BSD-102 Understanding Attic Ventilation Building Science Information

BSD-102 Understanding Attic Ventilation Building Science Information

Attics or roofs can be designed and constructed to be either vented or unvented in any hygro-thermal zone (Map 1). The choice of venting or not venting is a design and construction choice not a requirement determined by the physics or by the building code. The model building codes allow both vented and unvented roof assemblies. The applicable physics impacts the design of attic or roof systems as does the applicable building code but neither limit the choice.

Throughout the balance of this digest the terms attic and roof will and can be used interchangeably.

In cold climates, the primary purpose of attic or roof ventilation is to maintain a cold roof temperature to control ice dams created by melting snow, and to vent moisture that moves from the conditioned space to the attic (ventilation acts to bypass the vapour barrier created by most roof membranes). Melted snow, in this case, is caused by heat loss from the conditioned space. The heat loss is typically a combination of air leakage and conductive losses. The air leakage is due to exfiltration from the conditioned space (often because a ceiling air barrier is not present) and from leaky supply ductwork (often because ductwork located in attics is not well sealed) and from penetrations like non-airtight recessed lights. The conductive losses are usually from supply ductwork and equipment located in attic spaces above ceiling insulation (ductwork is typically insulated only to R-6—whereas ceiling insulation levels are above R-30). Conductive losses also occur directly through insulation, or where insulation is missing or thin.

In hot climates, the primary purpose of attic or roof ventilation is to expel solar heated hot air from the attic to lessen the building cooling load. The amount of cooling provided by a well ventilated roof exposed to the sun is very small. Field monitoring of numerous attics has confirmed that the temperature of the roof sheathing of a unvented roof will rise by a few to no more than 10 F more than a well ventilated attic.

 Map 1: IECC/IRC Climate Zones

The amount of attic cavity ventilation is specified by numerous ratios of free vent area to insulated ceiling area ranging from 1:150 to 1:600 depending on which building code is consulted, the 1:300 ratio being the most common.

Control of ice dams, moisture accumulation and heat gain can also be successfully addressed by unvented attic or roof design.

Why Two Approaches – Vented and Unvented?

Vented attic and roof construction has a long history of successful performance. Why change a good thing?

As the complexity of attic and roof assemblies increases, the difficulty to construct vented assemblies also increases. The more complex a roof geometry, the easier it is to construct the assembly in an unvented manner. With complex roof designs, multiple dormers, valleys, hips, skylights combined with cathedral construction with interior soffits, trey ceilings and multiple service penetrations (Photograph 1 ) it is often not practical to construct a vented roof assembly with an airtight interior air barrier at the ceiling plane.

Additionally, it is becoming more common to locate mechanical systems and ductwork in attic spaces in all climate zones. When such ductwork is leaky significant problems can occur (Figure 1 ).  There are significant energy advantages and durability advantages to move the thermal boundary and pressure boundary (air barrier) to the underside of the roof deck (Rudd,  Lstiburek, & Moyer;  1997 ) thereby locating these mechanical systems and ductwork within the building conditioned spaces (Figure 2 ).

Figure 1:   Ductwork Exterior to Thermal and Pressure Boundary

Supply ductwork and air handler leakage can be more than 20 percent of the flow through the system.

Leakage out of the supply system into the vented attic results in an equal quantity of infiltration through the enclosure. In cold climates the heat loss can lead to ice dam creation, in hot humid climates the infiltration leads to high latent loads due to infiltration into the conditioned space. In all climates this leads to thermal penalties – increased energy consumption in the order of 20 percent of the total space conditioning load (Rudd and Lstiburek, 1997) .

In hot humid climates condensation on ductwork and air handlers located in vented attics is common.

Figure 2:   Ductwork Interior to Thermal and Pressure Boundary

Duct leakage does not result in infiltration or exfiltration (air change) as ductwork is located within the conditioned space.

This results in significant energy savings compared to Figure 1 .

In high wind regions – particularly in coastal areas, wind driven rain is a problem with vented roof assemblies. Additionally, during high wind events, vented soffit collapse leads to building pressurization and window blowout and roof loss due to increased uplift. Unvented roofs – principally due to the robustness of their soffit construction — outperform vented roofs during hurricanes – they are safer.

In coastal areas salt spray and corrosion are a major concern with steel frames, metal roof trusses and truss plate connectors in vented attics.

Finally, in wildfire zones, unvented roofs and attics have significant benefits in terms of fire safety over vented roof assemblies.

Approach

The main strategy that should be utilized when designing roof or attics to be free from moisture problems and ice dams along with control of heat gain or heat loss regardless of ventilation approach is the elimination of air movement, particularly exfiltrating air in cold climates and infiltrating air in hot and hot humid climates. This can be accomplished by the installation of an air barrier system or by the control of the air pressure difference across the assembly (depressurizing a building enclosure reduces the exfiltration of interior air – pressurizing a roof assembly with exterior air also reduces the exfiltration of interior air).

Air barrier systems are typically the most common approach, with air pressure control approaches limited to remedial work on existing structures (Lstiburek & Carmody, 1994 ).

Vapor diffusion should be considered a secondary moisture transport mechanism when designing and building attics. Specific vapor retarders are often unnecessary if air movement is controlled or if control of condensing surface temperatures is provided.

Vented Design

Vented attics should not communicate with the conditioned space – they should be coupled to the exterior. Therefore, an air barrier at the ceiling line – such as sealed gypsum board — should be present to isolate the attic space from the conditioned space.  Ideally, no services such as HVAC distribution ducts, air handlers, plumbing or fire sprinkler systems should be located external to the air barrier (Figure 3 ).

Figure 3 :  Vented Roof Assembly

Roof insulation thermal resistance at roof perimeter should be equal or greater to thermal resistance of exterior wall.

1:300 ventilation ratio recommended.

The recommended ventilation ratio to provide for vented attic assemblies when an air barrier is present, is the 1:300 ratio (as specified by most building codes). This is based principally on good historical experience and simple psychrometric analysis (Handegord & Giroux, 1984 ).

In vented cathedral ceiling assemblies a minimum 2-inch clear airspace is recommended between the underside of the roof deck and the top of the cavity insulation. This is not a code requirement but ought to be (only 1-inch is typically specified in the model codes). It is the author’s experience that typical installation practices and construction tolerances do not result in an airspace of at least 1 inch and rarely is it “clear.”  Even when 2” clear space is provided, the rate of ventilation flow will be significantly less than in an open ventilated attic.

In addition to an air barrier at the ceiling line, a Class II vapor retarder (see sidebar) should be installed in Climate Zones 6 or higher (see Map 1 ).

Class I vapor retarders (i.e. vapor barriers – see sidebar) can be installed in vented attic assemblies in Climate Zones 6 or higher (see Map 1 ) but should be avoided in other climate zones as top side condensation can occur in summer months during air conditioning periods.

No interior attic assembly side vapor control is required or recommended in climate zones other than Climate Zones 6 or higher (see Map 1 ) for vented attic assemblies (note the distinction, this is not the case for unvented attic assemblies as will be discussed later). With vented attic assemblies moisture that diffuses into the attic space from the conditioned space is vented to the exterior by attic ventilation.

Unvented Design

Unvented attic design falls into two categories: systems where condensing surface temperatures are not controlled (Figure 4 ) and systems where condensing surface temperatures are controlled (Figure 5 ). The two categories essentially are the demarcation between regions where cold weather conditions occur with sufficient frequency and intensity that sufficient moisture accumulation from interior sources can occur on an uninsulated roof deck to risk mold, corrosion and decay problems.

No potential for condensation on the underside of the roof sheathing until interior moisture levels exceed 50 percent RH at 70 degrees F.

Figure 5 :  Condensing Surface Temperature Controlled

Potential For Condensation in Dallas, TX With Unvented Roof And Insulating Sheathing (see also curve).

Rigid insulation installed above roof deck.

No potential for condensation on the underside of the roof sheathing until moisture levels exceed 40 percent RH at 70 degrees F. when rigid insulation is not present.

Rigid insulation is recommended in this roof assembly to raise the condensation potential above 50 percent RH at 70 degrees F.

Ratio of R-value between rigid insulation and batt insulation is climate-dependent.

The key is to keep the roof deck – the principle condensing surface in roof assemblies (Figure 6 ) – sufficiently warm throughout the year or to prevent interior moisture laden air from accessing the roof deck. This can be accomplished in several ways:  the local climate may be such that the roof deck stays warm, or rigid insulation can installed above the roof deck, or air-impermeable insulation (typically spray foam – Photograph 2 ) is installed under the roof deck in direct contact with it.

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