What WRB/Air Barrier to use for CMU wall + interior cellulose insulated wall?
We are remodeling an old concrete block building into our future net zero (or almost net zero) retirement home. The house is located in Hillsborough, NC which is in climate zone 4A. Currently, the building is just a concrete block CMU building shell with a standard wood truss roof. In order to insulate and finish, we are proposing to build interior walls with a 4-1/2″ air gap to allow for 8″ of dense pack cellulose insulation yielding about R30. I am concerned about the interface of the cellulose and the inside of the CMU wall and am planning to install some sort of WRB/Vapor/Air Barrier there. The proposed wall section layers will be (outside in):
- Masonry Parging (for stucco look)
- 8″ existing CMU
- WRB/Vapor Permeable Air Barrier
- 4-1/2″ Air Gap
- 2×4 Interior Wall (3-1/2″ wall + 4-1/2″ gap will be filled with dense pack cellulose yield approx. R30)
- 1/2″ drywall, painted with latex
Thanks for any help you can provide
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Replies
CMU walls always seemed pretty water proof once painted inside and out.
If you are very concerned build, it like a basement wall with 2 in R10 sheet foam against the CMUs then a 2x6 stud wall filled with R19 of damp sprayed cellulose.
https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall
Walta
Remodeling an old concrete block building into a net-zero retirement home in Hillsborough, NC (climate zone 4A) is an exciting endeavor. Your plan to insulate and finish the interior while being mindful of moisture management is crucial for the long-term durability and comfort of your home. Based on your description, here are some thoughts regarding your questions:
Location of the WRB/Air Barrier
Correct Placement: Positioning the WRB/vapor permeable air barrier directly on the interior surface of the CMU wall is appropriate. This placement not only helps to manage any moisture that may wick through the CMU but also serves as an air barrier, reducing air leakage and improving the overall thermal performance of the wall assembly.
Product Recommendation
Prosoco R-Guard Spray Wrap MVP: This liquid-applied product is an excellent choice for your application. It provides a seamless barrier that can accommodate the irregularities of the CMU surface, ensuring complete coverage. Its vapor permeability is beneficial for allowing any moisture that does enter the wall assembly to dry to the interior, which is particularly important in climate zone 4A where humidity levels can vary significantly throughout the year.
Additional Considerations
Moisture Management: Ensure that the exterior masonry parging is properly applied to minimize water ingress. While the CMU acts as a rainscreen, attention to details such as flashing, weep holes, and proper parging can further reduce the risk of moisture penetration.
Interior Wall Assembly: The use of dense pack cellulose insulation is a good choice for achieving a high R-value and utilizing a sustainable insulation material. Ensure that the cellulose is densely packed to prevent settling over time and maintain its insulation value.
Air Gap: The 4-1/2" air gap between the CMU and the interior 2x4 wall filled with dense pack cellulose will help in managing moisture by allowing for some drying potential to the interior. However, ensure that the air barrier is continuous and well-sealed at all penetrations and transitions to maintain its effectiveness.
Thermal Bridging: While the focus is on insulation and air sealing, consider addressing thermal bridging through the 2x4 framing by ensuring continuous insulation across the frame where possible.
In summary, your approach and consideration of using a liquid-applied WRB like Prosoco R-Guard Spray Wrap MVP on the interior surface of the CMU walls are well-founded. This strategy, combined with dense pack cellulose insulation and attention to moisture management details, can contribute significantly toward achieving your goal of a net-zero or near-net-zero energy-efficient retirement home. As with any complex remodeling project, consulting with a local building science professional or architect familiar with sustainable renovations in your climate zone can provide additional insights and ensure that your project meets its energy and comfort goals.