What’s the best way to insulate your attic? or How I learned to think like a child

U.S. EPA Air infiltration poster

Many of my posts come back to air sealing and insulating the attic. Why is that?

If you’ve done any searching about home weatherization, insulation, energy efficiency or related topics, you’ve probably come across the picture shown above. And for good reason – it clearly shows all the common sources of air leaking into (infiltration) and out of (exfiltration) your home.

One subtle part of the picture is that the size of the arrows represents the relative amounts of air leakage from each location. See all those big orange arrows going up into the attic? Those tell you that large amounts of warm air from your house leak into the attic during the winter. This is why all us energy geeks keep spouting about the importance of sealing up the attic before you waste you time on things like replacement windows, sealing electrical outlets, and so on. You can spend thousands of dollars and countless weekends working on all these other areas and it probably won’t improve your home’s energy efficiency as much as just focusing on your attic.

The “conventional” way of insulating your attic

The little inset picture in the upper right shows you the “envelope” of a typical house. This is supposed to be where the insulation and air-barriers (typically your walls and ceilings) encapsulate your house, like a warm jacket. Notice how the orange line encircles the house? That’s the goal. Think of it like a balloon’s surface. And just like a balloon, if you poke holes in it, the air will leak out.

This is a pretty easy concept that seems impossible for builders to understand. I could probably explain it to a child, and they’d understand. If you don’t want air leaking out, you don’t poke holes in the balloon! So why does every builder poke thousands of holes in the “balloon” that is your home’s envelope? It drives me crazy!

First, a warning

Air sealing is more important than insulation and must be done first.

Ignore this warning at your own risk. If you insulate without air sealing, you run the very real risk of ending up with a moldy attic and a rotten roof!

Here’s why

Pretend that you’re a child and you actually listen to what adults say sometimes.

Remember that balloon? Well, if you insulate without air sealing, it’s like taking a hole-filled balloon, filling it with water and sticking it into your jacket. Seriously. This is exactly analogous to insulating your attic without air sealing the envelope first. EXACTLY THE SAME. Except that insulating your attic without air sealing can lead to thousands of dollars of damage to your house.

If I seem grumpy today, well, I AM. I’m constantly getting calls from people with rotten roofs and moldy attic who can’t step back and think like a child. They just can’t believe that letting the warm, moist air from a house escape into the attic, 24-hours a day, 365 days a year, could lead to a moldy, rotten attic and roof.

The air in your home is filled with moisture. I don’t care if it’s the dead of winter and you’re getting two-inch long sparks jumping from your fingers after you pet the cat – the air in your home is still filled with moisture. You cook. You breath. You water plants. You take showers. All of these things dump gallons of water into the air and that warm, humid air floats up in your house. When it gets to the ceiling, it finds holes and moves through them just as easily as  you walk through your front door. Easier in fact.

A water molecule is really small. REALLY small. So small a million water molecules would cross a gap that is one one-hundredth of an inch. Think about that – the tiniest crack is one million times larger than a water molecule.

This is why you must install a continuous air barrier before you insulate. This is why you must seal all the holes in your ceiling before you insulate. This is why you must rid your house of excess moisture so it won’t float up and rot your roof.

The Tyranny of the Ridge Vent

Unfortunately, the solution that builders and architects have been taught to deal with this problem is to add ridge vents to the attic. Guess what? That can make the problem worse because most of the time there aren’t sufficient soffit vents to supply air for the ridge vents. So what happens? The ridge vent sucks even more warm, humid air from the house so your roof rots out even faster. When that doesn’t work, they tell you to add a roof fan and that not only sucks even more warm, humid air out of your house, it uses electricity and drives up your heating bills! Arghhhh.

All these solutions are about as effective as trying to cure a paper cut with a machete and just as painful.

This is why you must install a continuous air barrier before you insulate. This is why you must seal all the holes you’ve poked in your ceiling before you insulate. This is why you must rid your house of excess moisture so it won’t float up and rot your roof.

I’ll say it again – if you insulate without installing an air barrier, you run the risk of ending up with a moldy attic and a rotten roof. Read this last section 100 times until you believe it.

Let’s start by looking at some of the worst offenders and how to fix them….

Note: click on any image to enlarge it for a clearer view

Bath fan venting into attic – horrible practice!

The unvented or improperly vented bathroom

This is perhaps the number one cause of excess attic moisture, mold and roof rot. Unfortunately, building code allowed excluding bath fans if there was an operable window in the bathroom. Whose stupid idea was that? Really, the time that bathroom moisture really matters is in winter, and do you open the bathroom window in the winter? I don’t think so!

Any builder who builds this way deserves to have their license revoked. Or at least, they should be forced to pay the $20,000 when the roof rots out and the attic needs a complete mold remediation done.

I wrote an extensive article on this on my other website. Please read it here.

The attic hatch

Thermal image of leaky attic hatch

The thermal image shown here gives you an idea of what your attic hatch looks like to the cold air. See all those black whisps around the hatch? That’s showing all the cold air rushing in during a “blower door” test. During this test, the air is slightly sucked out of the house, so that the outside air sucks into the house through all the gaps (the holes in the balloon). Under normal conditions during the winter, the air would be leaking out of the house, into the attic.

In another article, I showed how you could fix this particularly heinous area of home construction in an afternoon, probably saving hundreds of dollars in heating and cooling costs and maybe a rotten roof. And if you want an excellent commercial product that is built for this purpose, check out the Energy Guardian. That’s the only commercial product I’ve evaluated/seen that tackles this problem in a way that actually works. The other products are essentially worthless, about as effective as laying a piece of fiberglass over the hatch (which does nothing but cover you with fiberglass every time you open the hatch). Just remember – you’re trying to seal all those holes that you’ve poked in the balloon (your home’s envelope).

The recessed light

Oh, how I hate recessed lights! Sure, they’re elegant, unobtrusive and modern, but when you’re an energy and building science geek, you learn to despise recessed lights because of all the problems they cause.

Remember that balloon? How the heck are you supposed to make an air tight balloon when you’ve got holes for fifty recessed lights? Impossible! And they’re really a pain to retrofit properly. Go up to your attic and look at where they’re installed. After you’ve slithered through fiberglass on your stomach to get access to one, then tried to build an airtight box around it, you’ll come to loathe them as I have!

The picture here clearly demonstrates the problem. See the lamp housing? See the hole in the ceiling? Notice how the hole is about 3/4 of an inch larger than the fixture? And what about those holes that they punch in the metal housing for attaching accessories?

The easiest way to deal with these fixtures is to retrofit them with an LED retrofit light like the CREE CR6 that I describe wrote about earlier. The housings of those retrofit units are vastly more airtight than any recessed light housing, even the ones sold as air-tight. They really lie about that! Do you think that everything would be airtight if the fixture were installed like in the picture above? Instead, get the CR6 and then install a gasket between the fixture and the ceiling. If it’s a good gasket, it will vastly cut down on the total air leakage.

If you spend any time retrofitting fixtures in the attic by building boxes around them and air sealing those boxes to the ceiling sheet rock, you will quickly agree with me that installing the $50 CR6 and sealing it up is a bargain compared to the amount of time, effort and suffering you do in order to properly retrofit a recessed light fixture.

Don’t believe me about recessed lights? See this document. In it, they say,

“How much of an impact do recessed cans have on the total air leakage picture of a house? It is estimated that one conventional (IC or non- IC) fixture can be responsible for the loss of between $5 and $30 per year worth of energy and can dump about one-third of a gallon of water daily into a cold attic. These estimates are based on actual measurements performed by the Mechanical Engineering Department at Penn State University”

Here’s an article that has some good pictures of dealing with recessed lights.

If you deal with the issues discussed so far, you’re well on your way to sealing your envelope so you can insulate effectively the conventional way – by adding insulation to the attic floor. You might start understanding why homes usually aren’t built this way – it takes a lot of attention to detail.

But we’re not done yet!

Chase running up to the attic

Plumbing and electrical chases

Remember, we’re trying to make that balloon air tight, so we have to find all those holes where air and moisture are escaping into the attic. If you look around your attic, you’ll probably find some pipes sticking up through the insulation and going out the roof. Or, sometimes, like in this photo, they’re part of some other system. Unfortunately, they represent big holes in your attic that allow air and moisture to travel through the walls and into the attic. And, since they’re usually located in the bathroom wall where the humidity is high, the super-high humidity shower air often goes right into the wall and up to the attic. Worse, they often share a wall cavity with water pipes which can lead to frozen pipes and serious water damage.

Sometimes, the holes you find will be quite large! In the winter, you can actually feel the warm air coming up from the house.

For the larger holes, you sometimes have to fill in the hole with another air-tight material, caulked to the ceiling, like sheet metal or a board. You can also do a pretty good job air sealing big gaps by filling plastic bags with insulation to make a pillow. You then shove it tightly into the holes, sealing them up. This is the 5-minute solution that costs virtually nothing yet works really well. Just don’t do this around chimneys or other potentially hot things like flue pipes!

Gaps between the wall framing and sheet rock

Gap between wall framing and sheetrock

Gap between wall framing and sheetrock

This problem is a lot more subtle, so I’ll give builders some slack for making this mistake. Unfortunately, it’s so prevalent that it leads to a lot of attic moisture and energy loss issues, especially when the walls surround a bathroom.

What you’re looking at in this picture is the top of the wall framing and the ceiling sheet rock. The gap between the wood and the sheet rock leads right down into the wall cavity.

While this might not seem like a big deal, remember that water molecules are several million times smaller than this gap. See all the blackened fiberglass pushed to the sides? That’s black because of all the air and moisture that moves through the gap.

To fix this, you need to clean off the area around the gap. That means pulling back the insulation and carefully sweeping or vacuuming all the dust away from the gap so it’s clean. Then, you can use canned spray foam or caulk to get into the gap and seal it up. Details make all the difference here. If you’re going to spend the time cleaning it, make sure you seal the gap well.


This article has been pretty extensive, and I’ve probably lost most of you by now. Hopefully though, you’ll take home the basic point – before you insulate, you have to spend a lot of time air sealing the “balloon” of your house, or that insulation will do little good. In fact, adding insulation without air sealing may lead to rotting out your roof.

In the next post, I’ll cover less well known solutions that are much easier to implement and more sure of achieving the results you actually want!

More reading material

Home Energy magazine – a wonderful, classic article about identifying the types of problems associated with various construction styles.

HouseEnergy.com – air sealing basics. Neatly organizes and describes all the different places where air leakage occurs around the house.

New York energy code air sealing document – great how-to information


10 thoughts on “What’s the best way to insulate your attic? or How I learned to think like a child

  1. I really liked your suggestion of sealing every single crack on the ceiling before insulating because the tiniest crack is a million times bigger than a water molecule and water can ruin the insulation. That is really informative! Perhaps, I need to mention this to my parents since they are planning to insulate the roof soon. At least, by sealing the ceiling, we can ensure that the project is successful and foolproof.

  2. I have a typical split entry home that is 26’x54′ and I have installed baffles and used blown in insulation approx 16″ deep, the question is should the gable end vents be blocked or covered? Recently I had removed them for cleaning and left them off for a few days to paint and noticed that the the roof had created some dips and valleys, wondering if this allows to much warm air in the hot summer creating moisture?

    • leaving them open would be recommended. You’d have to determine what causes the roof to distort. It could be the high humidity but that will occur with the vents open or closed. You could do a temporary test and cover the opening with heavy plastic for a couple weeks to see what happens. But even then it’s hard to draw definitive conclusions since the weather conditions will vary. Note that during the winter humidity problems are much more prevalent and you don’t want to do anything that might hurt your roof in the winter

  3. I appreciate your emphasis on air leakage. Doing all this great work on insulating will be for nothing if that air comes in with moisture and causes mold. Thanks for the great tips. Spray foam insulation is also great a sealing up all those cracks you may have missed!

  4. You mention: “Gaps between the wall framing and sheet rock”. Its really hard to tell from that picture where you mean the gap is. Perhaps you can edit this picture and circle or arrow to the areas that need sealing.

  5. Pingback: The best way to insulate your attic – part 2 « Ted's Energy Tips

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