As an insulation contractor, I have conversations about cost vs. performance of spray foam on an almost daily basis. People are concerned about spending extra money for closed cell spray foam insulation, and want to be sure it is worth the additional cost. Believe me, I can appreciate their concern. No doubt about it, closed cell spray foam is more expensive than traditional forms of insulation such as fiberglass and cellulose. Inevitably, the conversation always turns to “R-value.”
I can’t tell you how many times I’ve been scoffed at by a potential customer comparing price per R-value of an “R-19/R30” fiberglass job bid to an “R-7/R-13” spray foam job bid. They have a hard time believing me when I say that when you go with closed cell spray foam, you don’t need nearly as much “R-value.” It’s as if I were trying to pass closed cell spray foam off as some sort of “magical miracle material.” Or maybe they think I am trying to swindle them? What’s ironic about that is I am actually trying to save them money, while providing them with a superior insulation material! I understand closed cell spray foam is a fairly expensive product, and to install it to a R-value specification developed and recommended for poor performing insulation like fiberglass would price it right out of the game for many people.
But the truth is that R-value is not the only factor to consider with insulation. In fact, it’s only one of many factors involved in the science of properly insulating a building. Unfortunately, “R-value” seems to be all anyone thinks about.
It’s mostly due to an industry with severely outdated standards, designed for outdated materials. Building code is notorious for being slow to accept change, and spray foam insulation technology is relatively new to construction, compared to older insulation materials. It’s also due in part to me not being able to adequately educate “would-be” customers on the science of insulating a building. Hopefully this article will shed some light on the subject.
“R-value”, in simple terms, is a measurement of how poorly a material conducts heat. The higher the number, the worse it is at transferring heat from one side to the other. So the more “R-value” the better, right? Not necessarily, and here’s why:
“R-value” ratings are determined using a standardized lab test, and “R-value” is strictly a measurement of “a material’s resistance to conductive heat transfer.” Don’t get me wrong, this is an important factor, but it’s not the only way heat is transferred in or out of a building. In fact, there are three primary ways this happens.
Conduction is the transfer of heat within a solid object or between solid objects in direct contact with each other. Think about frying bacon on your stove. When the pan is placed on the hot stove burner, the bottom of the pan gets hot. If you were to touch the inside of the pan after just a few seconds, it would already be hot. This is because your pan is a good conductor of heat. It would have a very low “R-value”. Now, when the bacon is placed in the hot pan, it begins to heat up and cook. Again, this is conduction of heat from the pan into the bacon, which is in direct contact with the hot pan. (Coincidentally, bacon would also have a fairly low “R-value.”) This is the only type of heat transfer that “R-value” pertains to. I will explain why this is a nearly irrelevant factor for comparing conventional insulation to spray foam in just a minute.
Convective heat transfer is what happens when a liquid or gas (air) comes in contact with something that is a different temperature. Think about a steaming hot cup of coffee or tea. If you were to try to drink it while it’s 140 degrees, it would transfer heat through convection into your mouth and burn your tongue! So instead, we take the lid off and blow air that is cooler than the coffee across the top, which in turn cools it down quicker than if we were to just let it sit there. The air you blow across the drink is transferring heat away from it by convection. What does this have to do with insulation in your house? Believe it or not, almost every house or building that has ever been built is full of small openings that air is able to pass through. Cracks between siding and window and door frames, soffit and roof vents, etc. (Don’t even get me started on metal buildings!) In the building science community, it is understood that a typical home may have so many places for air to leak that, if they were all put together, would be as big as an open door! Imagine leaving your front door open 24/7/365! Seems like it would waste a lot of energy and money, right? (More about that later…)
Radiation is when heat is transferred from one object to another through electro-magnetic waves. Let’s think about our microwave ovens for a second. When I was a little kid, I thought they operated on principles of magic. Well, as it turns out, they use electro-magnetic waves to transfer heat into my leftover baked potato. Now lets think about the hot sun in the sky, beaming down on us here in Texas almost every day of the year. Surely, all of us Texans know that in the summertime, the best parking spot is not necessarily the one closest to the front door, but rather the spot with the most shade. We can all relate to getting into a hot car or truck that has been parked in the sun, and getting burned by every surface inside the car that touches your skin. This is thanks to heat transferred into the car from the sun by Radiation. When you park in the shade, the radiation from the sun is blocked from reaching your vehicle. The same radiation is also transmitting heat to your house, which is absorbed by your roof, walls, and through windows. When you stand next to a wall that is warmer or cooler than the air temperature in the room, you can feel it. If the wall is hotter, you can feel the heat radiate to your body, making you warmer. If it’s colder, you can feel your body heat radiating to the wall, making you feel colder.
It is important to understand that heat will always make its way into cooler objects. This is an unstoppable force of nature! Even though we cannot completely stop it, we can slow it down. This is where insulation comes into the picture. Insulation’s job is to slow down the transfer of heat from the outside of an object (in our case, a house or building) to the inside.
Still with me? I know it’s a lot of information, but it’s important to know if you want to make an informed choice on insulation. Imagine if you were shopping for a new work truck, and you only focused on miles per gallon. The window sticker on your new truck claims an EPA rating of 20 mpg. When you load the truck up with a 4,000 lb. trailer, and check your mpg, it’s more like 10 mpg! What gives?! Turns out the EPA rating is done in a laboratory, with no weight in the truck, and doesn’t take into account wind resistance or any other real world factors. This would be like comparing insulation based strictly on “R-value” and price. It may look good on paper, with data collected from a lab test, but the real world results usually tell a different story.
R-value recommendations vary across the country, mainly based on the average climate of an area. Generally speaking, the colder the climate, the more insulation is recommended. (Coincidentally, traditional insulation like fiberglass and cellulose lose a tremendous amount of “R-value” as the temperature gets colder. More about this later…) Here in Central Texas, R-value recommendations are at minimum R-13 for walls and R-30 for ceilings. This is the lowest recommended value on the chart, because as we all know, it is HOT in Central Texas, for the majority of the year.
Here’s what is important to understand about “R-value”. As I mentioned before, it is strictly a measurement of how poorly a material transfers heat by CONDUCTION. It does not give any indication of how well the material blocks radiant heat flow, and more importantly, air movement that causes convection heat flow. Convection plays a MAJOR part in how insulation performs in the real world.
What is interesting is that an R-value of R-8 will reduce conductive heat flow by 90% compared to no insulation. That’s a huge improvement with a relatively small R-value! Double that to an R-16, and now conductive heat flow is reduced to 95% compared to no insulation. Double that again to an R-32, and conductive heat flow is reduced by 97% compared to no insulation.
Notice a trend here? We quadrupled the amount of insulation, from R-8 to R-32, and thus quadrupled the cost of materials, not to mention the added labor cost. But only gained an additional 7% improvement. Adding R-value follows a law of diminishing returns. So why do they recommend so much insulation? These recommendations are written with fiberglass batts and loose fill insulation in mind.
(For those of you who are Science Aficionados like me, I am working on a more detailed article on the science of thermal dynamics (heat flow) that explains R-value and its diminishing returns in more detail.)
Spun fiberglass and loose fill have long been the standards for cheap insulation. Before they hit the scene, there was basically no insulation used in buildings and houses at all! They were a big improvement, but these types of insulation have some fundamental flaws.
The biggest problem is that air can readily pass through them. In fact, spun fiberglass is also commonly used as an air filter medium, because air can pass through it so easily, and it does a good job at collecting the dirt, dust, pollen, etc. from that air. (Some insulation experts refer to it as “filterglass”.) Think about blowing on the coffee we talked about earlier. Guess what happens to the “R-value” of an insulation while air is flowing through it? If you guessed that it drops down to practically nothing, then you are right! Remember earlier, when we were talking about convective heat transfer, and all the air leaks that a typical building or house has? Think about what happens when the wind blows into all those cracks and crevices. It flows through these types of insulation and ends up exchanging heat with the air inside your house (the air you paid good money to condition.) This is known as “Air Intrusion”. In the winter this means your warm, inside air gets pushed outside and replaced with cold winter air. In the summer, it’s your nice, cool air being traded for hot summer air.
Convection also takes place within insulation types that air can pass through in the form of “Convection Loops”. This happens when the temperatures are different from one side of the insulation to the other. The greater the difference from one side to the other, the more heat is transferred through the convection loop. When the outside air is 30 degrees, and its 70 degrees in your house, that equates to a 40 degree temperature differential, and can result in a 25% reduction in “R-value” just by convection loops! Air touching the warmer side begins to rise, while the air in contact with the cooler side will fall. This creates a loop within the insulation and effectively transfers heat from the warm side to the cool side. This is the opposite of what we want to happen, and again, reduces the real world “R-value”.
Another factor that reduces an insulation materials R-value is moisture content. When conventional insulation absorbs moisture, (which it will, because the air passing through it is loaded with moisture), it now conducts heat much better. Think about putting a cool, wet rag on your forehead when you have a fever. If it were dry, it wouldn’t cool you down nearly as well, because it doesn’t conduct heat as well. Remember that “R-value” is how poorly a material conducts heat? So by adding moisture to the rag, we improve its ability to conduct heat, thus lowering its “R-value.” Cool rag on your forehead + moisture = good. Building insulation + moisture = bad.
Loose fill insulation, or “blown in insulation”, is typically installed on attic floors. Cellulose, fiberglass, and rock wool are the most common forms. They are popular among builders because they can create a very high “R-value” for a relatively low price. Once again, their “R-value” rating is seriously reduced by the same real world conditions; moisture, air intrusion, convection loops, etc. What is also important to know about loose fill insulation, is that the “R-value” rating assumes that it is actually “loosely filled.” Over time, gravity pulls the insulation down and compacts it, which also causes a major reduction of its “R-value”.
Hopefully by now you are beginning to understand what I mean when I say “R-value is not as critical as you think.” No doubt it plays an important part in the science of insulation, but certainly should not be the only factor to consider when comparing options.
So what does all of this mean to me and recommended “R-values”? Well it’s pretty simple, really. These recommendations are meant to try and compensate for the factors that reduce real world “R-value” of traditional types of insulation. It’s similar to putting on extra shirts when it’s cold AND windy to compensate for the “wind chill.” Putting on five long sleeve shirts will help you stay fairly warm, but you will still have cold wind coming in around your collar, sleeves, etc. Now imagine having a full body suit that completely seals out any wind. Wouldn’t need as many shirts to stay warm, would you? If any of you have ever ridden a motorcycle on a cold day, then you know exactly what I’m talking about!
So how do all of these factors affect closed cell spray foam? Glad you asked! Closed cell spray foam eliminates all the factors that reduce “R-value” of traditional insulation in the real world;
- Remember that convection is when a liquid or gas (air) moves across something and transfers heat? Remember all those cracks and crevices for wind to leak through that we talked about earlier? Spray foam fills these voids, and expands in place to ensure that “Air Intrusion” is stopped in its tracks, keeping outside air outside, and conditioned air inside. This is probably one of the biggest improvements of spray foam insulation over fiberglass and “blown in” insulation. As an added benefit, filling these voids also blocks all of the dust/dirt/pollen from entering these places, not to mention bugs, rodents, etc.
- So what about “Convection Loops”? The only “loops” in a spray foamed structure are the fruity ones in a cereal bowl! Since the interior surface temperature of the spray foam insulation is typically the same as the air temperature inside the room, there is no difference in temperature to create a convection loop. Furthermore, convection loops cannot occur inside the foam itself, because spray foam is considered an “air barrier”. Air is unable to pass through it, and air is unable to flow inside it.
- Well what about moisture content? Closed cell spray foam is considered a Class II Vapor Retarder at a thickness as little as 1″. What this means in a nutshell is that closed cell spray foam will only allow an EXTREMELY SMALL amount of moisture vapor to pass through it, and thus will never be able to accumulate any significant amount of moisture.
- While radiant heat transfer is not the worst offender of all, it is still a factor. Out of the three primary forms of heat transfer, radiant heat energy has the hardest time getting from one object to another. This principle is actually what makes closed cell spray foam such an outstanding insulation. Closed cell spray foam is formed by creating millions of tiny bubbles (cells) out of plastic resin. At least 90% of these bubbles are completely sealed up from each other(which is also what makes it such a good “Vapor Retarder”.) These bubbles are full of a special, inert gas, which conducts heat EVEN WORSE than regular, plain old air. Heat must pass from one side of each bubble to the other side by radiation, which it has a very tough time doing. This is why closed cell spray foam has such a high “R-value” per each inch of thickness. (Open cell spray foam, or 1/2 pound spray foam is not the same as closed cell spray foam. The majority of bubbles are not sealed up from each other, and they are filled with regular air from the atmosphere. Heat can flow through it much easier because the cells are mostly “open.” That is why open cell has a much lower R-value than closed cell spray foam, and also why open cell does not make a good “Vapor Retarder”. Furthermore, it may be possible for open cell foam to absorb and retain moisture, which would lower its R-value even more, and maybe even support the growth of mold. I have yet to read any compelling evidence to convince me otherwise.)
- In a structure insulated with spray foam, the interior temperatures of the walls and ceiling tend to be very close to the inside air temperature, so there is no radiant heat felt from them.
- This is the ultimate form of heat flow. This is what the term “R-value” was invented to describe in the first place. This is the ONLY FORM OF HEAT TRANSFER that the R-value test pertains to. Since closed cell spray foam is made up of millions of tiny bubbles, filled with an inert gas that is a very poor conductor of heat, it has a very impressive “R-value.” One inch of closed cell foam is rated at approximately R-6.5. That’s just ONE INCH! That is approximately twice the “R-value” per inch of all the other common forms of insulation, including open cell spray foam.
So now that we’ve covered what factors play a major part in the reduction of real world insulation performance, we are down to the grand finale of this article. This is where we put it all together and hopefully have that “a-ha” moment where, (if you hadn’t already figured it out), it all makes perfect sense!
Think back to when we discussed how, according to the lab test, a value of R-8 will reduce heat flow by approximately 90%, as compared to no insulation. This is approximately the R-value of just 1.2 inches of closed cell spray foam! That is a massive reduction in heat flow! And since spray foam blocks all the other heat transfer methods with a vengeance, you still end up with a 90% reduction! There is no wind blowing outside air into the building, and no conditioned air escaping to the outside. No convection loops transferring unwanted heat, no heat radiating through walls, and no moisture in the insulation reducing the “R-value.”
One inch of closed cell foam is what we usually recommend for buildings that will not be air conditioned, will only be air conditioned part of the time, or for residential applications where the budget is extremely tight. Keep in mind these applications are not for every climate zone. But here in Central Texas, I would choose one inch of closed cell foam in the walls and ceiling of my house over R19/R-30 fiberglass and loose fill any day.
Now let’s say you added another inch of closed cell spray foam, giving you a value of R-13. Since we already eliminated all other forms of heat transfer with the first inch of foam, all we stand to gain is more resistance to conductive heat flow. What we have now is a reduction of about 93% of all heat flow.
Let’s take a second to think about that.
The first inch made an improvement of 90% reduction in heat flow from conduction, plus virtually eliminates any other forms of heat flow. Now we have twice as much material, and more labor to install it. Of course this is going to increase the total price significantly, but only gains us an additional 3% reduction of conductive heat flow. So why would we even want to add that second inch? Good question!
In Central Texas, during the winter and early spring, the daily average outside temperatures occasionally drop down to the 20s and 30s. Granted, it’s usually only for a couple of days at a time during a cold front, but it does happen. When the outside temperature is drastically lower than the inside temperature for an extended period of time, the surface temperature of the insulation inside the wall begins to slowly drop. (Remember that it is impossible to stop heat flow, it can only be slowed down.) If the relative humidity inside the building is extremely high, the dew point inside the building may be raised to the point of the surface temperature of the insulation. If this occurs, water vapor can condense on the surface of the insulation. To prevent this from happening, we can add a second inch of insulation. The additional insulation effectively keeps the inside surface temperature of the spray foam closer to the inside air temperature of the building, and thus eliminates interior surface condensation concerns.
We strongly recommend two inches of closed cell spray foam for residential homes, and commercial buildings where there may be a concern of moisture condensation. Two inches of closed cell foam is generally the maximum amount of insulation needed for nearly all types of insulation applications we have done. Even though the R-value is “only” R-13, two inches of closed cell spray foam in the walls and ceiling of a building will outperform a building with “R-19/R-30” amounts of fiberglass and loose fill in the walls/ceiling. We consider two inches of closed cell insulation to be the optimum ratio of R-value performance and price for buildings that are conditioned full time.
Hopefully, most of you are still following along, and I have made some sense of the R-value misconception. It really makes sense when you think about the basic science of how heat is transferred. It also helps to understand a little bit about how buildings are built. The most important thing to understand and remember, is that the test to determine “R-value” does not even come close to duplicating real world conditions! The R-value recommendations based on traditional insulation reflect that statement. If you still aren’t convinced, ask some people with spray foam insulation. They will undoubtedly tell you how well it works. If you ask me, testimonials are the most convincing form of real world test results.
Hopefully, the building codes will eventually be re-written to account for the outstanding performance of spray foam insulation. It will probably take some time, as companies that make large profits from producing fiberglass, cellulose, etc. are doing everything they can to slow it down. They have been raking in the dough for a long time, and would like to keep it that way for as long as possible. But those that take some time to do the research, and learn from others who have already experienced it firsthand, can actually have insulation that pays for itself over time by way of energy savings compared to outdated insulation materials.
I’m going to conclude this article with one of my favorite examples of real life, closed cell foam insulation related food for thought; take your favorite ice chest/cooler. Yeti, Igloo, Coleman Extreme, etc. Some of them are able to keep ice frozen inside them for five days or more. How do you think they can accomplish this amazing feat? Cut one of them open and you will find out. You guessed it! Closed cell foam insulation! Now look at how thick the sides are. Many of them couldn’t have much more than an inch for insulation, yet they are able to keep ice frozen for nearly a week! Now imagine if you were to leave the lid propped open, even just a little bit. How long do you think the ice would last then?