{"id":3685,"date":"2013-07-12T15:33:08","date_gmt":"2013-07-12T19:33:08","guid":{"rendered":"http:\/\/www.constructionspecifier.com\/?p=3685"},"modified":"2017-01-20T11:15:50","modified_gmt":"2017-01-20T16:15:50","slug":"to-vent-or-not-to-vent-deciding-what-is-best-for-attic-applications","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/to-vent-or-not-to-vent-deciding-what-is-best-for-attic-applications\/","title":{"rendered":"To Vent or Not to Vent: Deciding what is best for attic applications"},"content":{"rendered":"
\"Photo<\/a>
Photo courtesy CertainTeed<\/em><\/figcaption><\/figure>\n

by Mason Knowles
\n<\/strong>For decades, designers of attics and crawl spaces have used cross-ventilation to minimize the potential for moisture accumulation and condensation. However, spurred by recent claims of energy savings and moisture control, unvented attics have become popular in both residential and commercial applications. While these attics can be used in many circumstances, this author believes there are reasons to use vented assemblies in many situations.<\/p>\n

Traditional methods of insulation materials and design call for using air circulation within the attic space to assist in drying excess moisture. In heating and cooling climates, this moisture could potentially travel through fiber insulation in building cavities.<\/p>\n

The traditional remedy to this wetting is to slow the influx of moisture-laden air into the cavity by using an interior vapor retarder, and by ventilating the roof cavity to the exterior in order to facilitate the carry off of moisture (i.e.<\/em> drying).<\/p>\n

\"CS_July2013.indd\"<\/a>When done correctly, attic venting can reduce the potential for condensation in winter and summer. During winter, the primary cause of attic moisture issues stem from warm moist air infiltrating into the attic space from the inhabited areas and condensing on cold surfaces. This can be intensified when lights, pipes, vents, and other penetrations pierce the attic floor. Too often, mechanical ventilation ducts from bathrooms, kitchens, or laundry rooms deposit warm, moist air into the attic instead of outside the building envelope.<\/p>\n

A combination of air-sealing and insulating the attic floor while providing ventilation considerably reduces the potential for condensation, as warm moist air is less likely to enter the space and condense on cold surfaces. As a result, cooler, less humid air from outside can be drawn in from soffit vents placed on the roof\u2019s lower portion, and flow through to rooftop or ridge vents, replacing warmer moist air that may have infiltrated into the attic.<\/p>\n

In the summer, warm moist air mostly comes from outside the building. As such, it would seem venting the attic would increase the potential for condensation. However, the opposite is true if the attic floor is air-sealed and insulated.<\/p>\n

Even when outside air is hot and humid, if the attic space is air-sealed from the interior, it is much hotter than outside air. The hotter the air, the more moisture (i.e.<\/em> absolute humidity) the air space can hold. Therefore, replacing the hotter attic air with cooler outside air\u2014even at a considerably higher relative humidity (RH)\u2014tends to dry the space, minimizing the potential for condensation.<\/p>\n

As Figure 1 demonstrates, if the outside air is 32 C (90 F) and 70 percent RH, it is drier than interior attic space which is 43 C (110 F) at 40 percent or higher RH.<\/p>\n

This design is less efficient when HVAC equipment and ductwork is in the attic space. In these instances, systems have a harder time maintaining the desired temperature. Air within ducts has a difficult time maintaining temperature when the space is overly hot or cold and must extend for long runs. In moderate climates, this does not pose a significant issue. However, in more extreme environments, both hot and cold, it can be a problem.<\/p>\n

\"Some<\/a>
Some attics can combine both vented and unvented assemblies. In this case, the sprayfoam unvented side is separated from the vented side with an insulated wall. <\/span>Photos courtesy Mason Knowles Consulting LLC<\/em><\/figcaption><\/figure>\n

For example, depending on roof color and orientation to the sun, attic air temperatures can exceed 55 C (131 F) when it is less than 38 C (100 F) outside. This hot interior can make the ducts and HVAC equipment work much harder to reduce the temperature to comfortable levels. This is more pronounced if the ducts are leaky and HVAC equipment is drawing air from the attic space itself. Also, if the exterior surfaces of the HVAC equipment or ductwork reaches 26 C (79 F), it only requires 21 percent RH to cause condensation.<\/p>\n

Unvented attic assemblies<\/strong>
\nUnvented attics rely on an air-impermeable insulation installed to the roof deck\u2019s underside (i.e.<\/em> attic ceiling) to stop airborne moisture from reaching a cold surface and condensing inside the building envelope. In this design, insulation effectively separates the interior and exterior spaces while slowing down moisture flow so the dewpoint is not achieved within the building envelope.<\/p>\n

The two products most often used in an unvented attic assembly are medium- and low-density sprayed polyurethane foam (SPF).<\/p>\n

In typical construction and climates, building code tables can be followed when using SPF as an insulation and air seal. However, in cases where a vapor drive is consistently moving in one direction\u2014such as cold storage applications or swimming pools\u2014it is prudent to conduct hygrothermal modeling or calculations to determine if the proposed design is right for the application.<\/p>\n

Medium-density<\/em>
\nMoisture calculations of building assemblies (i.e.<\/em> hygrothermal modeling) and field observations demonstrate medium-density (i.e.<\/em> 2-pcf) SPF eliminates potential for condensation in most climate zones and situations without venting or additional vapor retarder elements.<\/p>\n

<\/a>As per ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials,<\/em> medium-density SPF has a perm rating of approximately 1.5 to 3.0 per 25 mm (1 in.) and an R-value of about 1.05 per 25 mm (6.0 per 1 in.). It has also been tested to perform as an air-impermeable insulation.1<\/sup><\/a> This combination of low permeance, high R-value per inch, and air barrier characteristics effectively slows vapor flow, separates the exterior environment from the interior, and eliminates introduction of moisture-laden air.<\/p>\n

\"Some<\/a>
Some attics with ducts running for long distances may be better served with an unvented attic assembly, such as the ones shown here.<\/span><\/figcaption><\/figure>\n

The physical properties and performance characteristics of SPF allow for the design of unvented attics and crawl spaces with minimal potential for condensation within.<\/p>\n

Low-density <\/em>
\nHygrothermal modeling of building assemblies and field observations demonstrate low-density SPF can be used in warm and mixed climates without an additional vapor retarder element. However, in colder climates, an additional vapor retarder element is required to prevent the potential for condensation.<\/p>\n

Low-density SPF has a permeance rating of between 8 to 15 per 76.2 to 127 mm (3 to 5 in.) and an R-value of approximately 0.616 per 25 mm (3.5 per 1 in.). When tested as part of an assembly, low-density SPF can be an effective air barrier.<\/p>\n

The result is the physical properties of low-density SPF effectively separate inside and outside temperatures and minimize air infiltration, but allow a higher rate of water vapor transmission than medium-density SPF. This facilitates the design of unvented attics in warm and mixed climates without an additional vapor retarder, but requires an additional vapor retarder element in colder regions.<\/p>\n

Either of these systems comes with a premium cost\u2014typically two to three times the price of a blown fiberglass or cellulose vented attic assembly.<\/p>\n

Combination attic assemblies<\/strong>
\nModern residential design consists of elevations that can create various attic spaces within the same building. For example, this author\u2019s home has attic space accessible with a standard door on the second floor and an upper section that can be reached only via a ceiling hatch. Dormers are also attached, making it extremely difficult to insulate as an unvented assembly. Additionally, some of the attic space is over an outside deck while other sections are over the house\u2019s interior space. Furnaces, ducts, and air-conditioning (AC) equipment run throughout the attic spaces on all levels, except the dormers and the space over the outside deck.<\/p>\n

When determining how to insulate this attic space, various assemblies were taken into consideration and a hybrid combination of vented and unvented attic space was planned.<\/p>\n

The dormers and the attic space over the outside porch were sealed off from the rest of the attic by making a wall of plywood, then insulating the wall with closed-cell SPF. Following this, closed-cell SPF was also installed to the underside of the roof deck.<\/p>\n

Building codes and attic assemblies<\/strong>
\nSince 2004, International Code Council (ICC) supplements to the International Residential Code<\/em> (IRC<\/em>) mean unvented attic assemblies have been accepted by the building codes in residential, but not commercial, applications. IBC<\/em> requires ventilation in attics and crawl spaces and does not address the unvented attic concept. However, many building code officials have accepted unvented attics on a case-by-case basis when presented with compelling evidence \u2014such as hygrothermal modeling of proposed assemblies\u2014that the assembly will function properly. The requirements have changed slightly over the years, but many of the elements have remained the same.<\/p>\n

\"CS_July2013.indd\"<\/a>The 2007 ICC supplement, International Energy Conservation Code<\/em> (IECC<\/em>) 202, \u201cGeneral Definitions,\u201d introduced three new classes of vapor retarders:<\/p>\n