In the first installment on attic insulation, I discussed why it can be dangerous to add insulation to your attic without air sealing the attic floor first. Moisture can slip through tiny cracks in the attic floor and lead to rotten roofs. Given this information, we walked through the process of finding and sealing all those insidious air leaks in your attic, some easy, some difficult. But finally, after fixing all these problems, you could lay more insulation down on your attic floor, more confident that doing so wouldn’t lead to a humid, moldy attic.
But what if there’s an easier way?
Whether you’re building a new house or retrofitting an older one, you can make life much easier on yourself by using professionally applied spray foam insulation that air seals and insulates in one shot. There are two ways of doing this, each with their own benefits and disadvantages. We’re going to review both methods. One is spraying foam on the attic floor, instead of using loose fill or batt insulation. The other is spraying foam under the roof deck.
Spray foam on the attic floor
Recall all those little (and not so little) cracks shown in the photos from the last article? Those cracks that you have to painstakingly caulk or use canned foam to seal? Using spray foam, you don’t have to worry about any of these, making them much more likely to be dealt with correctly. They just spray a nice thick layer of foam all over the attic floor and it air seals every area it covers. Plus, it gives you high quality insulation that isn’t going to get kicked aside every time you go up to the attic.
The great benefit of this method is that it eliminates the fine detail work. You spray foam and it seals. Case closed. Almost. You still have to worry about the details. For example, some recessed lights can’t have insulation within three inches of the housing. Others can be insulated, but shouldn’t be foamed over.
You also still have to properly seal your attic hatch or all that other air sealing will be wasted because the gaps at the attic hatch are usually so huge. And so on. In addition, there are two types of foam products: open cell and closed cell. They have very different properties that you should know about.
Consistency: Open cell foam is soft like a sponge or the foam in your sofa. Closed cell foam is hard. Open cell foam tends not to shrink over time. Closed cell foam can shrink as it “cures”, but a proper job shouldn’t shrink more than a tiny amount but you may see rips in it in some places where it’s torn itself apart.
R-value: Open cell foam has an R-value around R-3.5 to R-4 per inch, a little better than fiberglass. Closed cell foam has an R-value of R-6 to R-7 per inch. Cost: Open cell foam is typically somewhat less expensive to install for a given R-value than closed cell foam.
Moisture permeability: this is an important one, unfortunately, it’s complicated. Open cell foam lets much more water vapor through than closed cell foam. Sometimes this is a positive, other times its a negative. Recall that an important part of the insulation process is air sealing so that the moisture doesn’t flood out of the house and into the attic. If that’s the case, then wouldn’t open cell foam be bad? Not exactly. Most of the water is carried along with the air, so if you block the air movement, you dramatically reduce the amount of water movement. Be careful – I didn’t say it would stop water movement, I said dramatically reduce it.
Open cell foam dramatically reduces the amount of moisture that escapes from the house to the attic by blocking air movement, but some water still passes through because of the high “vapor permeability” of the open cell foam. We’ll come back to why this is so important later. In most cases, open cell foam on the attic floor is vastly better than fiberglass, cellulose or any “loose fill” product. Remember, you’ll still needed a ventilated attic, just like you do with any normal insulation product because you want to allow it to flush out any moisture that does get in there.
Spray foam under the roof deck (the ‘ceiling’ of the attic)
The second application of spray foam is directly under the roof deck and around the attic, encasing the entire attic in foam.
Why would you want to insulate the ceiling of the attic rather than the floor? Take a look at the accompanying photo. This is a bedroom with a cathedral ceiling. There is no attic in the conventional sense because the sheet rock screws right to the rafters. But really, this is no different than an attic.
Suppose the room had a conventional flat ceiling installed just above the windows. The space above would be the attic. Remember all the painstaking work required to seal the ceiling as discussed in the last article? If you insulate directly under the roof and on the walls like shown here, you don’t have to worry about any of that because the attic is part of the “conditioned space” of the house. That means that the attic is roughly the same temperature and humidity as the rest of the house.
This is a great solution because you’re not worrying about filling in every little crack on the attic floor. You don’t care about having your air conditioner and ductwork in the “hot” attic. You can store your boxes in the attic and you won’t sweat (or freeze) – it’s comfortable in there! There are so many advantages to doing it this way, so why isn’t every house built this way?
The problem with this method is that it can be done wrong and, even when it’s done right, people blame it for major home problems. Many builders are “once burned, twice shy.” And there’s lots of misinformation or poor understanding of how to do it properly. Let’s examine some common myths/complaints/worries about insulating directly under the roof:
- If you don’t ventilate the roof, it will overheat and shorten the life of the shingles.
- If you have to replace your roof, it will ruin your insulation and you’ll have to do it again.
- If you don’t ventilate your attic, it will get too hot.
- If you get a roof leak, the water will be trapped in the insulation and your roof will rot out.
- If you don’t ventilate under the roof, you’ll get ice dams.
- If you insulate your roof improperly, it will rot out.
Let’s get the easy ones out of the way first…
- Shingle life: studies have definitively shown that a fully ventilated attic will only cool the shingles slightly compared to an insulated roof. Research shows that the color of the shingles and the direction which the roof faces has a far greater effect on shingle temperature. Next.
- If you replace your roof…: Why do you replace your roof? Because it has rotted out! You only replace your roof decking when something has gone seriously wrong. Insulating under the roof properly significantly reduces the chance of your roof rotting out. Next!
- Your attic will get too hot: um, how hot is your attic now? Probably 130 degrees. How hot will it be after insulating under the roof properly? It should be about the same temperature as the rest of your house, maybe 10 degrees warmer. Next.
The other issues get a bit more complicated.
Roof leaks It is true that roof leaks can be problematic. With closed cell foam applied directly under the roof deck, it is possible that water from a leak could repeatedly wet the roof deck, rotting out a section over time. Additionally, such leaks can be difficult to find. With open-cell foam, it is said that the water soaks through the foam then drips on your ceiling or the attic floor so the source of the leak would be visible. However, open cell foam lets interior moisture through and can lead to a rotten roof, so it shouldn’t be used in this application.
Ice dams Ice dams occur when the bottom layer of snow melts. The water then drips down the roof and freezes closer to the edge of the roof where it is colder. Most ice dams are caused by warm air escaping from the house that heats the roof deck, melting the snow. You often see this above recessed lights both due to air leaks and heat from the light.
With sufficient spray foam under the roof deck, it is very unlikely to have significant warming of the roof deck from below. However in cold, snowy climates, where snow sits on the roof for months, conditions can permit ice dams, even with considerable insulation. However, in these climates, conventional insulation would be vastly worse.
It can be very beneficial to have an air channel between the roof deck and the foam combined with proper soffit and ridge venting. This allows cold air to flow, keeping the roof deck cold which minimizes the chance of ice dams. A detailed description of ice dams can be found here. The key thing is that you have to do it in a way that’s right for your climate. What works in Philadelphia might not be appropriate for Minneapolis or Atlanta.
Rotten roof – I wrote about this before, but lets look at the common cause of rotten roofs – moisture from inside the house reaching the cold roof deck and condensing. How does this happen? Recall from the last article, warm, humid air from the house moves up into the attic through any little crack. If the roof and attic spaces are insulated with closed cell foam as shown in the photo above, most of the attic is warm enough that condensation cannot occur – except at the wooden parts which provide bridges for the cold.
In moderate climates, this isn’t a problem, however if the humidity is high enough, the wood can get cold enough for condensation to form. If you use open cell foam, moisture passes through much more easily than closed cell foam. So it is possible for enough moisture to move through the foam to cause condensation problems at the cold wood of the roof deck. It is a very bad idea in most climates to use open cell foam directly under the roof deck. See BuildingScience.com – Unvented roof assemblies BuildingScience.com – Ice dams Green Building Advisor – Can spray foam rot your roof?
In the next post, I’m going to show you a practical roof design that eliminates virtually all of these problems….