Unvented roof systems

Over the past five years several unvented roof insulation systems have been developed by the Building Science Consortium for use primarily in hot-dry and hot-humid climate zones. Designs and houses with unvented roof insulation systems have also been developed and constructed in cold and mixed climates.

The optimum systems developed to date are dependent on the climate location. The systems fall into two categories:

systems where condensing surface temperatures are controlled systems where condensing surface temperatures are not controlled The two categories essentially are the demarcation between regions where cold weather onditions 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.

The key is to keep the roof deck - the principle condensing surface in roof assemblies - sufficiently warm throughout the year. This can be accomplished either because of the local climate or as a result of design - principally through the use of rigid insulation installed above the roof deck. Where rigid insulation is installed above the roof deck, condensing surface temperatures are said to be controlled. Where the top of the roof deck is uninsulated, the condensing surface temperatures are said to be uncontrolled.

The demarcation is specified as a distinction between regions where the monthly average temperature remains above 45 degrees F throughout the year and where it drops below 45 degrees F during the year. The criteria was selected for three reasons.

First, by keeping the roof deck above 45 degrees F, condensation can be minimized or eliminated. Condensation will not occur unless the dew point temperature of the interior air exceeds 45 degrees F and this air contacts the roof deck. This interior dew point temperature is approximately equal to an interior conditioned space temperature of 70 degrees F at an interior relative humidity of 45 percent. These are interior moisture conditions that can easily be avoided with air change/ventilation or the avoidance of over humidification during the coldest month of the year in the climate regions specified.

Second, a monthly average temperature was selected, rather than a design heating temperature, as it is more representative of building envelope performance. Short term, intermittent "spikes" in parameters/environmental loads are of interest to structural engineers and in the sizing of equipment, but are not typically relevant to moisture induced deterioration. Wood based roof sheathing typical to residential construction has sufficient hygric buffer capacity to absorb, redistribute and re-release significant quantities of condensed moisture should intermittent condensation occur during cold nights when the sheathing temperature occasionally dips below 45 degrees F. The average monthly conditions more accurately reflect moisture content in wood based assemblies.

Third, this definition characterizes a region that is almost identical to the ASHRAE definition of hot-humid climates if a 20 inch annual precipitation criteria is also added. A powerful argument can be made for unvented roof construction in hot-humid climates in order to control interior moisture. This definition allows simple unvented roof design ("just don't install vents") in a previously recognizable climate zone (ASHRAE's hot-humid zone). Practicality is also enhanced. Average monthly temperatures are easy to obtain as are annual precipitation values. ASHRAE defines hot-humid climates as having a 67 degree F or higher wet bulb temperature for 3,000 or more hours during the warmest six consecutive months; or, a 73 degree F or higher wet bulb temperature for 1,500 or more hours during the warmest six consecutive months is difficult to apply.

Figure 1 is a map outlining the demarcation between regions that require the control of condensing surface temperatures and regions that do not. Note that both Hot-Humid and Hot-Dry Climate Regions do not require the control of condensing surface temperatures. However, all other regions require some form of control.

Control of condensing surface temperatures typically involves the installation of insulating sheathing above the roof deck. In residential wood frame construction this involves installing rigid insulation between the roof shingles and the roof plywood or OSB (Figure 2). The installation of the rigid insulation elevates the temperature of the roof deck to minimize condensation.

Figure 3 and Figure 4 illustrate the differences between the two fundamental systems. Figure 3 shows the potential for condensation of an unvented roof assembly in Houston, TX. Houston, TX is located in a Hot-Humid Climate Region. This roof assembly has no insulating sheathing installed above the roof deck. Figure 4 shows the potential for condensation of an unvented roof assembly in Dallas, TX. Dallas, TX is located in a Mixed-Humid Climate Region. Note that this roof assembly has rigid insulation installed above the roof deck in order to control the condensation potential.

The thermal resistance of the rigid insulation (thickness) necessary to control condensation depends on the severity of the climate. The colder the climate, the greater the resistance of the rigid insulation required. Note that the thermal resistance of the rigid insulation is based on the ratio of the thermal resistance of the insulation above the roof deck as compared to the thermal resistance of the insulation below the roof deck.

The key is to elevate the temperature of the condensing surface to 45 degrees F or higher during the coldest months of the year. Figure 5 shows the use of rigid insulation in a cathedral ceiling assembly in Washington, DC. A calculation procedure is presented that determines the temperature of the condensing surface. Figure 6 plots the temperature of the condensing surface. The graph shows that condensation within the roof assembly will not occur if interior conditions are maintained at 45 percent relative humidity or less at 70 degrees F during the coldest month of the year.

Figure 7 shows a roof design that is not as dependant on controlling interior moisture levels as the other roof designs presented. The absence of cavity insulation yields the highest condensing surface temperature of any of the designs presented. In this particular design, the condensing surface is the air barrier membrane installed over the wood decking. With this design interior relative humidities should be kept below 60 percent in order to control surface mold.

In Cold and Severe-Cold Climate Regions where there is likely snow accumulation on roof surfaces, there is also the likelihood of ice-damming. In order to control ice-damming, heat flow from the interior to the roof cladding must be minimized. In Cold Climate Regions the minimum total R-value for the entire unvented roof assembly should be R-40. In Severe-Cold Climate Regions this minimum R-value should be increased to R-50.

Note that in these types of unvented roof assemblies, interior vapor barriers are not recommended as these assemblies are expected to be able to "dry" towards the interior.

The optimum system developed to date where condensing surface temperatures are not controlled uses an air permeable cloth netting that is suspended from the upper chords of roof trusses or roof rafters. The cloth netting creates a cavity that is blown with loose fill insulation, either cellulose or fiberglass. The system provides cost and performance advantages over the installation of batt insulation by hangers or by wires. One of our industry partners - Greenfiber, is currently marketing the approach in Las Vegas, NV; Tucson, AZ; and Phoenix, AZ using cellulose. One of our builder partners - Pulte Home Corporation has adopted this approach as part of its production strategy in both Las Vegas and Tucson.

The system provides cost and performance advantages over the installation of batt insulation by hangers. The cellulose net approach is approximately 30 percent less expensive than batt insulation and hangers. It also appears to provide thermal performance advantages over batt insulation due to the elimination of the void areas typically found between batts.

A co-heating test was conducted between two buildings with identical floor plans over a two-week period. One unvented attic had approximately R-22 calculated thermal resistance netted cellulose. The other unvented attic had R-30 fiberglass batts supported at the underside of the roof sheathing with wires. Both houses were unoccupied and maintained at the same internal temperature. Within the range of experimental and test error, both houses performed identically.

The development of this insulation system reduced the incremental cost of constructing and unvented roof by approximately $200 to $250. In a typical unvented roof system using fiberglass batt insulation, the incremental insulation cost is approximately $1,000. A $250 savings is achieved by not installing roof vents resulting in an incremental cost of $750 for the overall approach using batts. Using the netted cellulose approach, the incremental cost of the insulation is approximately $750. Coupling this with the savings associated with not installing the roof vents yields a system incremental cost of approximately $500. Both approaches save around 1 Ton of cooling capacity. The cost of an installed Ton of capacity is approximately $500.

The cost implications are immediately obvious. There is no incremental cost for using netted cellulose when coupled with the downsizing of the HVAC system, but there is an incremental cost for using fiberglass batts.

With the success of the netted cellulose system, focus was turned back to batt insulation systems that could be installed in direct contact with the underside of roof decks. The reason for this shift in focus is that fiberglass batts remain the dominant insulation system in residential construction. Additionally, many insulation sub contractors refuse to use cellulose insulation due to financial and market ties to fiberglass manufacturers. Although we are pleased with the netted cellulose approach, there is currently insufficient cellulose sub-contractor infrastructure to service the production home builder market. Additional work in the batt insulation area was judged to be strategically important.

The following systems for batt insulation installation were considered:

Specially manufactured fiberglass batts with long mounting flanges that could be fastened to the underside of the top chord of roof trusses.

Metal straps installed parallel to the top chord of roof trusses on top chord/bottom chord connecting members used to support horizontal wood lath members that would in turn support the batt insulation.

Using 2x6 top chords on roof trusses with kraft-faced batt insulation.

The first two systems proved to be not cost competitive with the baseline approach of installing fiberglass batts with piano wire suspended from top chord/bottom chord connecting members. Additionally, neither approach successfully addressed the issue of convection around suspended batts.

The specially manufactured fiberglass batts added about $250 to the baseline fiberglass batt installation approach. The metal strap-truss modification approach saved on labor, but proved to be a "nightmare scenario" for truss manufacturers. Adding the strap disrupted production an unacceptable amount. An alternative approach of adding a wood truss member in place of the metal strap added too much in cost to be practical.

The third system was less expensive to install than the baseline approach - although additional costs were incurred due to the more expensive truss design. However, in production it is believed that truss design could be optimized using 2x6 top chords for no truss incremental cost. This approach also addressed the air convection issue due to the kraft paper facing on the batts acting as a convection suppressor.

Unfortunately, the major problem with this approach is the flammability of the kraft paper facing. No solution to this problem has been found at this time. A different facing material or a treated facing material is the obvious solution. However, no manufacturer is interested in manufacturing such a batt due to "limited market size".

No further work is planned in this area at present. The use of piano wire to suspend fiberglass batts appears to be the optimum approach at this time. Since this approach is more expensive than the netted cellulose approach, the cellulose installers are gaining market share at the expense of the fiberglass batt industry in the Las Vegas, Tucson and Phoenix markets. We wish them continued success so that enough "pain" will be inflicted to induce fiberglass manufacturers to reconsider their "limited market size" dismissal of a fire treated kraft faced fiberglass batt.

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• moisture content in roof insulation
• References related to roof and floor
• Self-drying roof
• vapor barrier during roof renovation
• vapor barrier during roof renovation
• vapor barrier during roof renovation
• Self-drying roof
• Self-drying roof
• References related to roof and floor
• References related to roof and floor
• moisture content in roof insulation
• moisture content in roof insulation