What is BSC?
Building Science Corporation provides consulting specializing in building technology. For more information, check out their website.
Where is the water control layer in an exterior wall made up of metal studs with exterior sheathing and air /water barrier and exterior ridged insulation containing windows?
The air and water barrier applied to exterior sheathing and taped to window mullion flashing acts as that control layer in this area of the country.
Also an exterior sheathing that has an integral air and water barrier then taped and sealed to the window mullion flashing can also act as that control layer.
Where should the dew point fall in the exterior wall assembly?
The dew point shall always fall outside of the water control layer of the building and never fall inside insulated studs or behind that water control layer.
What is more important for maintaining higher exterior wall performance, the water control layer or the air barrier?
The water control layer is important for moisture control but the air barrier installed properly helps improve the overall wall performance.
What type of exterior skins lend themselves to meeting the code related to continuous insulation?
All exterior skin materials can be integrated utilizing continuous outboard insulation by proper detailing for placement of exterior sheathing and an air / water barrier.
The mechanical fastening system must not create a thermal bridge through the ridged continuous insulation. Knight Wall Systems or equal provide a thermally broken skin attachment system.
When placing two lines of backer rod and sealant at window and door jambs with an air / water barrier taped to the jamb flashing where is the main line of defense?
Main line of defense is the second line of sealant in line with the air / water barrier using a breathable backer rod to allow sealant to cure.
Outboard sealant protects main sealant from the environment and acts as a first line of defense in line with the exterior face of glass and not block weeps.
Should exterior sheathing, the air / water barrier and outboard ridged insulation extend over the top of parapets and tie into the same on the opposite side of the wall?
Although this can be a challenge with attachment of wood blocking, it is best to enclose all exterior sheathing with the air / water barrier for continuous outboard insulation wall systems.
In cases where the UL for rated roof requires exterior rated sheathing laid on the metal deck the air /water barrier can continue under the roof insulation but must meet roofing system warranty.
Is there a code or NFPA requirement for the exterior materials and assemblies?
Yes, the code refers to NFPA 285 which identifies testing requirements for all components in an exterior skin assembly.
Is the placement of vapor barrier and/or air –water barrier to have continuous insulation outboard the same in all parts of the USA and World?
With no batt insulation in the studs and only an applied air/water barrier the thickness of ridged insulation is the largest changing factor. Dew Point Analysis will identify if the air – water barrier shall be permeable or not based on the climate and what side of the insulation it should be placed. Therefore that barrier acts as a vapor barrier as well in some climates.
Is there a source in St. Louis for assistance with questions related to building enclosures?
The St. Louis Chapter of BEC (Building Enclosure Council) web site is your portal to ask questions and have dialog with BEC board members.
Would you like more information on Vapor Diffusion?
Check out ROXUL’s website for an indepth look.