MOISTURE CONSIDERATIONS for INSULATED ROOF/ATTIC ASSEMBLIES June 2015

PENNSYLVANIA HOUSING RESEARCH CENTER

MOISTURE CONSIDERATIONS FOR INSULATED ROOF/ATTIC ASSEMBLIES June 2015 | Brian Wolfgang

INTRODUCTION In Pennsylvania, all 67 counties fall within Climate Zones 4, 5, and 6. These are primarily heating climates. This

www.autodesk.com/revit One of the main design considerations associated with results in attention being paid to interior water vapor

Consultant residential construction involves the building enclosure. Address Address primarily in the winter. Bulk moisture intrusion is a risk Address The building enclosure is made of up a series of assem- Phone Consultant year-round due to rain and snow events. Address Address Address blies (foundation, floor, wall, roof/attic) that separate Phone

Consultant Address Address interior conditioned spaces from the exterior environ- Address Bulk Moisture: Water Vapor: Phone

Consultant ment. Each building enclosure system is subjected to Rain, Snow, Ice Seasonal RH Address Address Address flows of heat, air, and moisture through the assembly. Phone Consultant Address Address Bulk Moisture: Address Much attention has been paid to the installation of ther- Phone Condensation mal insulation to reduce seasonal heat loss or gain in best practice and code requirements. Less attention has

No. Description Date been paid to the role of moisture within these insulated Water Vapor: assemblies. This document will outline some moisture- Occupants, Cooking, Bathing related considerations that should be taken into ac- Blank 1 1 1/2" = 1'-0" count during the design, construction, and inspection of Figure 1: Moisture sources at roof/attic assembly light-frame residential structures.

Owner DESIGN STRATEGIES MOISTURE SOURCES Project Name There are two main building enclosure design strategies Blank

As residential buildings have become more energy ef- that relate to moisture. Design to: Project Number Project Number Date Issue Date Drawn By Author ficient, the building enclosure has become more sensi- Checked By Checker tive to moisture-related damage. This is mainly due to 1. Prevent excessive wetting (material specifica- A5 Scale 1 1/2" = 1'-0" 6/8/2015 3:07:24 PM the lack of air and heat flow through various building tion, flashing details, etc.) assemblies that provided insurance against moisture 2. Maximize drying potential (material specifica- damage in the past. Having less heat and ventilation tion, weep details, etc.) available to provide drying of damp building systems requires greater attention to the design of the assembly In the context of roof/attic assemblies, many differ- up front. ent factors will contribute to the performance of the system. The following sections will walk through some In general, building enclosure assemblies are subjected of the main design options for this assembly and will to two different sources of moisture (see Figure 1): detail the moisture considerations associated with each 1. Bulk Moisture – Liquid water originating from option. moisture intrusion related to exterior precipita- tion or from condensation (condensation occurs INSULATION OPTIONS when water vapor condenses on a cool surface In this document, three different roof/attic insulation that is below the dew-point of the space) strategies are discussed: 2. Water Vapor – Moisture in gaseous form, typi- 1. Vented roof/attic assembly cally originating from interior sources (breath- ing, cooking, bathing) and from exterior sea- 2. Unvented roof/attic assembly with rigid foam sonal humidity 3. Unvented roof/attic assembly with spray foam

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VENTED ROOF/ATTIC ASSEMBLY openings in the upper (ridge) and lower (soffit) portions of the attic, but to also provide a clear path for ventila- Vented roof systems have long been a standard feature tion air to travel between these openings. This is typi- of light-frame residential structures as a means of heat cally accomplished through the use of baffles near the and moisture control. Roof/attic ventilation serves two eaves to prevent blown or batt insulation from imped- primary purposes related to moisture: ing the path of incoming ventilation air.

1. Keeping the roof surface cold in winter Ventilation is commonly provided through ridge vents, soffit vents, and other roof deck mounted ventilation 2. Providing ventilation of water vapor present in systems. The distribution of these is driven by roof the attic space design and product selection. Prior to the 2012 Interna- Why a Cold Roof? tional Residential Code (IRC), builders were encouraged to concentrate their ventilation openings in the upper One of the main moisture considerations for vented portion of the attic. This created a depressurized attic roof assemblies is the prevention of ice dams. Ice dams space, which commonly induced air leakage from inte- result from snow melting on the roof surface and re- rior space into the attic. Starting with the 2012 IRC, how- freezing at roof eaves. This creates a dam by which ever, these requirements changed to encourage a more subsequent snow melt can build up and penetrate the balanced distrbution, and in some cases, a greater con- roof assembly and enter the attic space. Ice dams can centration of ventilation openings in the lower portion be combatted in two distinct ways (see Figure 2): of the attic. This will slightly pressurize the attic space, 1. Air sealing drywall at attic floor thus reducing the likelihood of warm, humid interior air entering the attic space in winter. 2. Adequate ventilation Through proper installation of ventilation openings as By air sealing the top story ceiling drywall at seams and well as adequate air sealing, not only is warm air less penetrations (ceiling fixtures, especially recessed lights), likely to cause snow melt in winter, that same warm, hu- www.autodesk.com/revit warm interior air is less likely to enter the attic space mid air is less likely to condense on cool attic surfaces

and contribute to roof surface snow melt. in winter. A combination of ice dam and condensation Consultant Address Address prevention is necessary to ensure the durability of an Address In order to allow for adequate ventilation of the attic Phone insulated roof/attic assembly. space, it is important to not only distribute ventilation Consultant Address Address Address Phone

Consultant Ridge Vent or Ventilation Address Address Source in Upper Half of Attic Address Phone

Consultant Address Address Address Soffit Vents or Perforated Phone

Soffits Consultant Address Address Minimum Air Space Between Address Insulation and Sheathing Phone

No. Description Date Air Sealing at Critical Locations

FigureVented Roof Attic2: Details Critical moisture considerations in vented roof/attic assemblies 1 1 1/2" = 1'-0"

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Owner

Project Name VentedBIG

Project Number Project Number Date Issue Date Drawn By Author Checked By Checker A3 Scale 1 1/2" = 1'-0" 6/5/2015 2:51:07 PM PENNSYLVANIA HOUSING RESEARCH CENTER

UNVENTED ROOF/ATTIC ASSEMBLY minimized. When rigid foam insulation is used partially, WITH RIGID FOAM followed by the installation of air-permeable insulation such as fiberglass batts, it is important to provide The fundamental difference between a vented and un- a minimum R-Value in the rigid foam in order to keep vented attic is the location of the building enclosure. For the attic-side surface of the foam above the dew point. unvented roof/attic assemblies, the envelope is shifted Table 1 includes minimum R-Values based on the 2009 to the roof deck and now includes the finished roof ma- IRC for all climate zones in Pennsylvania. terial, underlayment, roof sheathing, and insulation. Table 1: Rigid insulation requirements for Rigid Foam and Unvented Attics condensation control in PA (based on 2009 IRC) Minimum Rigid Foam or One method to insulate the roof deck of an attic as- Climate Zone Air-Impermeable Insulation sembly is to install rigid foam insulation on top of the R-Value roof sheathing or on the underside of the roof sheath- 4 R-15 ing (see Figure 3). Rigid foam insulation is typically an 5 R-20 air-impermeable material and can provide appropriate 6 R-25 protection against condensation if designed properly. Another moisture consideration is related to drying po- The main moisture consideration related to unvented tential of the overall assembly. Since damp roof sheath- attics which incorporate rigid foam insulation is related ing will not be able to effectively dry toward the attic to condensation prevention. As condensation occurs when vapor semi-impermeable insulation is installed, when humid air comes in contact with a cooler surface it is critical to allow roof sheathing to dry toward the (below the dew point temperature), it is important to exterior. This will typically impact the selection of a roof www.autodesk.com/revit install an appropriate amount of air-impermeable insu- underlayment. Many traditional underlayments (i.e. felt lation in order to raise the temperature of the exposed paper) are vapor-open; however, some newer synthetic Consultant surface on the attic side to a temperature above the Address underlayments are Class I or II vapor retarders. An un- Address Address dew point. derlayment with a higher vapor permeability will allow Phone

for more effective drying of the roof assembly should it Consultant When rigid foam insulation is fully used to satisfy roof/ Address Address get wet from penetrations, storm damage, or ice dams. Address attic R-Value requirements, condensation potential is Phone

Consultant Address Address Minimum Air-Impermeable Ridge Vent Not Required Address Insulation R-Value per Table 1 Phone

Consultant Address Address Address Vapor-Open Under- Phone layment Preferred

Consultant Address Address Address Phone

Air-Permeable Insulation Permitted to Satisfy Remain- ing R-Value Requirements

Soffit Vents or Perforated No. Description Date Soffits Not Required Vapor Retarder on Underside of Air-Impermeable Insulation

Rigid Foam 1Figure 3: Critical moisture considerations in unvented insulated roof/attic assemblies using rigid foam 1 1/2" = 1'-0" www.PHRC.psu.edu Page 3

Owner

Project Name Rigid Foam

Project Number Project Number Date Issue Date Drawn By Author Checked By Checker A6 Scale 1 1/2" = 1'-0" 6/8/2015 3:07:29 PM PENNSYLVANIA HOUSING RESEARCH CENTER

UNVENTED ROOF/ATTIC ASSEMBLY Consideration for excessive wetting of roof assemblies WITH SPRAY FOAM impacts the selection of a type of spray foam. Open-cell foam is typically considered vapor-open with a vapor While rigid foam insulation can be used to create an permeability greater than 10 perms. Closed-cell foam, unvented roof/attic assembly, it is more common to see however, is tyipcally considered a Class II vapor retard- spray polyurethane foam used to pull the attic space er, with a vapor permeability between 1.0-10 perms, de- into the building enclosure (see Figure 4). Since it is pending on thickness and manufacturer specifications. uncommon to use air-permeable insulation in combination with spray foam, the main moisture consideration With a limited inward drying potential with foam in- with these assemblies involves wetting and drying stalled on the underside of the roof sheathing, it is through the transmission of water vapor. critical to prevent wetting of the roof assembly through vapor diffusion. Warm, humid air that is present in Open-Cell vs. Closed-Cell Foam the semi-conditioned attic space may introduce water vapor to the assembly without ensuring that a vapor As with rigid foam, another moisture consideration is retarder is present on the underside of the foam insula- related to drying potential of the overall assembly. It is tion. The same goes for rigid foam, however most rigid www.autodesk.com/revit critical to allow roof sheathing to dry toward the ex- foams are inherently Class II or III vapor retarders. Since terior. This will typically impact the selection of a roof Consultant closed-cell foam is also inherently a vapor retarder, Address underlayment. An underlayment with a higher vapor Address it tends to be the recommended selection. However, Address permeability will allow for more effective drying of the Phone codes typically allow open-cell foam to be installed as roof assembly should it get wet from penetrations, Consultant long as a vapor retarder coating or layer is installed Address storm damage, or ice dams. Address Address between the insulation and the attic space. Phone

Consultant Ridge Vent Not Required Address Address Address Phone Vapor-Open Under-

layment Preferred Consultant Address Address Address Phone

Consultant Address Address Address Phone

Soffit Vents or Perforated Vapor Retarder for Open Soffits Not Required No. Description Date Cell Foam (Coating or property of foam)

Figure 4: Critical moisture considerations in unvented insulated roof/attic assemblies using spray foam Unvented Roof Attic Details 1 This publication 1 1/2" = 1'-0" is available in alternative media on request.

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The Pennsylvania Housing Research Center (PHRC) exists to be of service to the housing community, especially in Pennsylvania. The PHRC conducts technical projects—research, development, demonstration, and technology transfer—under the sponsor- ship and with the support of numerous agencies, associations, companies, and individuals. Neither the PHRC, nor any of its sponsors, makes any warranty, expressed or implied, as to the accuracy or validity of the information contained in this report. Similarly, neither the PHRC, nor its sponsors, assumes any liability for the use of the information and procedures provided in Owner this report. Opinions, when expressed, are those of the authors and do not necessarily reflect the views of either the PHRC or its sponsors. It would be appreciated, however, if any errors, of fact or interpretation or otherwise, could be promptly brought to our attention. Project Name www.PHRC.psu.edu PHRC | 219 Sackett Building | University Park, PA 16802 Page 4 UnventedSFBIG

Project Number Project Number Date Issue Date Drawn By Author Checked By Checker A4 Scale 1 1/2" = 1'-0" 6/5/2015 2:51:13 PM