February 1, 2011

Abstract: 

The following report is an excerpt from the 2010 Building Science Corporation Industry Team Building America Annual Report. Many concerns, including the rising cost of energy, climate change concerns, and demands for increased comfort, have led to the desire for increased insulation levels in many new and existing buildings. Building codes and green building codes are being changed to require higher levels of thermal insulation both for residential and commercial construction. This report will review, and summarize the current state of understanding and research into enclosures with higher thermal resistance, so-called “High-R Enclosures.” Recommendations are provided for further research. For more information see Popular Topics/Foundations and Slabs and Popular Topics/High R-Value Walls.

Chapter 1: Introduction to High R-Value Enlcosures

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 and green building codes are being changed to require higher levels of thermal insulation both for residential and commercial construction. This report will review, and summarize the current state of understanding and research into enclosures with higher thermal resistance, so-called "High R Enclosures".

High R enclosures will reduce energy consumption for space heating in all climate zones. Their impact is largest in climates with cold temperatures for many hours, and smallest in climates with few hours per year at cold temperatures. However, High R enclosures are still important for enclosures exposed to the direct solar radiation in hot climates: the roof is the obvious example, especially if finished in dark colors that absorb solar radiation.

The introduction will present some definitions and background information. The balance of the paper is a review of current research by Building America research teams and other groups. Challenges remaining to be solved and recommendations for further research are addressed in the conclusion.

Why Control Heat Flow

Good thermal control is fundamental to good housing in all climate zones: achieving low-energy and ultimately net zero energy homes in a cost- and resource-efficient manner will require exceptional thermal control. Space heating and cooling remain the largest components of the energy use of typical new and existing houses. For the current stock of housing (see the Residential Energy Consumption Survey 2005, Table US14, excerpts below) space heating/cooling consumes more than half of the total site energy. In new construction, better thermal control has resulted in lower fractions (about 45%), but thermal control remains one of the lowest cost, most durable, easiest to predict, best developed means of reducing household energy use. 

Table 1: RECS 2005 Energy Consumption Data

The control of heat flow is important for more than just saving energy: by controlling interior surface temperatures, heat flow control helps ensure human comfort and avoid cold-weather condensation. As described later, controlling the temperature of various elements and layers within an enclosure assembly can be used to avoid condensation or enhance drying, both of which influence durability.

Thermal control is typically seen as being provided by insulation. However, air barrier systems, radiant barriers, solar control coatings, and thermal breaks are also part of any enclosures thermal control system.

Defining the High R-Value Enclosure

The term “High R” enclosures attempts to bring together what is known about delivering exceptionally good control of heat flow through walls, roofs, windows, and foundations.  High-R enclosures are more than just assemblies with a lot of insulation, they are systems that are airtight, have little thermal bridging, are buildable, manage solar heat gain, ensure human comfort, and provide moisture control to ensure durability and health expectations are met.

There are no widely accepted definitions of the term “High R”, but we will use the term here to describe much higher thermal control than the code mandates, usually offering around twice as much resistance to energy flow as more common approaches, but also meeting high standards of buildability, durability, health, and comfort.

R-value is commonly used to measure the thermal control of insulation products. However, this metric does not account for the impacts of thermal bridging, air leakage, installation quality, or thermal mass. It this multitude of factors that working together deliver good thermal control.

ORNL conducted work over a decade ago [ORNL 1994, 1995] in which they defined a number of useful terms:

  • Center-of-Cavity R-value: R-value at a point in the wall's cross-section containing the most insulation.
  • Clear-wall R-value: R-value of the wall area containing only insulation and necessary framing materials (i.e., for a clear section with no fenestrations, corners, or connections between other envelope elements such as roofs, foundations, and other walls).
  • Interface details: A set of common structural connections between the exterior wall and other envelope components, such as wall/wall (corners), wall /roof, wall/floor, window header, window sill, door jam, door header, and window jamb, that make up a representative residential whole- wall elevation.
  • Whole-wall R-value: R-value for the whole opaque wall including the thermal performance of not only the "clear wall" area, but also all typical envelope interface details (e.g., wall/wall (corners), wall /roof, wall/floor, wall/door, and wall/window connections).

High R enclosures must of course be measured by whole-wall R-values. But at the same time, higher levels of airtightness, good durability, and comfort must be added.  True R-value is what is actually required to measure performance, but this is not yet developed as a scientific measure.

Thermal continuity / Thermal Bridging

Continuity of the thermal control layer is important. Heat flow deviates from one-dimensional at corners, parapets, intersections between different assemblies, etc.  When heat flows at a much higher rate through one part of an assembly than another, the term thermal bridge is used to reflect the fact that the heat has bridged over / around the thermal insulation.  Thermal bridges become important when:

  • they cause cold spots within an assembly that might cause performance (e.g., surface condensation), durability or comfort problems
  • they are either large enough or intense enough (highly conductive) that they affect the total heat loss through the enclosure

Thermal bridging through insulation by wood framing causes significant heat loss. Reductions in R-value of 25% are typical for wood framed walls, and 40% reductions regularly occur. The trend to increasing complexity in building plans results in an increasing fraction of wood framing relative to studspace insulation.  This phenomenon is well understood in the scientific literature, and excellent tools have been developed to predict this impact with reasonable accuracy (for example, the DOE-sponsored THERM software package is widely used for this purpose by researchers and advanced practitioners).  Unfortunately, thermal bridging is poorly understood by the design and construction

industry, and residential building codes do not address it directly. Techniques to address dramatically reduce thermal bridging are available and reasonably well proven, but face many challenges in reaching widespread acceptable.  Advanced framing (see BSC 2010 Advanced Framing fro details) and insulating sheathing can make significant reductions in thermal bridging and thus increase thermal resistance without increasing “installed” R-value.  True R-values (e.g., including thermal bridging of realistic enclosure framing details) for more wall assemblies are needed and should be disseminated more widely.

Some new enclosure systems attempt to increase the thermal resistance by increasing the thickness of framed walls (e.g., by increasing from 2x6 to 2x8, or 10, 12, or 14” deep TJI roof rafters, or by the use of double-stud framed walls). These systems continue to suffer from thermal bridging, and hence their true R-value is constrained. Double stud walls (enclosure walls with two disconnected framed walls with voids filled with insulation) notionally reduce thermal bridging, but the extent of the reduction is highly dependent on the details at floor joists, basement interfaces, window frames, etc. 

It is practically acceptable to have some small local interruptions and reductions in the efficacy of the heat flow control. Penetrations such as nails and screws rarely affect the energy, durability, or comfort-related performance of the enclosure.  Even a rather small amount of thermal control, e.g. R2 to R4 (the thermal resistance of double-glazed windows) is tolerated over significant areas of many enclosures, although the energy and comfort penalty can be significant if the area covered is large.  The trend to greater window area without commensurate improvements in window thermal performance leads to both increased energy consumption and reduced comfort at the same time that such practices increase construction costs. 

Airtightness

As homes become better insulated, unplanned air leakage can consume a greater proportion of heating and cooling energy.  Hence, as insulation values are increased, airtightness should also be increased. At the same time, air leakage can result in damaging condensation, moisture damage, and poor Indoor Air Quality, in all climate zones, but especially hot-humid and cold climate zones.  The airtightness of American homes has improved significantly over the last several decades [LBNL study], but still lags behind that of some other countries such as Canada, Sweden, etc.  Perhaps more significantly, although most of the homes built in the US require specific minimum insulation values, there is no minimum airtightness level required, or even airtightness measurement.

Part of the challenge is that the effectiveness of air barrier systems are largely a result of careful design and workmanship.  Products can be specified, but it is difficult to enforce robust details and inspect the many small, and often hidden, details that can cause leaks.  Testing, usually via a blower door, has been shown to be a very effective means of testing the airtightness of a home, and assisting a builder in achieving specific targets. The challenge remains that too few homes are tested for airtightness. Even those new homes and retrofits tested for airtightness are often tested when most of the construction is complete: if targets are not met, it is difficult and expensive to find and repair hidden leaks. It would be useful to develop airtightness approaches, technologies, and testing protocols that allow for early airtightness characterization easily and less expensively. 

Durability

Reducing the control of heat flow across an enclosure can either increase its durability or decrease it relative to standard construction depending on how that heat flow reduction is achieved.  High R walls are no different. By adding insulation inside of wood sheathing / cladding, the moisture content of the sheathing / cladding will rise in cold weather, the risk of condensation increases significantly, and outward drying potential is reduced.  Adding insulation also increases the risk of condensation in the summer time, but on the interior finish, especially if vapor impermeable (vapor barriers, cabinets, mirrors, etc).

Solutions exist for these and other durability risks: use insulation exterior to the sheathing, using lower permeance insulating exterior sheathing, building a ventilation space outside of the

sheathing/behind the cladding, build a more airtight enclosure, provide better rainwater management (e.g. drained sub-sill flashing), etc.

Different enclosure assemblies will have different requirements to meet or exceed current durability expectations.  In general, drained and ventilated claddings, exterior insulating sheathing, and high airtightness combine to provide an enclosure that is more durable even when insulated to high levels.

Quality of Construction

The quality of construction can negatively impact thermal control. Some insulation systems are more prone to worker error than others although this has not been well characterized.  Progress has been made in encouraging the proper installation of insulation via inspection and the HERS rating scale. However, this approach must be more widely deployed to ensure expected performance matches actual performance. 

The actual energy impact of poor insulation installation has not been measured using standard ASTM hot-box enclosure tests at either very cold and very warm temperatures (when flaws have the most impact).  Without this information it is difficult to quantify the impact of defects.

Comfort

Delivering thermal comfort is primarily about maintaining interior air temperature and the Mean Radiant Temperature (MRT): the former is delivered by typical mechanical systems, measured and controlled by the thermostat, whereas the MRT is controlled by the thermal control of the enclosure.   ASHRAE 55 provides the tools to assess comfort.

Current building code requirements for opaque walls and roofs generally control the surface temperatures well enough for comfort. Poor thermal control, especially at windows, slab edges, and areas of framing congestion, can result in low enough or high enough interior surface temperatures as to compromise comfort. In climates with lower heating needs, enclosure thermal control is often seen as being less important.  However, we have had reports of thermal comfort problems in these climates because of solar gains heating the interior pane of solar control glazing, exposed slab edges, and even dark-colored walls with low, but Code-approved, thermal resistance.   Finally, air leaks can cause local jets of cold air that compromise comfort.  High levels of airtightness can solve most of these problems.

Economic Aspects

To conduct proper cost-benefit of life-cycle costing one needs to know, among other things, the future price of energy, the cost of interest (discount rate), the cost of materials and labor, the replacement intervals and maintenance costs for equipment and assemblies. Given this wide range of unknowns, High R enclosures can either by shown to be highly favorable (assuming 7% per annum increases in energy costs, low labor costs, and low discount rates) or very expensive (assuming no energy cost increase, high discount rates, and expensive materials and labor).  In short, without an agreed upon set of assumptions of what the future will hold, it is difficult to make decisions on current investments using life-cycle cost analysis.

A recent analysis by PNNL [Taylor an Lucas 2010] analyzed the savings achievable by increasing R-values in new residential construction. Energy cost savings of 30% were shown across many different climate zones. The analysis, like many similar studies, assumed current energy costs even though homes built today will likely last 75 years.  Similarly, the savings are based on the first years operation, which ignores the significant cost of replacing mechanical equipment over the life of the building.

Most analysis of “payback” in Building America are conducted using simple payback: savings per year/cost of upgrade = years to payback.  Other more sophisticated approaches use a fuel escalation rate, but this rate is often chosen to be 2-3%/yr: rarely is the 30 year average rate of 6%+ per year used.  According to the DOE Energy Information Agency 2009 Annual Energy Review, residential energy costs have increased at an annual compound rate of 6.5% per year since 1970. Hence, if detailed analyses are conducted, a rate considerably higher than 2-3% should be selected without evidence that future energy costs will rise more slowly than costs have risen in the past.

Analyses that ignore realistic fuel escalation rates, do not consider the replacement costs of HVAC equipment and appliances (e.g., typically on 15 yr cycles) relative to insulation lifespan (likely over 75 years) inherently result in significantly lower recommended R-values than the likely optimum.  NREL’s BEopt tool is capable of accounting for some of these factors, and, not surprisingly, recommends higher R-values for building enclosures than code on a monthly total ownership cost basis.

Life cycle cost analysis rarely account for the environmental cost of energy production and consumption. These externalities are difficult to price, and hence difficult to include in an analysis. However, this is an area of on-going research and will become more important as homes become more energy efficient.

Another approach used by BSC (and also reported on by Proskiw & Parekh 2010) to making investment decisions revolves around the idea of energy targets. Given an energy target (such as a code minimum, the Building America Benchmark, Net Zero, Architecture 2030, Energy Star, etc.), a designer list incremental design choices required to save progressively more energy, and ranks these based on a cost per Btu/yr saved. This approach avoids the need to know about future costs of any sort, but does demand a good understanding of construction costs and energy saving potential. The advantage of this approach is that it more fairly compares the cost of High-R walls to the use of efficient equipment (e.g., a ground-source heat pump in lieu of a gas furnace) or the cost of renewable energy production (e.g. photovoltaics).  One disadvantage of this approach is that it does not favor choices that have a very long service life (such as insulations) over choices with a short service life (such as a condensing hotwater heater). This approach has the decided advantage that it demonstrates very different cost-optimized solutions for different regions and different builders.

High R-value Enclosure Recommendations

Recommendations for high R performance targets must, necessarily, include factors such as climate, available enclosure and space-conditioning technologies, desired lifespan, comfort expectations, lifestyle, as well as the costs of construction, energy, environmental damage, and borrowed money.  Some of these factors can be well defined, but many cannot. However, based on currently available technology, experience with costs and expectations with production and custom builders from coast to coast, and the desire to provide new homes that will be ready to be powered by renewable energy sources immediately or in the future, BSC has developed a set of recommendations . . .

Download complete report here.