What is the difference between open cell and closed cell Polyurethane Foams?

This may be one of the most important pages on the website if your interest is in spray foam insulation. When it comes time to actually put the foam product in your home or commercial building structure, you must identify whether you will use Open cell foam, or Closed cell foam. This makes a big difference in cost, application methods, and performance.

With the open-cell vs. closed-cell issue, there are two major factors to understand and consider. The first is the nature of the foam. With open-cell foam, the tiny cells of the foam are not completely closed. They are broken and air fills the “open” space inside the material. This makes the foam weaker or softer feeling than closed-cell foam. Open-cell foam has an R-value around 4.21 per inch.
 
The advantages of closed-cell foam compared to open-cell foam include its strength, higher R-value, and its greater resistance to the leakage of air or water vapor. The disadvantage of the closed-cell foam is that it is denser, requires more material, and therefore, is more expensive. Even though it has a greater R-value, typically the cost per R is still higher than open-cell foam. The choice of foam can also be based on the requirements for performance or application specific characteristics.  Closed-cell foam has an R-value of around 6.7 per inch.
 
Both types of foam are commonly used in most building applications and the choice for which to use can depend on many of the factors discussed above. Some foam is inappropriate in specific applications. For example, you typically would not use open-cell foam below grade or in flotation applications where it could absorb water; this would negate its thermal performance.  Water vapor and moisture permeance is an area of great debate and misunderstanding in the insulation market and construction world as a whole. Most building inspectors and other industry “specialists” have very limited knowledge of it. As discussed earlier, closed cell foam has a low vapor perm rating (Less than 1.0 for 1.5” thick) and Open cell foam has a higher rating of around 10 (for 5”). Using generally accepted terms in the industry, closed cell foam is referred to as “vapor semi-impermeable” and open cell foam is referred to as “vapor semi-permeable”. A few points should be made prior to this discussion. First, moisture will move from areas of high pressure to areas of low pressure, which usually means it moves from warm to cold. Second, warm air can hold more moisture than cold air. Thirdly, this country has many different climates, so if you hear an expert from Texas tell you the “correct” way to insulate your home, you may want to consult a local expert. A good insulation solution in Minneapolis isn’t necessarily good for New York and a good solution for Miami isn’t the same as Seattle. The people you consult for building and insulation advice should have a thorough understanding of the local climate and building science. Unfortunately, most people in the industry don’t have this expertise. A Pentoir representative has the knowledge to make the best recommendation for you.
 
Any polyurethane foam will hold up well to liquid water relative to other building materials. They are plastics and when they get wet, they dry. Closed cell foam has stellar performance in these situations. In most cases, no significant amount of water will even enter the cell structure of the foam. Once the conditions that cause the water problem are removed and assuming the foam isn’t under water for months on end, the foam will simply dry out and return to action no worse for wear. On the other hand, the structure of open cell foam is not nearly as strong and it will take on water if a leak is bad enough. In the case of a roof leak or wind driven rain, open cell will simply dry out and return to normal once the leak is stopped. However, in the case of a flood that lasts for days on end whereby the foam is submerged, the pressure and weight against the foam will likely damage its cell structure beyond repair. What does all of this tell you? If you live near a creek or other flood prone areas, or if you have an older home with bad basement drainage, you should seriously consider paying the premium for closed cell foam. 
 
The exact values for vapor permeability are unimportant. What you need to know is: is it very permeable, somewhat permeable or very impermeable. A sponge is very permeable. Unpainted sheet rock is highly permeable. Painted sheetrock or foam board is slightly permeable. Plastic sheeting is basically impermeable.
 
Likewise for the air stopping ability, is it good at stopping air movement or lousy? If it’s lousy, you better have a really good air barrier in place before installing it, or the insulation will be next to worthless.
 

 Insulation moisture permeability varies significantly. Here are the most popular in our region:

TYPE

R-VALUE PER INCH

AIR BLOCKING ABILITY

PERMEABILITY

Fiberglass batts

3.25

non-existent

very high

Fiberglass, loose fill on attic floor

3.0

non-existent

very high

Fiberglass, loose fill in enclosed walls

4.0

non-existent

very high

Cellulose, loose fill on attic floor

3.5

non-existent

very high

Cellulose, dense packed in enclosed walls

3.4

good

very high

Spray foam, open cell

3.7

Excellent

High. ~16

Spray foam, closed cell

6.25

Excellent

Low. ~1

Board foam, expanded polystyrene

4.0

Excellent

Moderate. ~3.5

Board foam, extruded polystyrene

5.0

Excellent

Low. ~1.0

Board foam, foil faced polyisocyanurate

7.0

Excellent

Very low. 0.02

8″ Cinder block

1.11 (total R-value)

Lousy

very high

4″ Poured concrete

0.32 (total R-value)

Excellent

moderate

4″ Lumber

1.1

Excellent

moderate, ~3

Insulation Characteristics

An unvented attic with insulation installed between the rafters or above the roof sheathing. Moving the insulation from the attic floor to the roof plane turns the attic into conditioned or semi-conditioned space; this is especially beneficial in homes with attic ductwork. The term “cathedralized attic” usually refers to an attic that does not include finished space.

Closed cell foam forms in a similar manner, but a majority of the cells in closed cell foam are independent from the other cells in that they have their own cell structure and do not share any of their structure with other cells. The resulting foam has a much higher plastic content and less air/gas content. The foam is strong and rigid and typically weighs around 2.0 pound per cubic foot range with an R-value of 6.0-6.5. The general definition in the industry for closed cell foam is that greater than 90% of the cells are closed. 

Open Cell

· Water Blown Sprayed System: No VOC or Ozone Depleting Gases

· R-Value: SPF 1/2 lb foam’s R-value is approximately R-4.21 per inch (2x6 wood framed wall approx R-25)

· Air Infiltration Barrier: accounting for 40-50% of heat gain/loss in a typical house

· Noise Transmission: one of the best products for minimizing sound transmission

· Indoor Air Quality: significantly reduces dust and allergens

· Expands up to 100 times from liquid form: filling all gaps and voids in wall cavities

· Mold growth resistant

· Stable Over Time: unlike conventional insulation, the foam will not break down, shrink or deteriorate and the R-Value will not diminish

· Economical: costs less than closed cell SPF. Quick “payback” in energy savings over conventional batt or cellulous insulation

 

Closed Cell 

· High R-Value: SPF 2 lb foam’s R-value is approximately R-6.7 per inch (2x6 wood framed wall approx R-39)

· Vapor Barrier – stops moisture migrating into and out of the building (Open Cell SPF is not). In most cases additional vapor and air barrier “wraps” are not required.

· Exterior Applications – can be applied both above and below grade (under slabs, exterior side of basement walls) on Exterior Applications, as well as Interior (Open Cell SPF is only an interior application)

· Thermal Fire Resistance - Complies with FEMA requirements as a Class 4 insulation.

· Retro-Fit Exterior Walls – commercial and residential applications closed cell foam offers the benefits of air sealing, vapor barrier and high R-Value when applied to the exterior side of any wall and covered with cladding.

Structural Rigidity (racking strength) Tests conducted by National Association of Home Builders Research Center concluded that a 2” x 4” wall sprayed with polyurethane foam insulation had two times the racking strength of a wall filled with fiberglass batting insulation.
 

Closed-Cell Vs. Open-Cell

A side-by-side comparison of closed-cell to open-cell polyurethane foams is as follows:

Open-Cell Foam

Closed-Cell Foam

Open-cell foams are permeable to moisture and impermeable to air
R-value per inch: about 4.21
Although open-cell foam costs less than closed-cell foam, it has a lower R-value per inch, so a thicker layer is required. If the framing members are deep enough to accommodate your required R-value, open-cell foam may end up costing less.

Closed-cell foams stop air and moisture
R-value per inch: about 6.7
Closed-cell foam costs more, but provides a much higher R-value per inch than open-cell foam. Because of its density and glue-like tenacity, it also adds structural strength to a wall, ceiling, or roof assembly.

 

Closed-cell

Open-cell

Highest insulating “R-value” per sq. inch (>6.7 )

Good insulation value  (R = 4.21 )

Low vapor permeability (low perm)

Higher vapor permeability, but controlled

Air barrier

Air barrier at full wall thickness

Increases wall strength

Lower strength and rigidity

Resists water (is a WRB – water resistant barrier)

Not suggested for applications in direct contact with water

Medium density (1.75 – 2.25 lbs./ft.3

Low density (0.4 – 1.2 lbs./ft.3

Absorbs sound, especially bass tones

Best sound absorption in normal noise frequency ranges

 

Economical yield