Ask the Building Scientists

Question:

Thank you for publishing Working With Attic Knee Walls in the July edition of the Quality Home® Newsletter. We have two bedrooms in our attic which are difficult to keep cool in the summer. Your document describes my problem and the solution precisely but I do have one concern I’d like to ask you to address.

According to the document, I should “encapsulate the insulation by installing a layer of thin profiles sheathing, wood sheathing, or even house wrap on the backside of the wall.” In another place rigid foam insulation is mentioned. I like the idea of using the rigid foam because it adds significant R-value in addition to sealing the rooms; however Owens Corning states, “This product will ignite if exposed to fire of sufficient heat and intensity. This product should be installed only with a gypsum board on the interior side of the wall.” This is in the Product Data sheet for FOAMULAR 250. Obviously gypsum won’t work in our moist climate and plywood would be difficult to install. Should I be concerned about the Owens Corning warning in this situation, and if so, do you have another suggestion that would seal and add R-value and also be easy to install?

-Timothy J. Robinson
A homeowner in Tennessee

Answer:

Using rigid foam is a great idea, and the amount of protection depends on local building code interpretation and the conditions of your attic. If the attic is unoccupied, typically requiring limited access via a door less than 4 feet in height or via a ceiling hatch, and in some cases having no mechanical equipment in the space, extruded foam sheathing could be used. The fire separation requirement is for occupied space, so the interior wallboard meets this requirement. If the attic has mechanical equipment, or greater access via a larger door, then it would be considered occupied, and you would have to install a layer of wallboard over the foam, or use a layer of foil faced Isocyanurate sheathing instead. Actually a layer of wall board is easy to install, and you can practice your taping technique since aesthetics are not critical. Whether you choose an insulated sheathing, or a layer of thin structural sheathing, closing the sixth side is very important and will reward you with greater comfort and energy efficiency for the life of the home.

-Brad Oberg,
Chief Technology Officer

Question:

What is the recommended finish for the area below a weep screed?

-Don Carpenter
Carpenter Construction

How are other builders addressing weep screed issues when you have a gap between the weep screed and the foundation?

-Nova Taylor
Supervisor of Customer Service, Arvida

Answer:

Don, we recommend finishing the walls beneath a weep screed with a stucco scratch coat or paint tinted to match the stucco. If the weep screed has perforations, be sure not to fill them with paint or the scratch coat. It’s imporant not to extend the stucco finish below grade, as stucco will wick water through capillarity. The optimum distance between the grade line and the weep screed is six to eight inches.

Nova, we recommend dropping the weep screed one to one and a half inches below the sill plate. This will ensure that the stucco line is clean and doesn’t highlight any imperfections in the foundation. To seal the gap between the weep screed and the foundation, use a low expansion spray foam and then caulk the weep screed to the foundation wall.

-Anthony Grisolia
Building Performance Specialist