This post is in regards to which types of spray foam insulation are appropriate for the underside of a roof deck and why. It is another great open-cell versus closed-cell debate.

Unfortunately there is no clear cut answer to this question but I will summarize the benefits and disadvantages of each situation.

Using open-cell foam on the underside of a roof deck is an affordable option to obtain a very good R-value in a 10?+ joist cavity and to create that good air-seal which is the most remarkable benefit of all of the spray foam products and literally puts it miles ahead of many other insulation materials in my opinion.

The greatest debate on this issue is in regards to moisture. Most companies will inform clients interested in open-cell foam on the roof deck that it is recommended that an approved vapor transmission barrier first be applied so as not to saturate the foam and reduce its effectiveness. However, in reality it is no different than installing fiberglass batts or other types of insulation materials as they too would lose 90% of their R-value when wet. Companies normally explain this to the customer and unless required by the building inspector it is usually overlooked.

So where is the debate? Great question. As an experienced spray foam insulation professional I understand the cost-effectiveness of using open-cell products versus closed-cell. There are many SPF companies that ONLY offer open-cell foam because it is MUCH harder to sell consumers on the benfits of closed-cell when comparing the price.

Sadly, companies have been using the excuse that closed-cell foam will trap moisture behind the foam causing wood to rot and mold issues. This is entirely a selling tactic and completely false. If your roof is in a condition where it is leaking then those issues should be resolved before ANY type of spray foam is applied to the underside of that roof.

So here is the difference with how the two types of spray foam will be affected by a roof that develops a leak. Closed-cell spray foam acts as a vapor barrier and will not absorb moisture. It will stop the moisture from entering the attic. Due to the heating and cooling effects of the roof the wet portion of the sheathing will typically dry itself out within 24 hours and not lead to any rot or mold issues. This means you will have the same R-value and a dry attic space with no indoor air quality issues caused by intruding moisture.

Open-cell spray foam installed with a vapor barrier should act the same way that closed-cell does because it is not affected by the moisture and the moisture would be “trapped” as the open-cell salesman explained. Without the vapor barrier the open-cell foam would absorb the moisture much like fiberglass and lose 90% of it’s effectiveness until dry. While wet this could potentially lead to a mold issue in the attic due to the added moisture accumulating on organic materials.

So which one is better? You decide!