One of the hottest topics in energy efficiency and building science is “how should you insulate your attic?” Why? Simply put, the attic has more impact on your efficiency and comfort than any other single part of your home!
Let’s summarize why the attic is so important:
The attic is the hottest part of the house in the summer and is cold in the winter
Hot air rises up to the attic / cold falls drops into the living space
Moisture rises and accumulates in the attic
Central heating/AC systems and ductwork are often in the attic
Fiberglass insulation against roof deck plus moisture = roof failure
In the first post, we looked at how adding insulation could lead to frozen pipes if the insulation was put in the wrong location. But, just like a sweater, if you put everything you want to be warm inside the insulation, you can keep your house and pipes happy and energy efficient.
This time, we’re looking at how to install insulation properly so that you don’t rot out your roof. Unfortunately, the photo above shows how not to insulate under your roof!
In the first two posts of this overly wordy series, we saw a few ways to insulate an attic while avoiding some of the worst problems that can lead to moldy, rotten attics and roofs.
If you recall, the big problem is that moisture from the house rises up through the walls and all the little cracks around light fixtures, hatches, wiring, and the moisture condenses on cool surfaces. Over time, this will lead to mold growth and potentially, rotten roofs.
How do you know if you’ve got a problem? I’ll give you a hint – if you have ice forming under your roof like in this picture, you had better do something before you have to replace your roof!
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. Continue reading →
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.
A properly insulated attic is supposed to have about R-40 insulation everywhere. This means that the insulation reduces heat loss by a factor of 40 – pretty simple eh? That also means, R-20 roughly equals twice the heat loss as R-40. R-10 is four times the heat loss and so on.
So what’s the R-value of an area of ceiling with no insulation? As it turns out, bare sheetrock on the ceiling has an R-value of about R-1. This means that every square foot of uninsulated ceiling loses about forty times as much energy as a square foot of properly insulated ceiling!
Put another way, if you have one square foot of uninsulated ceiling, it’s losing as much energy as forty square feet of normally insulated ceiling. So what happens when you have a hundred square feet that are uninsulated, like in this photo? Well, the energy loss from this section of attic are about what the energy loss of a 4,000 SF attic would be!
The take home message is – details matter! Every square inch of your attic should be fully insulated. And if you’re an electrician, take the time to put insulation back carefully after you’re done running wires in the attic. If you don’t you’re basically robbing your customers.