This article will cover spray foam insulation R-values and why you should, or most likely not use it.
You might get some sticker shock if you price out spray foam insulation. It is expensive if done correctly by a professional contractor. So, you want to make sure it’s worth it.
The way most insulation is measured and compared is by R-value. The higher the number, the better at blocking the transfer of energy, or thermal resistance. Ideally, you want the highest R-value with the least material thickness and cost.
There are three different spray foam insulations with different R-values per inch.
Per-Inch Spray Foam R-Value
Most commonly used for roofing and exterior walls, high-density spray foam has an r-value that begins at 5.5 per inch—considered closed-cell at a density of 3 lbs per cubic foot.
Homebuilders will often use high-density foam when they need a high R-value insulation with strength, most often used in roofing applications.
Used for unvented attics and exterior and interior walls, medium-density spray foam has an r-value that begins at 5.7 per inch—considered closed-cell at a density of 2 lbs per cubic foot.
Medium-density is the answer when applications require the highest R-value foam insulation per inch. It can help create air, water, and vapor barriers while providing sound blocking.
In a typical 2×4 stud wall filled with medium-density spray foam, the r-value will be R-20. Going to a 2×6 stud wall will bump up the R-value to R-31.
Used in interior walls and unvented attics, low-density spray foam has an r-value that begins at 3.6 per inch—considered open-cell at a density of 0.5 lbs per cubic foot.
Low-density is also called “half-pound foam” due to its weight per cubic foot. It stays slightly flexible, unlike high-density.
In a typical 2×4 stud wall filled with low-density spray foam, the r-value will be R-13. Going to a 2×6 stud wall will bump up the R-value to R-20.
Problems with R-value of Spray Foam
There are a couple of main issues with spray foam and R-values. The first is just like some of the companies selling bubble wrap insulation, they have been known to be deceptive.
In 2010, Martin Holliday was writing letters about spray foam contractors skimping on thickness during installation. These companies could get away with it because most building inspectors do not inspect for airtightness, even though this is a part of the IRC code for energy efficiency.
They compared homes that leaked large amounts of air and were insulated with fiberglass. With spray foamed houses that were sealed better due to the nature of expanding foam.
Air leakage is the primary driver of energy efficiency as long as the insulation R-value is within range. You can get the same performance from fiberglass as spray foam if your air, vapor, and R-values as the same.
Spray Foam Type Doesn’t Matter in Stud Cavities
When you look at the charts comparing spray foam insulation R-values, you would think since closed-cell foam has the highest r-value, it is the best choice. Open-cell foam being R-3.6 and closed-cell R-5.5 per inch.
But you wouldn’t be seeing the whole picture. And to do that, we need to know how the different spray foams are installed.
Open-cell spray foam easily expands and is trimmed flush to the studs. So a typical 2×4 stud wall with a 3.5″ cavity will get completely filled.
Closed-cell spray foam is not as easy to trim flush. So spray foam installers will often stop short of filling the entire cavity, so nothing is left to trim.
When more of the studs are exposed, there is a more significant penalty for thermal bridging, which is transferring energy through a material. In this case, the studs.
Green Building Advisor created a chart that did a calculation. In a typical 2×4 stud wall using spray foam insulation, the added R-value benefits of closed-cell was only R-1.9.
So there is no significant difference in R-value when comparing the two types of spray foam in a stud wall. Due to thermal bridging and open-cell foam covering the entire cavity.
Often open-cell will be more cost-effective.
Also, when comparing the two, closed-cell foam is worse for the environment. Since it often uses a high global warming blowing agent.
Hydrofluorocarbon (HFC) blowing agents in spray foam and offset the energy savings from increased performance when looking at climate impact. Although, manufacturers are working to reduce the use harm of blowing agents.
There also may be other decisions to make when choosing open-cell or closed-cell spray foam other than R-value. Open-cell will not block moisture as well as closed-cell.
Third Type of Spray Foam – Tripolymer
There is also Tripolymer spray foam. It is made with water-soluble materials. It is usually used in existing buildings by injecting it into wall cavities. While being injected, it will flow around objects and obstructions before it starts to expand and cure. To ensure complete coverage.
Tripolymer spray foam can also be used for:
- Increased fire rating – Can get a 50-150% increase in fire-resistance rating without installing gypsum
- Sound deadening – Has a typical STC rating of 53
- Increased thermal performance of masonry blocks by filling cores
The R-Value of Tripolymer Spray Foam
Once Tripolymer spray foam has fully cured it has been stated to have an R-value of around R-5 per inch. With no thermal degradation during the life of the product.
Dangers of Tripolmer
There has been some concern of Tripolymer foams off-gassing formaldehyde in the past. A company in Canada was involved in a prominent lawsuit and was ordered to stop the import and sale of the product.
The product needs to be third-party tested to ensure health and safety standards are met. Unfortunately, this to date has not been done.
Spray Foam Exterior Walls to Increase R-Value
If you’re removing your sheetrock or siding. It might make sense to add a layer of spray foam.
Here is Peter Yost showing how this can be done and why:
This type of installation may be required in some homes. But, in most cases, you can accomplish the same outcome by installing an air and vapor barrier along with a rigid foam board or mineral wool.
Spray Foam vs Fiberglass Insulation R-Value
Fiberglass is going to typically have a lower R-value than spray foam. Fiberglass usually has an R-3 per inch value, and spray foam is usually R-5 per inch.
Spray foam will also have the advantage in a typical wood-framed home as many areas are challenging to air seal and insulate with fiberglass, which gives it a bad reputation. Dr. Energy Saver does a good job highlighting these issues in the following video.
Spray foam seems like a clear winner. But, the same sealing and performance can be achieved with fiberglass if done correctly.
Spray Foam vs Blown Cellulose Insulation R-Value
Blown cellulose is in the middle of spray foam and fiberglass. In cost and R-value. Cellulose has an R-value of around R3.4 per inch.
It also will fill a cavity very nicely to help block air movement. Here is a great video that shows the differences.
Keep in mind the above video does have some faulty assumptions.
You can achieve a perfect air seal with any insulation type. Insulations’ job is thermal performance (R-value) and not air sealing. So, while the video does highlight that different insulation types can improve air-tightness, don’t make the mistake of thinking you don’t need a proper air barrier.
Blown cellulose is an excellent product and should be used when possible.
Higher R-Value Solutions
While spray foam insulation is often thought of as the best way to get the highest R-value, there are other ways than may perform better with similar costs.
Continuous exterior insulation using rigid foam board or mineral wool can give you even higher R-values for walls.
It will offer all the benefits that spray foam companies tout, with even better performance.
For the most cost-effective exterior insulation, most homeowners choose EPS rigid foam.
Check out the “Perfect Wall” by Building Science Corporation. This eliminates thermal bridging and can offer great R-values. By not relying on the stud cavity, you can thicken the wall.
If you’re going to install insulation on the interior and the exterior, you need to keep in mind the vapor profile. There needs to be a high enough R-value on the exterior sheathing. So that your dew point is on the exterior and not the interior; otherwise, it could create condensation within the wall and start to rot.
If you have a flat vented design in your attic, blown-in cellulose is often preferred over spray foam insulation. You can pile the insulation up and create a high R-value attic. Most are R-60 and can be done with rental blowers to make cost-effective insulation.
Cellulose also doesn’t include any blowing agents that can harm the environment, or yourself.
Spray foam insulation R value numbers are impressive when looking at just the per-inch stats. And can be a great solution in the proper context, such as retro-fits and basement rim-joists.
But, it is expensive and can harm the atmosphere.
You can often get better performance from using a good air barrier and other insulation types.
Please don’t fall for spray foam companies that compare older homes with high air leakage since spray foam will perform better since it acts as an air barrier. As, over time materials expand and contract, and spray foam insulation may start to leak air as well. Lowering its energy performance over time.
Look at the whole wall r-value when comparing R-values and not just the insulation (don’t forget to factor in the penalty for thermal bridging). Then, compare the costs.
Most importantly, work with a knowledgeable home contractor who knows how to use materials properly to achieve the R-value you need with safe, cost-effective insulation. And be very leary of any spray foam company that claims foam is the holy grail of insulation and R-values. Their opinions are often biased.
If you do choose to use spray foam. Ensure the final product offers the correct R-value for your building codes and desired value by verifying installed thickness.
R-value of spray foam: www.whysprayfoam.org/spray-foam/r-values-and-performance/
Closed-cell vs. Open-cell Spray Foam R-Values in a stud wall: www.greenbuildingadvisor.com/article/installing-closed-cell-spray-foam-between-studs-is-a-waste
Tripolymer spray foam: