Open Cell Spray Foam vs. Closed Cell Spray Foam

Closed vs. Open Polyurethane Spray Foam

By David Braddy LEED GA

Last month we discussed the benefits of a hot roof system using either open or closed cell spray foam. If you missed it you can read it at http://LakeEcoGroup.com

Let’s also keep in mind that insulation is the key to energy savings and a cornerstone of an eco-friendly building.  Sustainable building practices require that we reduce the heating and cooling loads as much as possible. Many of the guidelines and codes used just a few short years ago simply do not cut it anymore, and the excuse “we have always done it that way or that’s how I was taught” doesn’t cut it either.

Central Heating and Air Conditioning Systems changed building methods forever, we are now starting to understand how this contributed to unhealthy homes by the formation of rot and mold in the building envelope.

Insulation has always been the thermal barrier of the building and we have typically focused on the R-Value of the insulation material only. This is simply the resistance of heat flow through the building envelope. Unfortunately this can be very deceiving because it is only part of the equation. How effectively insulation blocks the flow of air and moisture is equally important and R-Value means very little if you do not have an effective air and moisture barrier.

Most of you would be very surprised at how little energy it took to heat and cool a properly insulated and sealed home, and yes there are now ways to fill and seal existing wall cavities.

Energy Efficiency is why spray foam has become so popular, there are 2 types open and closed cell.


They are both superior products to conventional insulation methods, both are great air barriers. The major difference is in the R-value per inch and moisture permeability. The R-Value per inch of open cell is approximately 3.8 and the R-Value per inch of closed cell is almost double that at approximately 6.8 per inch. Closed cell spray foam also adds to the strength of a structure because it bonds to and becomes part of the structure. Closed cell is a true air and moisture barrier at 2 inches.

Do not believe any salesperson, trying to sell you open cell that says, “Since closed cell is solid, normal building movement will make it break lose”. This is simply not true, if applied correctly closed cell will greatly improve the structural integrity of any building and becomes part of the structure, it will not break lose.  Also do not believe a salesperson trying to sell you closed cell when he tells you that “open cell will wick water because it is sponge like”, it will only wick water when the cell structure is damaged. These are both sales tactics to sell one over the other.

Here is a comparison of moisture perm rating, the higher the rating the more moisture can pass through

  • Closed Cell has a perm rating of 1 for a 2” thickness
  • Open Cell has a perm rating of 10 for a 5” thickness
  • An unfaced fiberglass batt has a perm rating over 100

So which one is better? As you can see closed cell wins hands down for structural integrity and moisture resistance, but it will also be the more expensive of the two, they are both superior to conventional batts.  There is really no way to tell you which one is better for your particular situation. I am partial to closed cell as the better product, but it is more expensive. For sound control I would opt for open cell as the better product. Now if moisture was not an issue and I didn’t care about adding strength to my structure I would probably again opt for open cell. Climate will also play a role in this decision.

The people you consult should have a thorough understanding of local climate and building science, unfortunately many people in the industry do not have this expertise or are very resistant to change. This is why we still have musty, moldy, rotting homes. A properly sealed building envelope will not have these problems.

Hot Roof vs Cold Roof

What is a Hot Roof System?

By David Braddy LEED GA

 

Building science is changing rapidly and some of those changes are challenging traditional building methods. One of the most beneficial changes is also one of the most controversial, which is the hot roof system or the unventilated sealed attic.

A hot roof is a properly sealed unvented attic, instead of a traditional vented roof which consist of an insulated attic floor and open soffits where air enters and then leaves through the top of the roof.

 

A few years ago, the scientists at the Oakridge National Laboratory in Tennessee tested properly vented and totally sealed attics. They found that proper venting only reduced the roof deck temperature by 3 to 5 degrees Fahrenheit.

 

But, when an attic was insulated using the Hot Roof Theory, the attic temperature adjusted to within 10 degrees of the ambient temperature of the house. Because the attic is heated or cooled by air that would normally escape from the house, it does not raise the load on the heating and cooling system.

However a vented attic typically reaches 130 -140 degrees, when the attic temperature exceeds outside temperature shingle failure can occur and if you have duct work or HVAC equipment in an attic it has to operate in extreme conditions causing excess energy to be used. This also creates the perfect opportunity for moisture damage due to condensation forming on equipment and duct work.

 

Spray foamed attics have foam applied directly to the roof decking, and the attic space isn’t ventilated.  The lack of ventilation is why they are called hot roofs.

In a standard insulation system, ceiling insulation reduces the transfer of heat from the attic to the living space (in the summer). Attic temperatures often approach 140F during the day. Solar energy warms the shingles and sheathing and then transfers heat to the attic. The 140F temperature of the underside roof surface drives the heat transfer process.

 

By insulating the roof surface with spray foam, the surface temperature exposed to the attic (the temperature driving the heat transfer) is reduced dramatically.

 

The benefits of including the attic in the insulated space are:

 

* Duct leakage and heat loss/gain from ducts is much less of an issue.

* Air sealing is easier in the roof than in the ceiling.

* Dust and loose insulation are less likely to migrate down to the living space.

* Tests show energy costs are lower when the attic is sealed.

 

Further information is available from ASHRAE (8700-527-4723) in a publication titled ‘Vented and Sealed Attics in Hot Climates’.

 

For more information check out this video


 

The next question is what type of spray foam is best for a hot roof system; there are two types open and closed cell.

Both of these can be used if properly applied, but there are big differences between the two. Open cell is moisture permeable and should not be used in a hot roof system without a vapor retarder. It has a R value of approximately 3.5 per inch; on the other hand closed cell is moisture impermeable has an R value of 6.8 per inch and is a vapor retarder. Closed cell has excellent adhesion qualities as does open cell, but closed cell actually becomes part of the roof structure making it much stronger.

We will have more on the differences between open and closed cell foam next time.