Recommended Assemblies

Below we have included recommendations for the building envelope assemblies based on the Energy model results shown in previous sections. These are recommendations only and should be reviewed by the design team to ensure they are appropriate for the project.

The following assembly U-Values have been assessed using Flixo Energy v8.1. All assemblies are proposed based on the Energy model results shown in previous sections. Note: All 2-dimension heat flow analysis simulations have been executed following the protocols outlined in ISO 6946 and ISO 10211 where applicable.

Wall 01: Exterior [Target R-40]

• Install continuous XPS foam or Graphite EPS insulation outboard of primary wall structure:
  • Minimum: R-7.5 as per MA code 702.7.1.
  • Recommended: R-20
• Install foam board in min. of 2 layers, offsetting board seams.
• Install 3/4" wood battens over continuous insulation for siding attachment. Battens fastened through continuous insulation into wood studs min. 1-1/2".
• Use fully-adhered weather-resistive-barrier over primary wall sheathing for air-control and waterproofing, Pro-Clima Solitex Adhero 3000 or sim.
• Use 2x8 wood framing. Where possible, 24" OC with ‘advanced framing’ techniques.
• Stud-bay insulation batt to be cellulose, fiberglass, mineral-wool or open-cell spray foam.
• Use 1/2" GWB or sim on interior with typ. latex paint (Class III vapor-retarder).

Roof 01: Sloped [Target R-55]

Due to the complex roof shape and the cathedral ceiling spaces, it is recommended to use an ‘un-vented’ roof assembly throughout the home and to rely on the exterior underlayment for air-control. Comply with all local code requirements for un-vented roofs, as per Residential Code of Massachusetts | Section R806.5 Note: Rafter sizing for illustration only. Rafter sizing to be determined by structural engineer or architect.

• Install continuous XPS foam or Graphite EPS insulation outboard of primary roof structure:
  • Minimum: R-20 as per MA code 806.5.
  • Recommended: R-25
• Install foam board in min. of 2 layers, offsetting board seams.
• Install roof-sheathing over continuous insulation for roofing attachment. Alternatively: Use one-piece ’nailbase’ panel.
• Use fully-adhered roof membrane underlayment on primary roof sheathing for air-control and waterproofing, Pro-Clima Solitex Adhero 3000 or sim.
• Joist-bay insulation batt to be cellulose, fiberglass, mineral-wool or open-cell spray foam.
• Use 1/2" GWB or sim on interior with typ. latex paint (Class III vapor-retarder).

Floor 01: Ground Slab [Target R-25]

Note: Concrete floor shown for illustration only. Concrete floor thickness to be determined by structural engineer or architect.

• Target +/- 5" sub-slab XPS foam board (or Graphite-EPS) below concrete.
• Sub-slab air and vapor barrier to be installed directly above insulation. Use 15 Mil Stego-wrap or sim.
• Tape / Seal all penetrations and seams of sub-slab air and vapor barrier.

Recommended Details

Typ. Roof to Wall Connection

Note: See assembly details above for more information on recommended products.

1. Rafter bay insulation.
2. Air-barrier / roof underlayment over primary roof sheathing.
3. Continuous rigid insulation over primary roof sheathing.
4. Secondary roof sheathing for roofing attachment.
5. Stud Wall Insulation
6. Air-barrier / WRB over sheathing.
7. Continuous rigid insulation over sheathing.
8. 3/4" Wood battens running vertically, fasten through continuous rigid insulation into wood studs.

Typ. Roof to Wall Connection

Note: See assembly details above for more information on recommended products.

1. Stud Wall Insulation
2. Sill Gasket.
3. Air-barrier / WRB over sheathing.
4. Continuous rigid insulation over sheathing.
5. Non-metallic flashing.
6. Conc. frost-wall / footing as per Arch. / Eng.
7. Sub-Slab vapor-barrier.
8. Extend vertical insulation down to footing.
9. Cement-board or sim. cover board over insulation.

Building Airtightness

The primary role of air-tightness in buildings is to avoid interstitial condensation and mold/moisture damage to the structure during the winter and shoulder months. Additionally, in hot climates the air-tightness plays an additional important role in restricting warm outdoor air and moisture vapor ingress from the exterior. This helps to reduce energy consumption needed for cooling and dehumidification while improving occupant comfort and building resiliency.

As well as its role in ensuring building durability, air-tightness levels have a simple linear relationship to the building’s heat loss: the more air- tight the construction the less heat is lost in winter and the better the energy performance. In addition, the air-tightness of the building has a large effect on the indoor relative humidity during the summer months with a corresponding reduction of cooling energy consumption and dehumidification need.

Code-minimum construction in most states require an air-tightness rate of somewhere between 3 to 7 air-changes per hour (ACH), and the Massachusetts Energy Code 2021 / Zone 5 requires residential buildings in Climate Zones 3 to 8 to demonstrate an air-tightness level of less than 3.0 @50Pa. In order to meet the recommended building performance level, this project would have to achieve the extremely stringent air-tightness level of less than 1.0 @50Pa. The graph above shows the heat loss at various levels of air-tightness for the building. Improving air-tightness to this low level (1.0 @50Pa) is one of the best ways to improve performance and increase comfort and durability and is the recommended target for this building.

Building Redline Test

Illustrated below is an example of the ‘red line’ evaluation test. This test, commonly used in Passive House design, requires that a single continuous air barrier (commonly denoted with a red line) is able to be traced around the entire conditioned envelope of the home. Any area where the air barrier line is broken or ambiguous is marked for further development and clarification by the design team.

The goal of this exercise is to identify and clearly document all critical junctions, transitions and penetrations of the air barrier. In the diagrams below, the areas marked with callouts are judged to be critical details and we recommend that the detailing phase address these items and clearly identify the air-tightness layers, products and transition details. Establishing an unambiguous air barrier for this project with all needed project details will be critical to its success and achieving the high-performance targets.

1. Typ. Floor Slab: Use vapor-barrier as airtight layer. Tape / seal all joints, penetrations and seams. Sub-slab vapor barrier to be min 15Mil. sheeting or better.

2. Typ. Exterior Wall: Airtightness layer to be the exterior WRB underneath continuous insulation. Ensure all penetrations, transitions and junctions are detailed and clearly identified. Ensure all penetrations are sealed with airtight tape or sealant. Ensure all transitions are detailed and clearly identified.

3. Typ. Roof: Airtightness to be exterior roofing membrane underneath continuous insulation.

4. Roof-to-Roof Connections: Due to relatively complex roof geometry, roof-to-roof connections may be challenging in some areas. Ensure that roof underlayment is continuous, including through all hips, valleys, dormers, or other roof elements.

5. Roof-to-Wall Connections: Ensure roof-underlayment is connected to wall WRB at all eave connections.

6. Wall-to-Floor Connections: Ensure wall airtightness (WRB) is connected to frost-wall using sealant and / or tape. Ensure all transitions are detailed and clearly identified and products are specified. In particular, ensure that specified sealant/tape is compatible with any frost-wall waterproofing.