Typically, unfinished basements of new homes in NY have several supply registers that semi-condition the storage space. What starts as a storage space is often converted to livable space in the future. However, there are only two choices for characterizing a basement within energy modeling software (conditioned or unconditioned).
Newport Ventures recently worked on a high performance home for Gerber Homes, where the basement was treated as conditioned space in the modeling software. Modeling the basement as a conditioned space revealed significant energy efficiency gains could be obtained by moving away from standard basement insulation practices. While deciding to increase insulation values in a basement to increase energy performance may sound like low fruit on the energy efficiency tree, there is a wide chasm between specifying a system and actually achieving it.
First, Gerber’s typical basement insulation system consisted of precast concrete walls with integral R-5 continuous interior foam insulation. For this high performance home Newport recommended increasing the precast concrete walls integral foam from R-5 to R-12.5 and to fill the interior wall cavity with R-21 insulation. What seemed a few extra but simple steps was not as easy as adding more insulation would seem.
Un-faced fiberglass batts were initially selected to meet the smoke density and flame spread requirements of the building code. Though the un-faced batts conformed to code, local building code inspectors asked that a finish layer be applied over the batts due to concerns about ambient fibers. Newport and the builder worked with the code inspectors to try to find an innovative, cost effective solution to this issue but ultimately the builder was forced to use insulation with a PSK finish for the below grade walls to fully satisfy the inspectors’ concerns.
When it came time to installing the batts, the installers were faced with highly irregular stud spacing in the precast concrete walls. “Standard” stud spacing within the walls was listed as 24 inches on-center. But a large number of the cavities were much smaller than the 19.75 inches – dimensions of 11 or 9 or 7 inches were common – which required trimming many of the batts to avoid compression and maintain the rated R-value. All said, the low hanging fruit of basement insulation required coordination between manufacturers, code officials, the builder, and the contractor with the final installed system unable to provide the same performance as the specified system, due to the batt insulation issue.
The picture above shows the complete basement wall system. The picture to the left, is a close-up of the expanding foam used to seal the band joist. Because the basement used precast concrete walls and high density spray foam was used to seal and insulate the above grade walls, band joists, and penetrations the subcontracted HERS rater reported an infiltration rate of 2.0 ACH at 50 Pa for the home.
To learn more about Gerber’s high performance home read this Case Study (PDF).