removing any sources of moisture in the attic, and making sure there is adequate ventilation in the attic. Always vent dryers and bathrooms fans outside rather than in the attic, and check to be sure that soffit vents under eaves have not become blocked by insulation.
Basements and Crawlspaces
Q: How can I prevent mold and termites in of the crawlspace under my house?
A: Mold and termites can be a problem under homes, particularly in warm humid climates. Since both need water to thrive, start by checking for any plumbing leaks, condensation from ductwork, or rainwater seepage. Cover the crawlspace with sheets of 6 mil black plastic, overlapping them a foot and taping all seams. Run the plastic up the sides of the foundation walls with holes cut for the outside vents. Use a flashlight to inspect the joists and the subfloor periodically for signs of mold. Check the piers and foundation walls for mud tunnels make by termites. For more information, see our articles How to Prevent and Remove Mold, and Formosan Termites.
Q: How can I add a bathroom or laundry room in a basement?
A: Since plumbing uses gravity to drain, you will have to install what is known as a “grinder pump” or “lift station” to pump waste water up to the level of the drain pipe leading away from your house.
Q: What can I do to keep my basement walls from sweating?
A: Start by taping a one foot square piece of plastic to the wall and sealing it around all fours sides with tape. If moisture appears on the outside of the plastic, the problem is condensation due to excess humidity in the air. If water forms behind the plastic, then water is seeping in through the walls. To prevent condensation, start by reducing the humidity in the air as much as possible by venting dryers and bath fans to the outside and eliminating any plumbing leaks. If problems persist, consider insulating the exterior walls or installing a dehumidifier. See below if the water is coming from outside the basement.
Q: How can I stop a basement from leaking?
A: Start by being sure the land around your house is graded so rainwater is diverted away from the house. Install gutters and put extensions on existing downspouts. If that doesn’t help, you may have to excavate around the basement and waterproof the walls from the outside. For more information on basements, see our article Basement Refinishing.
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19 Comments on “Attic and Basement FAQ”
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March 26th, 2008 at 2:50 pm
Hi!
In my “cold room” where the water pump, sump pump and water filter is located, I’m getting condensation on the concrete ceiling, which is also the floor of the front porch. This only happens in cold weather. There had been a moldy, rotten piece of plywood there, probably left from forming the slab ceiling/floor, of which I removed. I’ve had a floor fan directed out of that room all winter, but am still getting condensation.
I’m considering using Nansulate thermal barrier paint. What do you think of this solution?
Thanks!
I
November 11th, 2008 at 12:05 pm
I have a home that was built in 1927. I have batt type insulation in the attic crawl space which has been installed with the vapor barrier toward the crawl space rather than toward the living area -It’s been that way for over 30 years. I would like to overlap some nonfaced insulation over the top but I’m wondering if it’s worthwhile to pull the existing insulation first.
November 23rd, 2008 at 10:54 am
Is it OK to or can you use attic insulation in the basement???
Thanks.
Ben Erickson Says:November 24th, 2008 at 9:08 am
RK,
Yes, you can use the same insulation in your attic or basement.
January 17th, 2009 at 9:29 am
Hi, I have a Home (built new in 2000), that has an exposed North Basement wall (8 ft tall x 28 ft wide). In the colder days of Winter we have a problem with that wall sweating. I think it needs to be sealed on the outside. Can you give me directions for best way to do this or how do I fix the Problem? Thanks Don
February 23rd, 2009 at 11:14 pm
I have basically a flat roof in Florida with no attic crawl space. I want to insulate and they told me it could be done through the fascia board by drilling 4 inch diameter circles in the fascia. Then they would insert a 3 inch pipe and blow insulation all through the house in each roof trust cavity. I went to home depot and they only had 3 inch round vents for the fascia. Could you tell me where I could get about 80 of the 4 inch diameter round vents for my fascia so I can insulate. Thank you
April 13th, 2009 at 12:33 pm
My basement framing was installed 9″ away from the wall. I want to finish the basement but Im not sure if it is wise to install insualtion as it wont be flush against the wall.Any suggestions?
Ben Erickson Says:April 14th, 2009 at 6:37 am
Hi John,
I don’t see a problem with having a gap behind the insulation. If anything it would be a good thing since the air gap would have an additional insulation factor and if there was any condensation on the wall it wouldn’t soak into the insualtion.
April 14th, 2009 at 8:11 pm
Thanks Ben,its the best news I have so far. One more question. Should I place a film barrier behind the Insulation in order to help keep it in place?
Ben Erickson Says:April 15th, 2009 at 8:15 am
John,
Assuming the insulation (fiberglass?) has a facing that is stapled to the studs, I wouldn’t think a barrier behind it is necessary.
July 1st, 2009 at 11:56 am
my home was built in 2006 . the roof has ridge vents. would i be helping any thing,like my energy costs or the life of the roof shingles by having power vent fans installed ? guy robinson
Ben Erickson Says:July 2nd, 2009 at 8:08 am
Hi Guy,
The most efficient system is to combine soffit vents with a ridge vent (using the natural flow of hot air rising) or soffit vents with a power vent, but not to use all three together. Using a power vent with a ridge vent disrupts the natural air flow of cool air coming in the soffit vent, rising, and going out the ridge vent. When you add a power vent to an attic that has a ridge vent, it tends to reverse the air flow and pull air (and possibly rain) in through the ridge vent.
December 1st, 2009 at 3:44 pm
I built my SIP (structural insulated panel) home in NH,in 2005, with a full foundation,(6 feet below grade). I have no leaks in the basement, as I did install a perimeter drain after they poured the foundation. I can smell a musty odor when I enter the home, both upstairs, and down in the basement. In the Summer, the pipes in basement sweat a lot.I am in the proces of insulating all the pipes in the basement. Even if I run a dehumidifier, I can still smell the odor. House is quite “tight”, along with energy efficient windows. Did I “overkill”, and what can I do?
Ben Erickson Says:December 28th, 2009 at 10:37 am
Ron,
Sounds like you have a mold problem somewhere that needs correcting. You might want to consider adding an air exchanger that lets in fresh air from outside without losing your heat or cool.
December 13th, 2010 at 12:49 pm
When my 20 year old house was built, it was built with gable vents in the attic. When a new roof was put on recently, the roofer added a ridge vent. Someone told me that the gable vents now have to be closed up. That you couldn’t use both gable vents and a ridge vent as they work against each other. Is that true? Should the gable vents be closed off?
Ben Erickson Says:December 14th, 2010 at 9:18 am
Hi George,
Gable and roof vents together should be fine, the more ventilation the better, what can cause a problem is combining a ridge vent with a power vent, since the power vent can reverse the natural air flow through the attic as heat rises and draw air (and possibly rain) in the ridge vent rather than expelling it through it. For the most effective attic cooling, you should also have soffit vents under the eaves to draw outside air in as the heat rises. More info is available in our articles on Combing an Attic Vent Fan and Roof Ridge Vent and Attic Ventilation.
January 25th, 2011 at 10:49 pm
Hi Came home from vacation and have rust stains on ceiling, which is under attic crawl space. We had a blizzard and lot snow on roof. Last years blizzard got this staining too. we have 2 gablevents, ridge vent almost whole length of a roof, and power attic fan, and a continous soffit vent, on this side of the crawl space. With the A roof and this crawl space has no circulation to the attic. small Mold has appeared on closet wall that shares this wall with crwal space. Husband has plugged in fan and has dried everything up. he has added more insulation to floor. Do we need another type of vent to vent this area and what else can we do. only seems like when we get these long cold spells and lots snow, the lack ventilation is a problem. Help!
February 28th, 2011 at 8:56 pm
Yes i was wondering if anyone knows if it matters if u leave a basement uncovered for a while before the house is put on?
August 26th, 2011 at 12:12 am
I grew up with an attic fan or whole house fan. I can remember that when it was turned on in the evening that it would cool the attic off. Thinking about this and the heat of my attic that I have now made me wonder if installing a whole house fan on my back porch would be a good way to push the hot air out of my attic on hot summer days. Is this a good idea? It seems that it would be a good way to bring air in from the bottom of the attic and push it to the top.