*If you don’t want to read the entire article, scroll to the bottom for a quick summary of the pros and cons*
I’m going to be using a little bit of spray foam here and there in the new house I’m building, so I thought it might be a good time to talk about the pros and cons of both open and closed cell spray foam.
The kind of spray foam we use today started to emerge in the 1980s, and its roots can even be traced back as far as the 1930s; however, it seems to have only gained a lot of popularity since the turn of the century. Today, many people believe spray foam is the very best insulation, but that’s not true. The truth is, no insulation is the best insulation, because they all have their pros and cons, which is another reason I’m writing this post.
By the end of this article, you should be able to decide if spray foam is the right insulation for your next building project. I also suggest reading my articles about Rockwool vs. fiberglass and denim vs. cellulose.
Before I get into the pros and cons, though, let’s talk briefly about the difference between open and closed cell…
The Difference Between Open Cell and Closed Cell Insulation
The main difference between these two is right in their names: one has open cells, the other has closed, but it doesn’t stop there…
Open cell spray foam is about 97% air, which means it is far less dense. Open cell spray foam also has a much greater and faster expansion rate. Some amount of excess always needs to be screeded off because it expands so much. Open cell is also far more air and vapor open than closed cell.
Closed cell is much more rigid and dense than open cell. For this reason, despite being more expensive, you need less of it to achieve the same results. Closed cell is air tight and has a perm rating of less than 1 per 2″, which also makes it a class II vapor retarder.
As we move through the pros and cons list, I’ll discuss some more of the nuanced differences between the two, as they are rather important when going over their strengths and weaknesses. I should also mention that there is no reason you can’t use a combination of both – a layer of open over a layer of closed – and many builders do.
Pros
R-Value and Insulating Properties
Spray foam has a good R-value, but that doesn’t tell the full story… Open cell spray foam has an R-value or about 3.5 per inch, whereas closed cell has an R-value of about 6 per inch. However, spray foams also have air sealing properties that can enhance the effective R-value even more, especially when applied continuously throughout an assembly.
In other words, the testing method that determines the R-values of open and closed cell spray foams does not take into account how the air sealing also improves thermal efficiency.
Air Sealing
Since I just mentioned the air sealing capabilities of spray foam, now would probably be a good time to expound. Air tightness is of great importance in modern buildings. If we can’t control how air enters, exits, and moves around a building envelope, we’re going to waste lots of energy trying to control the temperature and climate within the structure.
The easiest way to imagine just how problematic this can be is to simply imagine the difference between open and shut windows. If you’re running your AC or heater but all the windows are open, what good is that doing? All of the air you’re cooling or heating would be flowing out of the building faster than your mechanicals could keep up with it.
One of the greatest pros spray foam has, be it open or closed cell, is its ability to create air barriers. It takes just a little more than 3″ of open cell spray foam to create an air barrier, and only 1 to 1.5″ of closed cell to do the same.
Lower Energy Bills
When installed properly, spray foam insulation can reduce heating and cooling costs by up to 40%. Again, this is due to its high R-value and air tightness. Conditioned air stays in the building longer than it would compared to other types of insulations acting alone, so there’s no need for your HVAC, mini split, etc… to run as frequently.
Even if you’re living off the grid, like I am, and using a wood stove and solar panels for heating and cooling, you’ll still benefit greatly from the energy savings. You won’t need to burn as much wood to heat your home, nor will you need as much passive or stored solar energy to power your AC.
Structural Reinforcement
Closed cell spray foam actually increases the shear strength of a building. It is so rigid, that it is essentially like adding extra studs or concrete to your walls and ceilings. It is said to increase the shear, or racking strength, by as much as 300%.
Open cell spray foam, however, does not improve the strength of an assembly.
Water and Vapor Resistance
Closed cell spray foam is a water and vapor barrier. It has a perm rating of less than 1 per 2″ depth, which makes it a class II vapor retarder. It is also hydrophobic and does not absorb liquid water to any significant degree.
That said, make sure you do your homework on vapor retarders in your climate zone before you decide to use spray foam as an all-in-one water, air, and vapor barrier. Putting it on the wrong side of the assembly could spell disaster down the road.
Open cell spray foam, on the other hand, is vapor permeable and will soak up bulk water like a sponge – I’ll talk about that more when we get to the cons.
Longevity
Both types of foam have long lifespans and will not sag and compress, or thermally drift, over time like most other types of insulation. Spray foam is estimated to maintain its efficacy for 80 years, and it may even outlive the building it’s insulating.
Inorganic
These days, most people want everything to be organic, but inorganic insulation is actually a good thing. Spray foam does not supply a source of food for rodents, insects, and microorganisms, such as fungus.
Cons
Cost
Spray foam insulation is cost prohibitive, to say the least. Although open cell is usually half the price or less than closed cell, both varieties are still more expensive than most other types of insulation. Expect to pay about three times more than fiberglass.
With that said, it’s worth mentioning that the high cost is upfront. Eventually, you can expect to make your money back through the energy savings, which may take about five to 10 years.
Noise
Spray foam is not a good soundproofing material. In fact, it can actually amplify midrange frequency. When comparing the Sound Transmission Class – STC – and Noise Reduction Coefficient – NRC – ratings, both open and closed cell spray foam lose to stone wool and fiberglass, respectively, by a long shot.
For example, spray foams have an STC rating ranging from 36 to 39, whereas fiberglass has an STC rating of 52. Similarly, open and closed cell spray foams have an NRC of .70, while fiberglass has an NRC of 1.10.
If acoustics are of great importance to you, spray foam is not a good choice.
Difficult to Install Correctly
Anybody can fill a small gap with a can of Great Stuff, but if you’re spray foaming an entire house, or even just an attic or basement, you really should have it done by a professional because it’s an entirely different animal.
Sure, you could buy a DIY kit from your local home center store, but those can easily result in failure. This is because two-part spray foam is more difficult to mix than you might think. If it is not mixed perfectly, it will not adhere sufficiently.
The preparation of the building is also a nightmare. Every surface must be perfectly clean and dry, or again, the foam won’t adhere.
When you consider how expensive spray foam is, it isn’t worth the risk of screwing up. If it isn’t done correctly, you’re throwing a lot of money down the drain – or more accurately, stuffing it in your walls.
Lastly, there’s also the risk of working with hazardous chemicals. The PPE required for working with spray foam is no joke. It is essentially a hazmat suit.
I could write an entire article just about how hard it is to do spray foam correctly and all the things that can go wrong, but for the sake of brevity, hopefully I have said enough to convince you it is not a DIY-friendly product.
No Do-Overs
You only get one chance to do spray foam correctly. Yet another reason it should be done professionally. Spray foam, unlike all other forms of insulation, is permanent. You can’t just take it out of the wall cavity or peel it off the side of your building.
This is particularly risky if you if you install it on the inside of your assembly because it has to be sprayed over electrical and plumbing. If and when you have a plumbing or electrical failure, it’s not going to be an easy fix if it’s covered in foam.
Strict Code Requirements
Due to the way R-value is measured and the code is written, a few inches of spray foam might not be enough to meet your code requirements, despite the fact that it is giving you adequate insulation. This could put you in a situation where you’re stuck spending a lot more than you budgeted for just to pass inspection.
High Potential for Moisture Damage
Closed cell spray foam is hydrophobic and nearly vapor impermeable, but open cell sucks up water like a sponge. Even worse, if open cell does happen to soak up bulk water, it stays wet for a very, very long time.
I’ve seen open cell foam removed from homes that were flooded, and it’s an absolute mess. Far worse than fibrous insulation.
Another potential risk for moisture damage is condensation. If you’re using closed cell, and the foam is not installed correctly and continuously, you could possibly create condensation points within the assembly.
Whereas open cell is susceptible to bulk water damage, it is actually closed cell that can create condensation problems. That is because closed cell is a vapor retarder and must be treated as such. Put it in the wrong spot for your climate and you’re almost guaranteed to have problems down the road.
Ventilation Problems
Spray foam is so airtight that it sometimes creates ventilation problems. For example, vent hoods, fireplaces, wood stoves, and anything else that relies on combustion or suction, needs to draw in fresh air to work properly.
It is not uncommon for a homeowner to have their attic sealed with spray foam, only to discover the next day that their appliances and mechanicals are no longer working. The only solution to this problem is to pair the spray foam with an appropriate ventilation system for the building assembly, such as an energy recovery ventilator – ERV.
Similarly, spray foamed homes in humid climates are prone to being too humid, and therefor require a dehumidifier.
Environmental Concerns
In the past, spray foams had a concerning carbon footprint, but these days it’s not nearly as bad as you might think. It used to be made with greenhouse gasses, however, that is no longer the case. Open cell is now blown with water, and closed cell is blown with a gas that has little greenhouse potential.
There is also the environmental concern of using a polyurethane plastic, which is what spray foam is; though it is important to remember its long lifespan. It is not a plastic grocery bag or package that is going to be used once and then thrown in the trash.
Spray foam is intended to last many decades, and the energy savings can outweigh the eco drawbacks.
Considerations
Continuous Installation
Spray foam works best when it is installed continuously, either around the outer envelope of the building, or behind the wall studs. For example, if you are spray foaming a shipping container, and you don’t want to spray the outside of the container to preserve its industrial appearance, you should spray the entire inside of the container before your wall studs go up.
Combining Open and Closed Cell
Due to the high cost of closed cell foam, you may want to consider combining open and closed cell to make it more affordable. You can first apply one or two inches of closed cell followed by another 3 or 4 inches of open cell.
Flash and Batt
Another technique you can implement to reduce costs is the “flash and batt” method. This works by applying a thin layer of either closed or open cell, and then filling the rest of the cavity with less expensive batt insulation, such as traditional fiberglass.
| Pros | Cons |
|---|---|
| High R-value per inch and excellent insulating properties. | More expensive than other types of insulation. |
| Creates and air seal and air barrier. | Bad at reducing noise and soundproofing. |
| Significantly lowers energy bills. | Needs to be installed professionally. |
| Adds structural reinforcement to assemblies (closed cell only). | Permanent. No room for mistakes or do-overs. |
| Water and vapor tight (closed cell only). | May be hard to pass code if not thick enough. |
| Long lifespan. | May cause moisture problems if done incorrectly (closed cell) or is exposed to bulk water (open cell). |
| Not a food source for rodents, insects, and microorganisms. | May require the installation of a high-performance ventilation system and/or dehumidifier. |
