BUILDING ENCLOSURE: NEW CONSTRUCTION

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When to use this basis of design section:

This BOD section should be used for new construction projects. A high-performance building enclosure will minimize loads on heating/cooling systems and is essential to building durability, resident comfort, and energy efficiency at POAH properties. This section is intended to outline criteria for high performance building enclosure in new construction projects.

High Performance BUILDING ENCLOSUREs: CONTROL FUNCTIONS

POAH expects its design consultants to design and contractors to construct high performance building enclosures for all new construction projects.  POAH’s requirements in this regard may – and most likely do – exceed minimum code requirements. 

A high-performance enclosure will excel at providing the following 5 critical control functions:

• Water Control

• Air Control

• Thermal Control

• Vapor Control

• Critter Control

Requirements relative to these enclosure control functions are outlined below.  Note that the outline is not exhaustive.  Additional measures beyond those listed may be required to achieve performance objectives.  Some of these measures may not apply to specific projects (e.g. step flashing would not apply to a membrane roof).  Consult with POAH Design + Building Performance regarding deviations or questions of applicability.

GENERAL REQUIREMENTs:

• Building enclosure details in design documents must clearly designate the components providing these control functions. 
  Note: Color coding of control layers in details is required.

• Provide building sections that demonstrate continuity of Water, Air and Thermal control layers.
  Note: Color coding of control layers in sections is required.

WATER CONTROL REQUIREMENTS:

ROOF

• See the Roofing section of the Basis of Design for more information.

• Provide self-sealing water control membrane between wall sheathing and ledger boards or other structural attachments.

• Provide robust flashing at all wall openings such as window and door openings.

• Sill flashing drains to the OUTSIDE (use slope and back dam) over the water control layer or cladding.

• Jamb flashing continuous up full height of jamb.

• Head flashing diverts water away from the opening.

• Provide diverter flashing and opening protection at all penetrations through the water control layer, such as for pipes, conduits or structural members.

• Saddle flashing over penetration

• If using sealant, sealant laps over and adheres to (check compatibility) water control layer.

• Apply two-stage joint where appropriate/feasible.

• Provide proper flashing at all roof-wall intersections

  • A self-adhering polymer-modified bitumen sheet shall be installed at roof-wall intersections behind step flashing and extending at least 12” up the vertical face of the wall sheathing.

  • Step flashing integrated with roof cladding

• Top edge of flashings to be terminated with compatible tape or mastic or lapped shingle-fashion by a water control membrane.

For additional details applicable to windows, doors and roofing, see their corresponding Basis of Design Sections:

WALL - CLADDING

• Install all wall cladding and trim in a back-drained or “rain screen” configuration.

  • Brick veneer to have 1” clear drainage space behind

    • Through-wall flashing to be supported if not a rigid material (unsupported membranes can form troughs that inject water at open seams).

    • Provide weep openings 16” o.c. at base of wall and above through-wall flashings.

  • Fiber-cement or wood siding to have minimum 3/8” drainage/ventilation gap behind

    • Furring or spacers strips to be installed in vertical orientation only.

    • Do not caulk lap joints of lapped siding.

  • Vinyl siding without insulation backing is considered a vented and back-drained cladding.

• Maintain a separation of at least 2” clearance between wall cladding and roof surface or horizontal surface such as decks or paving.

FOUNDATIONS - SLAB

• Use the slope of grade to divert surface water away from the foundation.

• Provide a capillary break membrane between concrete and ANY wood member regardless of whether the wood is pressure treated.  Wood shall not be in direct contact with concrete.

• Provide a drainage layer/capillary break of at least 4 inches of washed gravel beneath slabs.

• Provide a vapor control/capillary break membrane in direct contact with the underside of the slab.

• Provide a capillary break between the footing and foundation wall or between the footing and soil.

• Provide protected damp-proofing at below-grade, ground-contact foundation surfaces.

• Provide drain tile draining to daylight or to sump/drywell around perimeter of footings.

• Protect drain tile with layer of clean gravel wrapped in filter fabric.

• Back fill above perimeter drain tile with free-draining backfill.

AIR CONTROL AND INSULATION REQUIREMENTS:

POAH has adopted the PHIUS+ standard to guide building enclosure thermal and air control of new construction project.  POAH has found that the R-values for opaque assemblies in a Passive House enclosure are not far beyond current IECC requirements, although POAH acknowledges that the R-values required may be beyond current local practice standards. 

PHIUS+ certification for the project is encouraged, but not required unless otherwise directed. The thermal performance (R-value and U-value) parameters listed below are provided for guidance. Refer to the Passive House Institute US (PHIUS) website (https://www.phius.org/phius-certification-for-buildings-products) for specific information.

Exterior Enclosure Air Control Requirement

Provide a continuous air barrier around the entire building enclosure to meet a tested performance of no more than 0.06 cfm air leakage per sf enclosure when tested at a pressure of 50 Pascals. 

Note: If areas of the building are excluded from the whole-building air barrier (e.g. compactor rooms, commercial space, etc.) the separation between the residential program and these spaces must meet the air tightness requirement.

Airtightness is sometimes regarded as a proxy for overall construction quality. The airtightness of a building and of apartments within buildings are properties that can be measured. 

The common method for testing and verifying the airtightness is through the use of a calibrated fan and pressure measuring gauges. In the construction industry the common term for this testing apparatus is “blower door”. The widespread availability of blower door equipment and qualified technicians allows for quantitative airtightness targets to be established for new construction and renovation projects.

See Compartmentalization section of the Basis of Design for in-unit air control measures.

THERMAL CONTROL - OPAQUE COMPONENTS (R-VALUE) REQUIREMENTS:

Thermal bridge-free.  Minimize thermal bridging through the insulation layer.  Note that insulation between metal girts cannot be considered continuous insulation.  Any insulation layer that is subject to thermal bridging must be discounted accordingly.

High-R-Value Assemblies. Provide a building enclosure (including the integrated performance of the whole enclosure) capable of meeting Passive House heating and cooling performance requirements. The table below is provided for guidance. A Passive House analysis can be used to refine the specific requirements for the project.

Below-Grade Insulation. In order to actualize continuous insulation for a building, it is important to consider below-grade insulation. Rigid foam insulation is preferred for this application.

For additional requirements for windows and doors, see their corresponding Basis of Design Sections:

VAPOR CONTROL REQUIREMENTS:

Vapor retarders are classified according to their vapor permeability:

• Avoid use of low permeability (i.e. Class I vapor retarder such as polyethylene) vapor retarders or wallpaper to the interior side of any wall assembly.

• In warm climates (climate zone 3 and below) using a Class II or Class III vapor retarder to the exterior of the sheathing is recommended.  Many weather-resistive barriers (WRB) or air-and-water barriers are also vapor retarders, check product data sheet.

• In all climate zones, provide sufficient insulation to the exterior of the sheathing to allow the use of Class III vapor retarder (gypsum wallboard with latex paint) at the interior side of framed wall assemblies.  Generally, this will mean that from 30% (in warmer climates) to 45% (in cold climates) of the total wall assembly R-value is located exterior to the sheathing.

CRITTER CONTROL (PEST CONTROL) REQUIREMENTS:

Necessary measures for pest control depend on local conditions and local pest pressures.  Follow recommended pest practices for the locality.

General requirements (all locations):

• Provide a robust termite barrier between any below grade insulation and non-concrete building components.

• Do not plant vegetation such that, when mature, it will be within 1’ of the building.

• Provide screening or metal closure at the bottom of exterior wall assemblies such that exterior insulation and ventilation cavities of exterior walls are not exposed.

• Seal around all penetrations through foundations.  Pack larger gaps (wider than 3/8”) with copper mesh before applying sealant.

• Use a flashing membrane to seal the bottom of sheathing to the foundation wall.  Bituminous flashing membrane with aggressive adhesive works best.  Prime the foundation wall as needed for effective adhesion.