Sometimes, attics and insulation can be confusing. Usually they make a lot of sense, but occasionally, they’re really confusing! So when people ask “how do I insulate my attic?” – the only correct answer is: “it depends…”

Suppose you purchased this house this past October. You get your first electric bill and it’s  really high, so, after reading Ted’s Energy Tips,  you go up to the attic to look for problems and find this. “Holy cow!” you think – not only is there a space heater up in the attic, there’s all this missing insulation.”

First, you unplug the space heater, then you start insulating. Since you know that a little bit of missing insulation can lead to lots of wasted energy, you do a careful job. You air seal all the holes and you add insulation so the entire attic is well insulated. Satisfied that you’ve done a great job, you eagerly await the bills for November. When you receive it, sure enough, it’s half what it was. “Yay!” you think, mission accomplished!

But then, during a cold snap just before Christmas, disaster strikes! A pipe bursts in the ceiling, raining thousands of gallons down into your house. The ceiling is ruined. The wood floors are warped. And it costs you thousands of dollars to repair the problems and your holidays are ruined. Ugh. What happened?

Take a close look at the photo above. See the black insulated pipes? When the previous owner did renovations, they got lazy and ran water pipes through the attic. But the pipes probably froze once, so they added a space heater to keep them warm. Plus, they removed insulation from the attic floor so that some heat would come up from the house and keep the attic warmer. What a dumb solution!

You did two things that led to the frozen pipes. First, you turned off the space heater. Second, you added insulation to the attic floor. Both of these are good solutions if the pipes weren’t in the attic, but now the attic was allowed to get really cold, so the pipes froze.

How should you fix this problem?

There are a few approaches to fixing this problem.

Solution 1: Re-route the pipes

The “right” solution is to re-route the pipes so that they’re not in the attic. Water pipes really, really don’t belong in the attic! If at all possible, they should be re-routed. Even if you have to rip open some walls and do some things that might be a little ugly in the house, it’s worth it. More houses are destroyed by water problems than fire..

The problem is, when they did the renovations, they made it extremely difficult to route the pipes. After consulting a plumber, you decide that you can’t do that. Even after reading the above paragraph, your plumber refuses to listen and insists that the pipes must stay where they are. So now what?

Solution 2: Insulate above the pipes

What causes pipes to freeze? They get cold. Why do they get cold? Because they’re exposed to the cold temperatures in the attic. You want to keep them warm, but you don’t want to “heat the attic” so you have to keep the pipes warm by insulating above the pipes and not let cold air come in contact with them.

The pipe running by the space heater is easy. You can just lay insulation batts in the space, making sure there’s no insulation between the pipe and the ceiling below. That allows that pipe to be “inside the thermal envelope” of the house. It’s like putting a sweater on – everything inside the sweater is closer to your body than the outside air, so it’s warm. But, just like a sweater, you don’t want any gaps where cold air could get in. Think about it. When you wear a sweater, you want it nice and snug. If it’s too loose, and cold air blows up at your waist, or down the neck, it’s really chilly! Insulation is exactly the same. So you have to ensure that no cold air can get under the insulation or it bypasses the insulation and the pipes will still freeze.

The second pipe is a much bigger problem. Notice that it runs on top of the ceiling beams. When you normally insulate, you insulate between the beams. That would be like putting something on the outside of the sweater, so the pipes will freeze.

This particular configuration is difficult to deal with properly. You could add a LOT of insulation on top of the beams, being careful not to insulate below the pipe where it runs. One way to do this would be to use rigid foam board and build a box around the pipe, so that the pipe is inside the box (inside the sweater). You have to pay attention to details because the box would have to run between each set of beams and be caulked or foamed to the ceiling below. Remember – you want the heat from the house to come up and warm the pipes without being lost to the attic. The trick is, every place the pipe runs over the beam, you have to be careful to insulate above, protecting it from the cold of the attic. It’s a lot of work and prone to installation error. Even small gaps could let the cold air in and allow the pipes to freeze.

What now?

Solution 2a: Insulate above the pipes with spray foam

Rather than using fiberglass, and going through all the trouble of manually sealing every little gap, which is nearly impossible, you could use spray foam.

The best thing would be to remove all the insulation from the attic floor and spray the entire floor with foam. Not only does it insulate much better than fiberglass, it air seals everything, getting in all the nooks and crannies. But be careful! Remember, there can’t be any insulation between the ceiling below and the pipes (putting the pipes outside the sweater). Many foam installers would miss this detail and spray lots of foam under the pipe running across the beams. That would allow that pipe to get cold and freeze. So you still have to create a barrier around the pipe so that no insulation is put in the space between the pipe and the ceiling below. However, unlike before, the main thing is that it just has to keep the insulation from getting in there – it doesn’t have to be perfect, so you could install cardboard or plastic over top of the pipe and tape it to the ceiling below, forming a little hat that protects the pipe and the space below it from filling with insulation.

Now, when they spray foam, it will go around the pipe, keeping the pipe on the right side of the insulation (inside the sweater).

The added advantage of this solution is that unlike fiberglass, which is easily removed, foam is essentially forever. It’s a solid mass that someone would have to chip away. With fiberglass, someone else might move it away from the pipes leading to frozen pipes again. With foam, you’ve created a permanent solution.

Solution 3: Change how the attic is insulated

This is a more radical solution that many people have a hard time understanding, but from the building-science perspective, it’s totally rational.

In this case, you’re changing the location of the “sweater.” Imagine that you’re face is getting cold. What do you do? You put on a hat and scarf. The analogy isn’t perfect, because you’ve still got some skin exposed and you’re breathing, so you need to leave some holes. But bear with me. Imagine that you pull the neck of the sweater up and above the top of your head then close the neck hole. That’s like insulating the attic. Now your head is inside the sweater. Or as we say in building science terms – inside the thermal envelope of the house.

The way you’d do this is to remove the insulation from the attic floor and move it up under the roof. Basically, you’d be turning the attic into part of the heated space of the house.

“What?!” you think, “now you’re heating all that attic space. Isn’t that really inefficient?”

In fact, no. If you do it right, it’s not bad and can save energy.

The attic ceiling of many homes is much easier to insulate than the attic floor. The attic floor might have pipes, ducts, electrical wires, attic stairs and so on. The ceiling (under the roof) is usually much easier.

Lots of people have written about this. So instead of rehashing what others have said, here’s some articles to read on this approach:

Creating a conditioned attic

Building Science FAQ – Conditioned attics

Can Conditioned Attics be Too Big?

Conclusion

Hopefully, I’ve done my job well enough that you have a better idea of how to insulate attics. Sometimes it’s easy, but other times, your best intentions can lead to disasters. By understanding the way to think about insulation, you can avoid many of the common pitfalls people make when insulating.

This is in no way a complete discussion of all the issues that can arise, but it’s a start. There are other problems that occur associated with attic insulation, such as condensation in the attic. But those are food for more posts.