Thoughts of Home: Put a Lid On It

Attic insulation and air-sealing tips for professional home performance contractors

This is the third edition of Efficiency Vermont’s “Thoughts of Home” series as written by Dave Keefe. It is intended for building science professionals in Vermont and not intended to be used as guidance or assistance directly for homeowners. Comments should be directed to

Attics are miserable places. They’re cramped and dark, dusty and dirty. They’re hard to get into and hard to get out of. Most of the time they are either too hot or too cold. There’s nothing to stand on, even if there is room to stand, which there isn’t. Often there isn’t even room to sit, even if there is something to sit on, which there isn’t. Nails are sticking through the ceiling. If you step in the wrong place you fall through. The air quality is awful. There’s a bunch of old nasty insulation in the way, some of it asbestos. There are hornets up there. And mice, bats, and pigeons. And the feces thereof.

So why would anyone want to go up there? Trick question—no one wants to go up there. But home performance contractors do go up there. It’s where the action is.

Attics are simple in theory. Make the attic floor airtight and install a nice thick layer of insulation. But reality is more inconvenient. As the saying goes, theory and practice are the same in theory, but not in practice. So let’s explore a few attic details and how to deal with them.

The opportunity

Sometimes air leaks are hard to get to, and you wonder whether it is worth the trouble. One way to think about this is that all the leaks are going to be much harder to fix once the cellulose blower is on. Chances are that if the leaks aren’t fixed now, they will never get fixed. Since the work done in the attic should be good for the life of the building, any leak left unaddressed will probably be leaking for decades to come.

The leaks that are missed (or neglected) are more likely to cause moisture problems than they were before this work. The attic will run colder now, so the surfaces will be more at risk for condensation. The house air is usually more humid after attic work because the house is tighter. Any leaks left will cause wetter air to hit colder surfaces.


Before going up there, take a look from below. Where are the critical spots? If the old insulation isn’t removed, make a map. Draw the partitions. Locate the major landmarks—the chimney, the hatch, and so on. Mark where the ceiling fixtures are and any other places that will need sealing. Then when you are on your knees in the dark you won’t have to root through the old insulation everywhere, just where the leaks are.

A clean slate

If the budget allows, removing all the old stuff first and starting fresh is just heaven on earth from an attic retrofitter’s point of view (admittedly a low bar). That way you can get at everything and do it right. You’ll have improved working conditions, better adhesion for the sealants, greater speed, and probably better R-value when you’re done, since you’re getting the less effective insulation out of the way.

The hatch

Small hatches should be insulated with foam and weather-stripped. The bigger the hatch, the more the R-value matters. Pull-down stairs are essentially impossible to insulate and weather-strip, so contractors build insulated boxes over them. If the hatch isn’t down near the ceiling but instead up near the top of the dam, pay attention to the sides of the dam near the top. If the dam isn’t insulated, it can be a weak link.

The fans

The house will be tighter when you’re done—maybe a lot tighter. The home will need more ventilation than it used to. Right now is the cheapest and easiest time to replace the bath fan, instead of after you’ve sealed it in and covered it with a bunch of cellulose. Besides, you are likely redoing the ducting anyway with something that’s smooth-walled and airtight. Try to convince the homeowners that now is the time to upgrade to a quiet, efficient fan that they won’t need to replace a year down the road.

The chimney

Only non-combustible materials apply here. Use sheet metal (aluminum or galvanized steel) and high-temp caulk for the air seal around the chimney. Appropriate clearances to combustibles depend on the chimney material; for instance, it’s three inches for masonry chimneys. Pay attention to the framing around the chimney, because there are several seams where the air is trying to get around your air barrier by sneaking between the two-bys. (If you are unconvinced, depressurize the house and puff some smoke around the base of the chimney after you think you have gotten everything. It can be humbling.)

Recessed lights

If they’re not IC (insulation contact) rated, replacing them is best. It’s a good time to upgrade to LEDs. You can get surface-mounted LEDs that look a lot like recessed cans. If the lights are IC rated and you’re not replacing them, make them airtight and heap a bunch of insulation on them so they run warm and don’t condense.

The eaves

Eaves are hard to get to, so they take some extra effort. In the area just above the wall top plate, you want as much R-value as possible because there’s not much height. Don’t stuff a chunk of fiberglass here—that’s basically using the least-effective insulation available and installing it incorrectly in a critical location. It’s best to provide a vertical barrier above the outside edge of the wall in order to get good coverage over the top plate. You should air-seal the top plate also. This is a pain to do, but it’s possible, chest-down on your plywood working deck, even with a 3/12 or 4/12. Some contractors spray foam in this area from top plate to vent chute to get a good air seal and the most R-value possible where the height is limited.


We know that attic ventilation doesn’t remove moisture very effectively, but that doesn’t mean it’s worthless. It can be beneficial, and many folks think it’s really important. That’s why it’s in the codes. So refrain from blocking off any preexisting ventilation as you are working on the insulation. If there is a moisture problem in the attic after your work and you have blocked off the ventilation that was there, there’s a good chance you will be blamed.

Checking your results with pressure

You’ve found and fixed a bunch of leaks in the attic floor, and now you are going to cover everything 18 inches deep. A couple of hours from now it will be a lot harder to get at anything you might have missed. Wouldn’t it be nice if you could verify that you got everything before you covered it all up? Well, you can. Depressurize the house to 50 Pascals (50 Pa) and measure the pressure difference between the attic and the house. You want the attic to be “outside.” There’s a 50 Pa pressure difference between the house and outside, so any space that is 50 Pa different from the house is outside. If the pressure difference to the attic is less than 50 Pa, then the attic is less than completely outside. This is the basic idea behind what is called zone pressure diagnostics.

If you measure that pressure difference before you start and then again along the way, you can tell how much progress you have made. Let’s say you start with only 25 Pa between the house and attic. This means the attic is half inside and half outside. As you air-seal the attic floor, the pressure difference will get larger. If you move from 25 to 35, you have made progress but are not there yet. If you move from 25 to 49, maybe it’s time to turn the cellulose blower on.

Certain things can give you the wrong impression. For instance, the number you get depends not just on the air leakage in the attic floor, but also the air leakage through the barrier between the attic and outdoors. If there is a lot of venting, the attic can appear to be mostly outside even if the attic floor isn’t all that tight.

And the baseline pressures matter. If it’s cold and the stack effect is strong, you should take baselines. Leave a hose hanging down from the attic through the hatch. Measure the baseline pressures at the blower door and at the hatch. Change the house / outside pressure by 50 Pa and measure how much the attic / house pressure changes. It’s all about the change. When you change the house / outside pressure by 50, how much does the attic / house pressure change? That’s the number to use.

This is also a handy technique for attached or tuck-under garages. Again, you want the space to behave as if it is outside. If you’ve never tried this, ask your Efficiency Vermont assigned consultant to show you how it works. The person who does your quality assurance inspections routinely measures these pressures—you can, too.

Top-floor interior partitions

If you’ve done a good job of air-sealing, there should be no airflow between the partitions and the attic. This is pretty easy to check. Establish a 50 Pa pressure difference between inside and out, then check the partitions. Under depressurization, there should be no flow at outlets or baseboards. Under positive pressure there should be no smoke drawn in at those locations. If you depressurize for a few minutes and then look with infrared, you can tell if air is being drawn down by the temperature of the individual wall bays, especially at the top. This works as long as you have a temperature difference in either direction (it works great on summer afternoons, not just in cold weather). If you check this before you go into the attic, you can get a pretty good idea of where to focus your efforts, and if you check it afterward, you can verify that you addressed those leaks. This is another thing that your QA inspector checks on most visits.

Of course, you need a blower door to run this test. So if you manage a company that has people sealing attic floors, you might consider giving them the means to check their own work. (This spring, Efficiency Vermont is offering free training on blower door diagnostics to employees of current Home Performance contractors.)


When you’re in the far corner of the attic and something needs to be fixed, but it’s going to be hard and no one is ever going to see it, it takes a certain strength of character to decide that you are going to do it. Not everybody can do this work.

If you are one of the ones who can and does, one of the folks who crawl into the unpleasant places and do the difficult work that will never been seen, you might feel like an unsung hero. But your work makes a difference, not only to your clients and Efficiency Vermont, but for the state as a whole. Please keep it up. Vermont needs you.

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