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-dimensional heat flow analysis simulations have been executed following the protocols outlined in ISO 6946 and ISO 10211 where applicable.

SIP Panels:

Assembly Types:

Wall 01: Exterior [Target R-45]

  • As per InsulSpan requirements PIB212, use fully-adhered weather-resistive-barrier over primary wall sheathing for air-control and waterproofing. WRB to be:
  • As per InsulSpan requirements PIB212, install 3/4" wood battens over exterior WRB for siding attachment.
  • Recommended: Add 2-1/2" ‘Service Cavity’ inboard of the SIP panel for running all plumbing and electrical. This will help ensure the continuity and integrity of the interior OSB airtightness layer.
  • Optional: The ‘service cavity’ may be insulated with typical batt insulation (fiberglass, mineral wool, etc…) if additional R-Value is desired.
  • Use 1/2" GWB or sim on interior with typ. latex paint (Class III vapor-retarder).

Roof 01: Sloped [Target R-55]

  • As per InsulSpan requirements PIB203, use fully-adhered weather-resistive-barrier over primary roof sheathing for air-control and waterproofing. WRB to be:
  • Recommended: Add 2-1/2" ‘Service Cavity’ inboard of the SIP panel for running all plumbing and electrical. This will help ensure the continuity and integrity of the interior OSB airtightness layer.
  • Optional: The ‘service cavity’ may be insulated with typical batt insulation (fiberglass, mineral wool, etc…) if additional R-Value is desired.
  • Use 1/2" GWB or sim on interior with typ. latex paint (Class III vapor-retarder).

Roof 02: Flat [Target R-55]

  • For low-slope roof, install sloped insulation exterior to SIP panel in order to establish drainage. Target 4" of insulation min.
  • As per InsulSpan requirements PIB203, use fully-adhered weather-resistive-barrier over primary roof sheathing for air-control and waterproofing. WRB to be:
  • Recommended: Add 2-1/2" ‘Service Cavity’ inboard of the SIP panel for running all plumbing and electrical. This will help ensure the continuity and integrity of the interior OSB airtightness layer.
  • Optional: The ‘service cavity’ may be insulated with typical batt insulation (fiberglass, mineral wool, etc…) if additional R-Value is desired.
  • Use 1/2" GWB or sim on interior with typ. latex paint (Class III vapor-retarder).

Floor 01: Ground Slab [Target R-17]

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

  • Target +/- 4" 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.

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-season 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.

Winter Heat Loss due to Envelope Air Leakage

Code-minimum construction in most states require an air-tightness rate of somewhere between 3 to 7 air-changes per hour (ACH), and the ID ECC 2020 / Zone 6(B) requires residential buildings in Climate Zones 3 to 8 to demonstrate an air-tightness level of less than 5.0 ACH@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 1.3 ACH@50Pa. The graph above shows the heat loss at various levels of air-tightness for the building. Improving air-tightness to this low level (1.3 ACH@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.

Redline Diagram

  1. Typ. Floor Slab: Use sub-slab vapor-barrier as airtight layer. Tape / seal all joints, penetrations and seams. Sub-slab vapor barrier to be min 15Mil. sheeting or better. Install sub-slab vapor-barrier above insulation, in direct contact with concrete.

  2. Detail: Slab to Wall: Turn sub-slab vapor-barrier up wall min 12" and seal with elastic sealant and tape. Use termination bar to ensure perm

  3. Typ. Wall: Airtightness layer to be INTERIOR layer of SIP OSB sheathing. Seal all joints, seams and penetrations as per manufacturer’s instructions. Recommended: Install ‘service-cavity’ to avoid penetrations through this OSB layer.

  4. Typ. Roof: Airtightness layer to be INTERIOR layer of SIP OSB sheathing. Seal all joints, seams and penetrations as per manufacturer’s instructions. Recommended: Install ‘service-cavity’ (drop-ceiling) to avoid penetrations through this OSB layer.

  5. Detail: Roof-to-Window Connections: Consider how the roof and window-wall will connect and how the airtightness will be robust enough to handle the movement of roof and window elements over time?

  6. Detail: Floor-to-Window Connections: Thermal-bridge-free and airtight detail at the window-wall base connection to floor.