Air Sealing

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By Energy Circle Staff - August 18th, 2009

START HERE to learn about Air Sealing... because chances are you are losing more than air through the cracks and crevices of your home. You’re losing money, too. Find out what to do about it.

Homes burn through a mortifying 21% of the energy used in the United States. A  lot of that energy is spent heating and cooling your house. What worse? A whole lot of that energy is spent heating and cooling your backyard and front porch, through leaks and holes in your building envelope.  In fact, if you do some serious work sealing cracks, you can expect to net about $600.00 in annual energy savings. Sealing air leaks will garner most of us significantly greater savings than replacing windows, in part because the cost of materials and installation are much lower.

I. Overview:  Sealing up your home keeps more than air inside (it keeps cash inside, too).

Nothing personal, but the warm air in your house wants out. The silent (and costly) exodus of your warm air is made easier with the help of leaky doors, leaky windows and poorly sealed interior or exterior walls. Some of those holes and leakage points are visible to the untrained eye. The truth is, however, that even the draftiest old houses suffer most of their air leakage in places you don't see, due to the chimney effect, which we discuss in more detail below.  It's because of those hard-to-see leaks that we strongly recommend investing in an energy audit.  Some leaks require a home energy auditor's keen eye, structural know how and infrared technology, and others are just confoundingly hard to spot.  But first, we'll tell you a little bit about air sealing, why it's necessary, and how the flow of air works in your house.

II. Air Sealing Basics: Your money could be going up the chimney—even if you don’t have one.

Here's how the chimney (or stack) effect works: Cold air infiltrates your cellar through leaks and cracks in the foundation and walls, and begins rising. It works its way up through the floors and/or walls, then up into your attic through structural defects, holes in your ceiling, recessed lighting, leaky duct work, the furnace flue, the plumbing stack, a poorly insulated attic floor or improperly sealed areas in general. In some homes (even those that have no chimney at all), the chimney effect can be so pronounced that the effect is like leaving your front door open all winter long!

Something to Think About

It’s best to start your efforts to improve home energy efficiency with a home energy audit by a certified professional. You will find how much energy your home consumes and wastes, how you can best conserve energy, where the health and safety concerns in your house are, and estimate how long it will take you to solve the major issues. We have found out the hard way that a house is an interconnected ecosystem, and sometimes a complex one.  Learn what to take on first. You may well find out that you can keep your old windows and just seal up around the edges. You'll very likely come away with some attic and basement projects. Then again, you might find out you need a new roof….

While the chimney effect is the most common air leakage contributor, there are three other air leakage drivers you should be aware of: wind pressure, chimney/exhaust pressure and duct pressure.

1. Wind Pressure. (No, you don't have to live on a boat to care). Wind blowing against an exterior wall can push cold air into your home on the prevailing side and create negative pressure on the leeward side. The wind pushes air through holes and cracks in the windward wall, then exits through the leeward wall. Contributing factors include the exposure of your home, porch roof, inside corners and other features that trap the wind and surrounding vegetation. The end result is a cold and drafty home.

2. Chimney/exhaust pressure. Not to be confused with the chimney effect, chimney or exhaust pressure is created by active ventilation such as exhaust fans and clothes dryers. These fans can be so efficient at removing air that "make-up air" is drawn through holes or cracks in exterior walls or even down your chimney, creating a potentially dangerous condition known as backdrafting.

3. Duct Pressure. In homes with forced hot air heating systems, the furnace blows heated air out into living areas and is resupplied with air through a system of return ducts. If the return ducts system is leaky to the extent that the flow of air back to the furnace is restricted, it may double or triple air leakage through exterior walls compared to not using the furnace at all. It is essential to seal as many sources of duct leaks as possible to keep your energy costs low.

III: Take it on

Now that you know about some of the most common sources of air leakage, how can you tell which ones apply to your home? As always, we recommend starting with a home energy audit that includes a blower door test. The auditor will install a blower in one of your exterior doors, turn on a powerful fan that simulates an especially windy day and start depressurizing your home by creating suction. This will allow the technician to measure all of the sources of air leakage in your home. The audit will reveal air leakage in your home in terms of CFM (cubic feet per minute) or ACH (air changes per hour). The blower fan is so powerful that you can often feel air being drawn from the outside through cracks or holes in your walls. Ask to follow the technician around your home so he or she can point out and help you prioritize problem areas, critical information to have whether you plan on plugging the holes yourself or having a contractor take it on.

As you get started, turn to our air sealing buyer's guide to insure that you are using the right tools for the job.

The primary tools of the trade are expanding foams, non-expanding foams, caulk, insulation, and various types of weatherstripping. While caulking is widely available, it is definitely worth your time and money to invest in professionally rated materials that will keep cracks and holes properly sealed for years. Our advice: before stocking up on sealants and foams intent on sealing every last infiltration point in your home, take a few minutes to step back and look at the big picture. Check out our buyer's guide for more information about which sealing product makes the most sense for your needs.


thank you for the information Posted by Philip on Apr 22, 2009 6:40pm
I really like looking through a post that will make people think. Also, thanks for permitting me to comment! Posted by Cash Flow on Aug 27, 2012 1:09pm
Very informative article. Re; 3. Duct Pressure, I would think that in addition to air being sucked into the house through the walls to compensate for an inefficient system, the system will also work harder and thus use more energy as well. The thermostat would be set higher. Posted by Nick on Sep 5, 2009 9:26am
I've reviewed the above and find the info helpful. Do you have a recommendation for sealing a gap between the outside brick chimney and the house. The gap is up to 1.5 inches wide. Posted by Jeff Sarsons on Oct 25, 2009 9:17pm
is a two part silicone sealant considered or certified as an air seal? Posted by philip solomon on Jul 2, 2010 3:21am

Can you recommend someone to assist with air sealing on a leaky post & beam house?

Posted by cferland on Dec 17, 2010 10:28am
Gosh ... isn't it nice how there are NEVER any responses? Posted by Anonymous on Mar 2, 2012 2:08pm

Hey Anonymous--

When someone asks a very local question, or one that entails an opinion (such as a contractor recommendation), it is our policy to respond privately (if, of course, the commentor leaves contact information.)


Posted by Peter Troast on Mar 2, 2012 3:42pm
The easiest way to make your home more energy efficient is to seal any air leaks, and one that is often overlooked is the bathroom ventilation fan and exhaust vent. The back-draft flap these units come with do a very poor job of stopping leaks. To address this issue, I use a replacement insert fan from the Larson Fan Company (online). Their fans has a true damper built in, that does a great job in keeping warm air in during the winter and hot, humid air out in the summer. This product has reduced my annual energy bills by over ten percent. It saves the most when air conditioning is being used. Posted by Steve Davidson on May 6, 2012 10:11pm

Thanks Steve. I hadn't seen the Larson Fans before, but we will definitely check them out. Our typical approach is to test products within our PRO network, where there are many companies/individuals with considerable expertise in ventilation. 

Posted by Peter Troast on May 7, 2012 8:00am
What to do about a house which is very old and badly built but which I cannot afford to do much to? The crawl space is so low that I got claustrophobic just looking in there so couldn't get in there with plastic to cover the soil. So I have been stuffing garbage bags full of recycled styrofoam in there. I've foamed the gap between the foundation and the siding but the room above is just full of drafts. The windows are fairly new and don't seem to be part of the problem. The house is on what is basically a zero lot line on the north and anyway I can't afford to strip the siding and insulate and then replace the siding. The roof is new. Can rooms be insulated from the INSIDE and a new wall put on the INSIDE over it? I realize this would mean a loss of a few inches of room space. I have been looking and looking and cannot find any information on this. I cannot afford to have anyone come out and do an audit as there is virtually no money so whatever I can scrounge has to go to DOING something that will help. Knowing what to do is not much use when you cannot make use of the information because of finances. I am a pensioner living on $1000 a month and utilities alone take up a good amount of that, even when I don't use them much.Thank God I don't have a mortgage. The cost of getting the services each month is usually higher by far than the amount of energy actually used, except in the coldest months of the year, when I move into one room and spend a lot of time in bed to keep warm. So anything I could perhaps do myself would be a huge help but I don't know what won't solve one problem but raise others. Posted by Anonymous on Oct 7, 2012 7:23am

@Anon: without knowing more, or what climate you are in, or having a professional do a full audit, it's hard to advise. But more often than not the easiest and highest impact measures start with air sealing and improved insulation in the attic. Adding insulation to the walls is possible, but in my experience rarely the highest return. And, depending on your state, there may be audits available at no cost. 

Posted by Peter Troast on Oct 7, 2012 8:51am
Are there any studies/reports available that judge the ROI on insulating around recessed lights? I'm talking specifically about the insulated boxes that are available that keep a safe distance from the housing, but seal off the huge air gaps from the attic space? They run about $7-10 each. I'm considering them for a house that has over 75 rec. lights throughout. Thanks!! Posted by Anonymous on Dec 3, 2012 7:36pm

@Anon--we're not aware of specific ROI studies on insulating can lights. I think the unique circumstances of each house make it hard to produce numbers in a generic way. But 75 is a lot of cans, and it is a fair assumption that they represent a significant source of air leakage and should be fixed. That said, a couple things to consider in addition to the cost of the light covers: though they are typically fire resistent, many of the one's made of mineral wool are not approved for use with hot lights like halogens and other incandescents. So your math may also need to consider bulb replacement. And what type of insulation you're working around in the attic has a lot to do with the labor component.

Here's a somewhat geeky article, Air Leakage in Recessed Lighting, from the PA Housing Research Center.

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