January 26, 2015

Abstract: 

This Measure Guideline describes a high performance enclosure retrofit package that uses mineral fiber insulating sheathing. It describes retrofit assembly and details for wood frame roof and walls and for cast concrete foundations. Exterior insulation retrofit is important to the goal of net zero energy ready homes. Mineral fiber insulating sheathing can provide enhanced moisture durability for the exterior enclosure. Mineral fiber also represents a viable solution for high performance home builders, designers, and clients who wish to use an alternative to foam plastic insulation.

Introduction

This Measure Guideline provides details and procedures for high performance retrofit using insulation board composed of mineral fiber material. Specifically, the guide demonstrates techniques for retrofitting three major enclosure components: roofs, walls, and foundation walls. The strategies and procedures developed in this guide are directed to wood-framed residential buildings with sloped roofs and cast concrete, block, or brick masonry foundation walls. The approach supported in this guide could also be adapted for use over masonry-clad or bearing masonry structures.

Table 1 shows the minimum assembly R-value that would constitute a high R-value enclosure for the various International Building Code (IBC)/International Energy Conservation Code (IECC) climate zones and pertinent enclosure components. These values are based on work by Straube (2010). A high performance retrofit, sometimes referred to as a deep energy retrofit (DER), results in enclosure components that meet these high R-value targets.

Table 1. Recommended Minimum R-Value for High R-Value Enclosures

Climate
Zone
Building Enclosure Component
WallCompact
Roof
Basement
Wall
 
Slab Edge
110355None
21540105
32045107.5
42545157.5
530501510
635602010
740652515
850753520

DER has been shown to provide a path to net zero energy ready homes for existing homes (Gates and Neuhauser 2013, 2014). High performance enclosure retrofit can position an existing building to perform at the level of best-in-class new construction. Retrofitting a wood-framed enclosure with thick layers of foam plastic insulation board is a proven high performance retrofit technique.

An exterior retrofit is generally more favorable than an interior retrofit because it is less disruptive to the living space and typically allows a structure to remain occupied during the project. Exterior retrofit also offers significant advantages for building durability by reducing the likelihood of cold weather condensation within the structure.

However, Ueno (2010) observed that although thick insulation board virtually eliminates the risk of condensation from outward vapor drive (cold weather condensation), vapor-impermeable insulation board can make a wall more vulnerable to water leaks than walls without insulation board. Homes built more recently often incorporate engineered wood sheathing (oriented strand board, OSB) and rim boards. When these elements are insulated to the interior with closed-cell spray-applied polyurethane foam (SPF) insulation, drying to the exterior becomes critical and foam plastic insulation board applied directly over the wall could present a problem (Lstiburek 2013b). Mineral fiber provides a vapor open insulation board that is key to making these assemblies both high R-value and moisture safe.

This Measure Guideline is important to the high performance retrofit industry because it demonstrates techniques for using vapor-open, noncombustible, mineral fiber insulation board in a high performance enclosure retrofit.

This Measure Guideline includes a review of decision criteria pertinent to the selection of a mineral fiber-based retrofit strategy. These criteria include relative cost, combustibility, moisture performance, cladding attachment, trade resources, as well as other considerations associated with the insulation material.

Risks that must be addressed with a high performance enclosure retrofit generally, and with a mineral fiber-based enclosure retrofit specifically, are outlined. Proper techniques for moisture management as well as climate-specific considerations are reviewed. Cladding attachment considerations are also discussed.

Although the Measure Guideline does not include any recognized fire resistance rated details or assemblies, it does present methods to improve the fire resistance of the retrofit assembly.

This Measure Guideline is intended to support contractors implementing a mineral fiber-based high performance enclosure retrofit as well as designers looking to design a high performance enclosure retrofit using an alternative to foam plastic insulation. The Measure Guideline may also be helpful to building owners wishing to learn more about strategies available for high performance retrofit of wood-framed residential buildings. 

1 Assess the Home

1.1 Risk Identification

1.1.1 Identify Site Risks

Before a significant retrofit project is conducted, the home must be assessed to determine whether significant, if hidden, problems should be addressed as higher priorities than an enclosure performance upgrade. Also, changes to the building enclosure will have a significant impact on the dynamics of water, air, vapor, and heat flow within the home. Certain measures should be implemented prior to, or as part of, any significant enclosure retrofit project to ensure that these changing dynamics do not have negative ramifications for health and safety or for building durability. Measures to be implemented prior to or as part of the enclosure retrofit outlined in this Guide relate to: 

  • Remediation of existing hazardous conditions
  • Combustion safety
  • Indoor air quality

The measures described to address indoor air quality include mechanical ventilation and soil gas control. These are not the only means to ensure good indoor air quality. However, effective mechanical ventilation and soil gas control are essential aspects of the building (as contrasted to furnishings, equipment, materials, etc. that people might bring into a building) that provide the occupants a means to control contaminants and respond to indoor air conditions.

1. Remediate hazardous conditions

Before a DER project begins, any hazardous conditions must be remediated that will be affected (e.g., exposed or aggravated) by the planned work. Also, it is wise to ensure that more serious problems are not lurking. A high performance enclosure retrofit—however worthy the goals may be—should not divert resources from where they are more urgently needed.

  • Before embarking on a project as outlined in this Guide, inspect and assess the building for:
  • Structural integrity of the frame and foundation
  • Safety and serviceability of the electrical system
  • Hazardous materials (e.g., lead, radon, asbestos)
  • Rainwater, groundwater, or plumbing water leaks
  • Rot or decay in framing
  • Insect and other pest damage and activity.

The structural system must be sound before high performance retrofit work can begin. Similarly, the electrical system must provide basic electrical safety. Knob-and-tube wiring may preclude retrofit of the enclosure; a licensed electrician should be consulted to discuss the nature of the retrofit and determine whether knob-and-tube wiring should be replaced. Ideally, the electrical system would be brought up to date with current code and brought into alignment with current and projected building needs. 

Hazardous materials that will be affected by the planned work or that may impact the indoor air quality must be remediated and/or removed to avoid exposure to occupants or workers. 

Any bulk water issues, whether these relate to rainwater leaks, groundwater infiltration, or plumbing system leaks, must be repaired before the enclosure retrofit project proceeds, unless the planned work will remove the source of the bulk water issue (e.g., if the roof surface or leaking section of plumbing is to be replaced as part of the project).

Any framing that shows signs of rot or decay should be replaced. This frequently applies to the framing sill in older buildings. Also, any framing that exhibits preconditions for rot or decay— i.e., damp wood—will require remediation.

Past insect and pest damage must be evaluated for impact on the building. Ongoing insect and pest activity must be remediated.

Follow applicable laws and industry procedures for mitigation of hazardous materials. Engage the services of a qualified professional when needed.

2. Ensure combustion safety.

Atmospherically vented (or naturally aspirated) combustion appliances are not appropriate for high performance homes. With the exception of gas cooktops, ranges, and wall ovens, combustion appliances should be direct-vented or direct exhaust-vented equipment. These types of appliances are significantly more energy efficient than natural draft appliances. If replacing equipment is not feasible, draft inducer retrofit kits can provide fail-safe forced-draft performance for some types of combustion appliances. This can address the combustion safety issue; however, adding forced-draft operation alone is unlikely to have a significant impact on energy performance.

Gas cooktops, ranges, and wall ovens must be equipped with a range hood that is ducted to the outside. 

Solid fuel-burning stoves (e.g., wood stoves and pellet stoves) if not equipped with a fail-safe draft inducer, must have a tight-fitting loading door (and no permanent openings or leaks into the firebox or flue) and outdoor combustion air ducted to the firebox. If any combustion appliance vents through a chimney, the flue should be inspected to ensure that it is in good condition. The flue must be repaired if it exhibits any deficiencies.

Attached garages present a potential source of carbon monoxide and other contaminants that could infiltrate into the home. Consult the Indoor airPlus guides for specific guidance on treatment of attached garages.

If the house has combustion appliances of any kind or an attached garage, carbon monoxide alarms complying with Underwriters Laboratories 2034 should be placed in reasonable proximity to the combustion appliances, in any regularly occupied space adjoining an attached garage, and outside each separate sleeping area in the immediate vicinity of the bedrooms.

3. Ensure adequate ventilation.

Mechanical ventilation is an essential feature of high performance homes. Ventilation is needed for source control at the location of contaminant sources (e.g., kitchens, bathrooms, craft areas) as well as for distribution of fresh air and dilution of general contaminants. The distribution of fresh air and dilution of general contaminant is referred to as background or whole-house ventilation. 

The 2012 International Residential Code (2012 IRC) provides code minimum requirements for ventilation systems, including the ventilation rate capacity for background/whole-house ventilation and source control ventilation. This Guide aims to provide additional guidance toward meeting the high performance objectives of DER projects.

4. Control soil gas.

Before any enclosure retrofit project begins, measures must be taken to prevent entry of soil gases into the home. Radon is a soil gas with significant and well-documented health risks. Air moving through soil also carries significant moisture and may carry other contaminants such as mold spores, herbicides, pesticides, and methane. 

As a minimum soil gas control measure, cracks and holes in the foundation walls and basement slab must be sealed. Because of the need for soil gas control, it is important that any penetrations made through ground-contact assemblies (slabs and foundation walls) are sealed. Sump pits must have airtight and gasketted covers.

Implementation of a soil gas venting system is strongly recommended. Any significant work to the roof or basement/crawlspace provides an opportunity to establish such a system.

A soil gas venting system essentially provides the soil gas a direct pathway from below the house to the outdoors so that it does not move through the indoor living space. If needed, a fan can be added to the vent pipe to actively move soil gases from beneath the basement floor to the outside.

For additional information on soil gas control, see Radon Fan and Vertical Radon Ventilation Pipe on the Building America Solution Center website.

1.1.2 Identify Work Risks

In addition to the hazardous material risks indicated above, other risks to worker and building safety must be addressed before work commences.

1. Locate gas and electric utilities.

The foundation retrofit measures described in this guide require excavation adjacent to the foundation. Before foundation retrofit work begins, any underground gas or electric utilities in the vicinity of the excavation must be located.

Overhead wires could present a safety hazard for wall and roof retrofit measures. Work and staging in the vicinity of overhead wires must be carefully planned to provide safe working conditions.

2. Assess staging needs.

The wall and roof retrofit measures described in this guide require ladders, staging, or mechanical lifts. Carefully review the area around the building to assess the suitability of the ground to support ladders, staging, or mechanical lifts.

1.2 Asses the Homoe to Determine Suitability of Options and the Need for Additional Control Measures

1.2.1 Roof Measure

Investigate the roof assembly to determine whether it has vapor retarders, and if so, what type. Examples of vapor retarders of concern could be class I and class II types such as OSB sheathing, closed-cell SPF insulation to the underside of the roof sheathing, kraft or foil facings on fibrous insulation, a polyethylene vapor retarder, and plaster finish with oil-based paint.

Assess the roof and wall structural systems to determine whether they can support a “chainsaw” approach. If the roof overhang framing is part the roof truss or if roof rafters are supported beyond the top of the wall on cantilevered framing, a “chainsaw” approach is not recommended.

1.2.2 Foundation Measure

The exterior foundation retrofit described in this Guide requires that the foundation wall surface be relatively flat. If the foundation wall is constructed of field stone or fractured block, other retrofit strategies should be pursued.

The exterior foundation retrofit described in this Guide also requires excavation around the perimeter of the building. Assess whether there are impediments to excavation. Excavation necessitates locating underground utilities in the vicinity of the planned excavation. 

It will also be important to assess how surface water and roof runoff can be drained. Does the site allow a drain to daylight? Will a storm water collection or drywell be required?

2 Decision-Making Criteria

2.1 Cost and Performance

Cost and performance are intricately linked and have to be studied in conjunction to determine the best choice per the decision-maker’s goals and objectives. Installation costs for the retrofit solutions described in this Measure Guideline can be expected to vary widely from estimates in the referred sources to field experience depending on such factors as contractor experience, prevalent region practices, material costs, and the particular circumstances of the project. It is worth noting that the range is sometimes a factor of 5 to 10.

2.1.1 Roof Retrofit

As with the walls, the National Residential Efficiency Measures Database (NREMD) does not directly provide a cost estimate for retrofitting insulating board to wood-framed roof assemblies. The retrofit measures provided that are relevant to a conditioned attic include both cavity insulation and insulation board. For a retrofit situation, it may reasonably be the mode that pre- retrofit case involves an insulated attic space; therefore, insulating within rafter cavities would be a necessary part of the measure. The NREMD does not provide costs for removal of roofing but does provide cost estimates for replacing an asphalt roof with a unit cost range of $1.90/ft2 to $4.30/ft2 (see Table 2).

Table 2. NREMD Unit Costs for Insulation Board and Cavity Insulation Roof Retrofit ($/ft2)

Retrofit Description
(Cavity insulation,
Insulation board)
Nominal
R-Value
Cost Range
($/ft2)
R-19 fiberglass, 3 in. XPS*342.10 - 3.60
R-19 fiberglass, 4 in. XPS392.50 - 4.20
R-30 fiberglass, 3 in. XPS452.40 - 4.10
R-30 fiberglass, 4 in. XPS502.80 - 4.70
R-38 fiberglass, 3 in. XPS532.60 - 4.50
R-38 fiberglass, 4 in. XPS583.00 - 5.10
R-38 fiberglass, 5 in. XPS633.40 - 5.80

* Extruded polystyrene

The National Grid DER pilot projects that Gates and Neuhauser (2014) evaluated pursued a roof/attic insulation target of R-60. The projects that met this target with a strategy including both insulation board and framing cavity insulation, all used either 4 in. or 6 in. of polyisocyanurate (PIC) insulation board. The reported unit costs for this strategy (excluding outliers) ranged from $10.05/ft2 to $21.84/ft2.

The target thermal performance in the National Grid DER pilot might correspond to the nominal values of the NREMD for R-38 fiberglass plus 4–5 in. of XPS insulting sheathing; however, the costs reported for the National Grid DER pilot projects would reflect elements beyond the insulation. For example, the reported pilot project costs would include removal of existing roofing, installation of an air and water control membrane over the existing sheathing, installation of a second layer of roof sheathing above the insulation board, and, in many cases removal and subsequent reconstruction of roof overhangs. Actual costs for the roof retrofit solution described in this guide are expected to be generally higher and more variable than the estimates derived from NREMD.

2.1.2 Wall Retrofit

The NREMD does not directly provide a cost estimate for retrofitting an existing wood-framed wall with insulation board; however, a composite of measures can be used to develop an estimate from this source. The NREMD provides a range and an average cost for various exterior insulation measures. The database also provides a figure for the cost of removing wall cladding ($0.66 regardless of cladding type), and estimated cost ranges for installation of new cladding. . . 

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