November 30, 2010

Abstract: 

The following report is an excerpt from the 2010 Building Science Corporation Industry Team Building America Annual Report. The goal of this research is to find optimally designed, cost effective roof insulation systems that can be included with other enclosure details to help reduce whole house energy use by 70%. This report will compare a variety of roof insulating strategies and present their advantages and disadvantages according to several comparison criteria.

Preface

Building America Program

The objective of the U.S. Department of Energy’s Building America Program is to develop innovative system engineering approaches to advanced housing that will enable the housing industry in the United States to deliver energy‐efficient, affordable, and environmentally appropriate housing while maintaining profitability and competitiveness of homebuilders and product suppliers in domestic markets. For innovative building energy technologies to be viable candidates over conventional approaches, it must be demonstrated that they can cost‐effectively increase overall product value and quality while significantly reducing energy use and use of raw materials when used in community‐scale developments. To make this determination, an extensive, industry‐driven, team‐based, system‐engineering research program is necessary to develop, test, and design advanced‐building energy systems for all major climate regions of the United States in conjunction with material suppliers, equipment manufacturers, developers, builders, designers, and state and local stakeholders.1

Building America research results are based on use of a team‐based systems‐research approach, including use of systems‐research techniques and cost and performance trade‐offs that improve whole‐building performance and value while minimizing increases in overall building cost. This report describes the Building America Program research teams’ current state of knowledge of High R‐value Enclosures.

A. Introduction

Many concerns, including the rising cost of energy, climate change concerns, and demands for increased comfort, have lead to the desire for increased insulation levels in many new and existing buildings. Building codes are improving to require higher levels of thermal control than ever before for new construction and retrofit programs are helping many people to add insulation to their homes.

This report considers a number of roof insulation strategies that can meet the requirement for improved thermal and moisture control in colder climates. By code, roofs in DOE climate zone 6 require an insulating value of R49. In this report, higher R‐value (High R) roofs for zone 6 climates are those that approach or exceed R60 for compact roofs and R75 for vented attics. In a warmer climate High‐R may be considered less and in a colder climate high‐R may be considered more.

A successful roof (or roof‐ceiling assembly) will perform the following tasks

  • Provide a water management system to keep precipitation out
  • Provide an air barrier system between the indoors and outdoors
  • Provide a thermal control system to keep the heat out during the summer and retain heat during the winter
  • Provide a vapor control system to maintain a durable environment that does not allow condensation and does not promote mold growth

Roof failures typically occur due to leakage of bulk water (precipitation), or vapor diffusion condensation. By designing the roof enclosure system properly, the majority of all failures can be avoided in new construction.

In retrofit work, the order of work to be considered during construction or home improvements is important. Health and safety issues must be addressed first and are more important than durability issues.2 And durability issues are in turn more important than saving energy.

This study is an extension of the previous Building America study of High R wall assemblies (Straube and Smegal 2009) and High R foundations (Straube and Smegal 2009); all of these studies have the goal of improving the overall building enclosure and achieving greater energy savings. This study compares 20 roof enclosure designs and—through computer‐based simulations and field experience—demonstrates differences in energy consumption, thermal control, and moisture‐related issues.

The optimal true R‐value of roofs is often more complex to define than that for walls because of the impact of solar heating. Roofs in warm and hot climates experience high temperatures due to solar exposure for many months of the year: dark shingle roofs experience surface temperatures of 140°F to over 160°F on every sunny day. This generates daytime temperature differences of 60°F or more, which is similar to winter night‐time temperature differences experienced in a Zone 7 climate. Light‐colored / reflective roofs will experience much lower temperatures and may allow lower R‐values. The actual impact of low‐solar roofs on the choice of R‐values in High‐R roofs has not been conducted, although there has been considerable work investigating the performance of cool roofs by ORNL and LBNL and on the choice of R‐values of compact roofs (commercial) [Bianchi et al, 2007].

Unvented attics / cathedral ceilings are always significantly more expensive to build in a durable manner than ventilated attics. This additional expense is the reason for lower target R‐values for these roofs in our study; historically, code values have been lower due to geometric constraints and costs. Research has shown [BSC and others] that a significant energy penalty is incurred if a builder locates ductwork and equipment within a vented attic. Therefore, some may choose to use an unvented cathedralized attic or ceiling to allow HVAC equipment to be located within the conditioned space. Locating the ducts in conditioned space saves more energy than the reduction in recommended R‐value and increased surface area increases energy use, given typical air leakage of ductwork and air handler systems.

The analysis section of this report is divided into eight sections. These sections introduce the comparison criteria for the analysis, provide BSC’s high R‐value targets, analyze of the energy implications of high R‐value roof systems in two climates and compare a set of roof assemblies based on five criteria. These five criteria are; simulated R‐value, durability, buildability, material use and cost.

1. OBJECTIVE

The goal of this research is to find optimally designed, cost effective roof insulation systems that can be included with other enclosure details to help reduce whole house energy use by 70%. This report will compare a variety of roof insulating strategies and present their advantages and disadvantages according to several comparison criteria.

2. SCOPE

This study is limited to roof systems for cold climates; comparisons to warmer climates are made for reference. Previous studies were conducted for wall systems and basement/foundation systems in 2009. In general, only cold climates are considered in this report since enclosures in cold climates benefit the greatest from a highly insulated building enclosure; however, important conclusions can also be drawn for other climate zones.

This study does not deal specifically with retrofit strategies, but application of these assemblies in retrofit applications will be mentioned for any relevant insulation strategies. Many of the solutions that are examined in this report would be suitable for use in a retrofit application.

3. APPROACH

The quantitative analysis for each roof system is based on a two‐dimensional energy modeling program and a whole house energy model. Minneapolis, MN (IECC climate Zone 6) was used as the representative cold climate for most of the modeling due to the combination of the cold winters and fairly warm and humid summer months.

4. ROOF ASSEMBLIES EXAMINED IN THIS REPORT

There are a large variety of roof assemblies considering local practices, climate, the architect’s design or the general contractor’s preference. An attempt was made to choose the most common and most recommended roof systems, and to comment on possible alternatives during the analysis. This list of chosen systems is explained in more detail in the analysis section.

Roofs are constructed in many different forms: vented, unvented, cathedralized, attics, with dormers, etc. The different approaches taken to provide insulation and airtightness for this diverse set of assemblies have profound implications in terms of cost, durability and performance.

Vented Attics with Insulation at Ceiling Level

As is now well understood by the research community (but not always by the code or construction community), fully‐vented pitched attic assemblies (Figure 1) are the lowest cost, highest R‐value, and most durable roofs in all climates zones (except perhaps Zone 1 and Zone 2 due to coastal high humidity) if and only if no ductwork or major air leakage (e.g. recessed light fixtures, discontinuous ceilings geometries) are present above/in the ceiling plane. Given the low cost of ventilated attic insulation (loose‐fill fibrous insulation), rather high R‐values are justified even in climates with moderate exterior air temperatures, such as Zone 2, and very high levels (R60 to R100) are affordable and economically justified in Zones 5 through 8.

Figure 1: Vented Attic—The lowest-cost, highest thermal performance roof system

The incremental cost of changing from code‐mandated levels to R‐values of 60 to 100 is very small given that blown fibrous insulation has the least marginal cost per R‐value of all products (e.g. the cost for an additional R10 given R30 or 40 is to be installed is very small), and thermal bridging does not have much impact, if any, as the ceiling joists are all covered by a reasonable depth of insulation. Other than requiring an airtight ceiling, the only changes required to achieve twice current code levels of R‐value are the provision of “high heel/raised heel” trusses or rafter designs to accommodate the increased amount of insulation at the end of the roof.

Providing a truly airtight ceiling plane is very important, and is the most difficult task as it requires changes to how designers design and builders build. As the installed R‐value of insulation increases to R50, 60 or 80, the influence of even very small air leaks takes on great importance. Thus eliminating attic ductwork must be the first step, as well as sealing around all lights, partitions, access hatches, etc.

Good attic ventilation is necessary to remove whatever moisture may leak into the attic space so that it does not accumulate. The roof sheathing will drop below air temperature (by 5 – 20°F) every clear night, making condensation on the sheathing and framing of any water vapor in the airspace almost inevitable. Solar heating by the sun during the day can drive this moisture from the sheathing, but ventilation is required to remove it.

Vented Cathedral Ceiling Assemblies

Vented cathedral ceiling assemblies have long been used in housing. These assemblies operate under the same fundamental principles as vented attics. However, choosing a cathedral ceiling reduces the performance in a number of ways:

  • the depth of the structural framing members limit the depth of insulation that can be applied: more expensive insulation materials must be used to achieve higher R‐values
  • because the ventilation space is constrained, ventilation flow encounters more resistance, and hence the flow rate is less, decreasing moisture removal, and thus increasing the risk of moisture accumulation in the sheathing
  • Ventilation requires a direct path from the soffit to the ridge: this only occurs in simple gable roofs over rectangular plans with no dormers
  • The air barrier can easily be compromised (and often is, in typical construction) via penetrations through the interior ceiling finish or bypasses
  • thermal bridging reduces the true R‐value

The limitation on ventilation is the most severe compromise in this design. Very few plans are pure rectangles, not all roofs are gable roofs, and many roofs have dormers, hips, valleys, etc. Typical roof designs intended to be ventilated are not effectively ventilated in practice, and hence cannot recover well from small air leaks depositing moisture in the assembly.

The roof shown in Figure 2 would be appropriate for a High R roof in a warmer climate as its R‐value would be limited to roughly R40 with a 12” engineered wood I‐beam. However, by using a 16” engineered wood I-beam almost R60 can be achieved and the assembly could be used in colder climate: the foam layer is used in this assembly as a baffle to support the fibrous insulation and provide a deep clear ventilation space, and so higher R‐values are provided by thicker fibrous fill. It has the additional benefit of providing a more positive air barrier between the air‐permeable insulation and the ventilation space. The use of more than 12” engineered wood I‐beams are rarely justified on the basis of structural need, so the cost of the thicker engineered wood I‐beam is part of the cost of increasing the R‐value to 60. Airtightness at the ceiling plane is still critical to achieve, and the levels of airtightness required for a High R assembly are difficult to achieve in practice. Some necessary measures might include installation of interior partition walls after the ceiling drywall is installed, banning the use of any ceiling fixtures, and similar measures.

Figure 2: Ventilated cathedral ceiling

Another approach is to apply interior layers of air‐impermeable insulation (i.e., rigid foam board stock) as shown in Figure 3. This increases the R‐value and decreases thermal bridging. Again, building partitions after the interior layer of insulation has been installed, and the avoidance of any ceiling fixtures is required for this approach to be successful.

Again, it is important to note that both of these compact vented roof systems can only be vented if the roof is a simple gable with no dormers, valleys, hips, or other obstructions, or if specialized products are used to provide in‐roof ventilation at these obstructions.

Figure 3: Vented cathedral ceiling

Unvented Attic Assemblies

Unvented attic assemblies, or cathedralized attics, which move the insulation and airtightness planes to the slope, have been developed to overcome two major problems with vented attics (Figure 4). These problems are:

  1. locating ducts/air handling units in the attic space causes major air leaks of conditioned air (and thus forced infiltration/exfiltration), and heat/loss gain through the ductwork
  2. designs with complex coffered ceiling planes, numerous penetrations by lights, speakers, vents, etc. make it practically difficult to achieve the excellent airtightness required just below the insulation layer.

Figure 4: Cathedralized or unvented attics

Because High R roofs can be seriously impacted by even small leaks of indoor air, even ducts with 5% leakage are unacceptable: it is difficult to imagine a sensible scenario with a High R roof and ducts in unconditioned space.

If the ducts are to be installed in the attic, then an unvented cathedralized attic is a recommended solution. Alternate solutions for vented attics include covering all ductwork, and the ceiling plane with an uninterrupted layer of spray foam for air sealing (2” of closed cell foam or 3” of open cell foam), after which this foam is covered with thick layers of fibrous insulation. This is expensive, difficult (because of the geometry), creates equipment servicing difficulties, and may not be allowed by equipment manufacturers.

Unvented attics can be comprised of any unvented cathedral ceiling assembly (see below) but do not require a finish. However, the lack of a finish means that gypsum wallboard (GWB) cannot be used as a fire control layer. Code requirements for finishing cathedralized attics vary significantly across the country. . .

Download complete report here.

Footnotes:

  1. More information about the Building America Program, including other research publications, can be found at
    www.buildingamerica.gov
  2. The Attic Air Sealing Guide written by Joseph W. Lstiburek, Ph.D., P.Eng. provides the background and approach for the preparatory work necessary prior to insulating an attic or adding insulation to an existing attic. The guide focuses on combustion safety, ventilation for indoor air quality, and attic ventilation for durability. The Attic Air Sealing Details section of the guide provides a scope of work and specification for the air sealing of many points of air leakage in common attic spaces.