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Project Team: Boston Green Building, Builder and DER Lead; Building Science Corporation, DER Consultants and Technical Support; National Grid, Massachusetts, DER Pilot Program Administrator/Sponsor
Location: Arlington, Massachusetts
Description: Comprehensive retrofit and upward addition of over-under duplex; 2,340 ft2 pre-retrofit; 3,430 ft2 post-retofit; 6 bedrooms, 4 baths total; 3 stories plus unconditioned basement
Completion Date: March 2011
Estimated Annual Energy Savings: Predicted source energy reduction is 57% from pre-retrofit conditions
Overview
The project for this owner-occupied two-family residence started with the idea of enlarging the upper unit to accommodate a growing family and renovating the lower floor unit for the mother of one of the owners. With support of the National Grid DER Pilot program, the owners were able to realize these objectives while dramatically reducing energy consumption.
The project involved removing the roof and adding a full third floor. Exterior insulation was added to the existing walls as well as the newly constructed walls. Windows were replaced throughout. The renovated apartments received new heating systems with new distribution, new water heating systems and heat recovery ventilation systems. The interior of the building was gutted during the course of the renovation. The project was staged such that the first floor apartment interior work and exterior insulation was completed first, then the family moved into this lower unit while work progressed on the upper floors.
This project provides an example of a major addition and renovation that incorporated super insulation and other higher performance enclosure and mechanical system measures. It also provides an example of the difficulties in achieving robust air and thermal control at an existing basement ceiling.
Design
To accommodate the desired increase in space, the design called for demolition of the roof and half-story to make way for a new 3rd floor and roof to be framed on top of the 2nd floor. The design for the retrofit of the existing enclosure as well as the new structure is intended to provide a high level of thermal performance.
Initially the design called for a vented roof with deep layers of cellulose insulation on a flat top-floor ceiling and vented cathedralized ceilings. The builder then determined that an unvented roof assembly, with insulation to the interior of the roof deck, would be more feasible.
For the exterior walls, the design provided a thick layer of exterior insulation over a housewrap on the retrofit walls and over a Zip System™ wall at the 3rd floor addition. Open-cell spray foam was specified for insulation and air flow control in the wall cavities of the first floor apartment unit. Wall cavities of the upper apartment unit were insulated with a fiberglass batt insulation.
Where acceptable to the client, the builder selected casement windows with the intention of reducing air leakage through window units.
Against the recommendation of BSC, this project decided to exclude the partially-finished basement from the thermal enclosure. With the basement excluded from the thermal enclosure, robust airflow control would be needed at the floor over the basement as well as at the stair access from each apartment to the basement. This configuration also placed the air handler for the first floor apartment and some of the ductwork in an ostensibly unconditioned space. The initial design for the floor over the basement was to apply a flash coat of closed-cell spray foam to the underside of the subfloor, a continuous layer of taped foil-faced rigid insulation to the underside of the floor framing, and a dense-packed cellulose cavity fill. For cost reasons, the closed-cell spray foam was limited to application of canned foam as a sealant at penetrations through the subfloor and at the perimeter of and penetrations through the rigid insulation layer. The builder used open-cell spray foam in the walls of the basement access stairs to isolate these from the apartments.
Parametric Study
Enclosure Design
Roof Assembly: R-58 (nominal) unvented roof assembly: 9” closed-cell spray foam; ¾” roof sheathing, roofing felt; asphalt shingles.
Roof Assembly
Wall Assembly: R-38 retrofit assembly 1st and 2nd floor: open-cell spray foam or fiberglass batt in 2x4 wall on first floor; board sheathing with housewrap; two layers of 2” polyisocyanurate insulating sheathing, joints offset and taped. R-41 new construction assembly 3rd floor: fiberglass batt in 2x6 wall; taped Zip System™ wall sheathing; one layer 2” XPS; one layer 2” foil-faced polyisocyanurate with seams taped.
Wall Assembly
Window Specifications: New EcoShield triple pane, low-E, argon fill, vinyl framed, double-hung and casement windows; U=0.22-0.21, SHGC=0.21-0.18; window installed proud of drainage plane on strapping.
Window Specifications
Air Sealing: Housewrap with lapped and taped seams; taped exterior insulation layer; open-cell spray foam at 1st floor framing cavities and basement access stair walls; closed-cell spray foam in roof rafter cavities extended onto back side of wall insulating sheathing; taped foil-faced rigid insulation at basement ceiling; closed-cell spray foam to underside of enclosed porch floor.
Air Sealing
Foundation Assembly: R-30 (nominal): dense-packed cellulose in floor framing cavities; 1” foil-faced polyisocyanurate to underside of floor framing with seams taped; one-part foam sealant at perimeter of and penetrations through rigid insulation layer.
Foundation Assembly
Construction
The staging of the project required that exterior wall retrofit measures and interior work for the first floor apartment be essentially complete before work could commence on the renovation and addition of the 2nd and 3rd floors.
The project budget did not allow for detaching the open porches at the rear of the building in order to allow the air and thermal control to be applied in a continuous layer behind the porch connection. To address the concern for airflow control and insulation, the builder cut back a strip of porch roof adjacent to the exterior wall of the building so that spray foam could be applied against the wall and around the porch roof framing.
The installation of windows over strapping rather than in plane with the face of exterior insulation created challenges for proper flashing of the windows. A sequencing problem emerged where vertical strapping adjacent to windows prevented window head flashing from being connected back to the drainage plane. To provide head flashing across the top of the window, the adjacent vertical strapping would have to be cut and the upper piece temporarily removed.
At both the newly constructed roof at the 3rd floor and existing lower roofs over conditioned space, projecting rafters created a condition of thermal bridging and difficult airflow control transitions. At the newly constructed roof, exterior insulating sheathing was notched around rafters and extended up to the underside of the roof sheathing. This allowed the closed-cell spray foam of the roof system to seal between roof sheathing and wall insulating sheathing and the framing top plate. At the overhang of existing roof sections where there was not access from the interior, the transition of airflow control was more challenging. At these locations, exterior insulating sheathing was notched around projecting rafters in order to allow one-part foam sealant to seal between the projecting framing and the exterior insulation.
Mechanical Design
Heating and Cooling: 96.6% AFUE variable speed condensing furnaces located in 1) insulated mechanical space in unconditioned basement, and 2) conditioned mechanical closet inside apartment; refrigerant coil in air handlers prepped for future A/C.
Condensing furnace
Ventilation: 3 speed HRV, 65-200 cfm nominal capacity, ducted to heating distribution system; one for each apartment.
Heat recovery ventilator (HRV)
Space Conditioning Distribution: 1st floor apartment with air handler and ductwork in unconditioned basement, partially within insulated mechanical space; upper apartment distribution entirely within conditioned space.
Ductwork in upper apartment
DHW: 0.95 EF, gas-fired, condensing, on-demand water heater, one for each apartment located in insulated mechanical space in unconditioned basement.
On-demand water heater
Lighting: ENERGY STAR® CFL or LED lighting throughout.
Appliances: ENERGY STAR® dishwasher, refrigerator and clothes washer.
Testing and On-Site Technical Support
BSC conducted site visits to review construction details with the builder, point out errors to repair in flashing or other elements, and to identify viable simplifications to the project.
BSC conducted blower door testing to assess the air leakage of the structure both before retrofit work and after substantial completion of the project. Because the plan for this two-family structure excluded the basement from the thermal enclosure, BSC also performed a series of diagnostic tests to assess the significance of leakage across the basement separation relative to the air leakage of the thermal enclosure as a whole.
The pre-retrofit fully unguarded test of the apartment space found it to be rather air leaky at 8730 cfm50 or 26 ACH50. Comparison to guarded testing of the apartment space and basement suggests that about 45% of the combined apartment leakage is at the basement-apartment separation. After substantial completion of the retrofit scope, BSC found the unguarded leakage measurement for the combined apartment enclosure to be 3586 cfm50 or 7.3 ACH50 with leakage across the basement-apartment separation at least 1750 cfm50.
Moving Forward
Much was learned from this project in terms of, for example, the effectiveness of basement separation, the importance of sequence in windows installation and flashing, and connections between roof and wall assemblies. Because of the experience of this project and measurements that BSC was able to provide, the builder has revised assumptions about basement insulation and adopted a different approach to window installation on subsequent DER projects.
Design Challenge: Whether to Include or Exclude Basements Basements present a host of challenges to high performance retrofit. Basements tend to be cool, damp and musty spaces. Often low framing heights render the spaces unsuitable for habitable space. Basements can also be a source of soil gas or other air borne contaminants. What is often not adequately appreciated is that basements tend to have fairly strong airflow connections to living spaces above. Insulation and air sealing at the ceiling over the basement may initially seem a more cost effective thermal enclosure retrofit than properly insulating the entire basement. However, many factors make it difficult to provide effective airflow control between a basement and adjacent spaces. The difficulties in achieving a robust separation despite strong efforts were evident in this project. Although the overall leakage of the basement space was significantly reduced as a result of the retrofit measures, the basement remained nearly three times more leaky to the apartment spaces than to the outside directly.
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