Q: We need insulation for the attic in our 1920s home, which currently has no insulation at all. I have heard about bio-based foam insulation, but the company I am considering says their R-rating is about 23, and I have heard that ideally it should be R-49. The person who came to give an estimate said that it actually performs about double the R number so I am confused if this is a more green and efficient insulation, compared to other forms such as blown-in fiberglass, which offers a rating of 49. Which kind would you recommend?
...Other issues are VOCs, which they say are low because it's based on soybean oil, but I don't have the number for that either.
Sent by Amy
Editor: Here's what our friends at Green Home Guide say.
Answered by Andy Ault, CLC, Little River Carpentry, LLC
Wow, where to start? By my count, you actually have 8 critical questions packed into that one small paragraph. Here's how I break them out:
- How should we deal with an existing uninsulated attic?
- What's the real deal with so-called "bio based" insulation?
- How can I verify the R-values claimed by an insulation salesman?
- What is the correct R-value for my area of the country?
- I am confused about green vs. efficient?
- How do the various types of insulation compare to each other?
- What concerns should I have about insulation VOCs in my home?
- Given all of those factors, what product(s) should I be considering?
These are all exactly the right questions to be asking before spending thousands of dollars to put lots of potentially helpful or harmful materials into your home. And the wildly exaggerated marketing claims made by manufacturers don't help clear things up at all.
So let's put "marketing science" aside for now and focus on some "building science 101."
Building Science 101
At the base level, all insulation materials work the same way. They trap air in one form or another to create pockets which resist the flow of heat. The level of resistance they create is what we call an R-value. The higher the R-value, the higher the level of heat resistance.
Next, heat does NOT rise. The thermodynamics of heat are that it will always seek cold. The heat doesn't care whether that is up, down, or sideways. Hot AIR will rise because it is lighter (less dense) than cold air. But don't let someone tell you that heat itself rises because this is false.
The reason the directional flow of heat matters to you and your home has to do with how your home's "envelope" will perform throughout the year. Here in the mid-Atlantic region, where you are, we have it tougher than many other areas of the country when it comes to how we insulate.
- In northern climates, they are predominately worried about winter heat escaping from their home to the outdoors.
- In southern climates, they are trying to prevent the outdoor heat from coming into their homes.
- Here in our mid-Atlantic area, we are referred to as a "mixed-humid climate." This means -- for your home in Alexandria, Virginia -- you have to think about heat trying to both escape AND come in depending on the season.
The fact that in a mixed-humid climate we have to consider multiple directions of heat flows and building behaviors makes it fairly difficult, but not impossible, to develop the right balance of:
- trapped air (or heat resistance value) and
- air permeability (what helps our homes dry during the very humid summers).
If there are large volumes of uncontrolled air being freely exchanged between our homes and the outdoors (which is usually the case with homes built in 1926), it doesn't make any difference what insulation strategy you choose because none of them will work very well. Again, for any form of insulation to be effective, it must trap the air into pockets. So if the air is moving too freely, then the insulation won't be able to achieve its stated R-value.
So, now that you have a feel for how insulation works, how heat moves, and the effect that air movement has on those two factors, you probably know more about thermodynamics than half of the salespeople you're likely to encounter.
Now let's look at your specific questions...