Newly Revised ASHRAE 90.1 Standard Addresses the Thermal Performance of Metal Building Envelopes

By Chris P. Crall
NAIMA Metal Building Insulation Technical Subcommittee

The American Society of Heating, Refrigerating, and Air Conditioning Engineers has approved Standard 90.1-1999, "Energy Standard for Buildings except Low-Rise Residential Buildings". This is the latest revision in the 90 Series originally published in 1975. The 1999 revision is significant in that, for the first time, metal building walls and roofs are treated as distinct envelope elements.

Also for the first time, the standard establishes envelope criteria based on life cycle economics. The previous version, published in 1989, established maximum U-factors based on the consensus professional judgement of the standards committee and did not differentiate metal buildings from other commercial construction types.

The revised standard establishes criteria for metal building walls and roofs for each of the 26 climate zones defined by heating and cooling degree-days. The criteria are based on cost and performance estimates of actual constructions so that the prescribed criteria are achievable and cost effective.

The ASHRAE Standards Committee utilized data originally publicized by The North American Insulation Manufacturers Association (NAIMA) in the brochure entitled "ASHRAE 90.1 Compliance for Metal Buildings". The document is available through NAIMA and may be ordered from the NAIMA web site (www.naima.org). NAIMA members also consulted with the 90.1 Standards Committee in the development of the revised standard.

U-factors

As in previous versions of the 90.1 Standard, the envelope thermal performance criteria for walls and roofs are expressed as maximum U-factors. The U-factor is the overall heat transfer coefficient of the envelope element. It is understood to include air films on both inside and outside surfaces, and it accounts for all the construction details of the building element being described. By convention in the building industry, it is evaluated at a mean temperature of 75 °F. Units are Btu/(hr•ft²•°F). In steady state, the heat loss or gain through an envelope element can be calculated by multiplying the U-factor by the area and by the temperature difference between inside and outside air. This will give the heat loss (or gain) in Btu's per square foot per hour.

The goal is to describe, in a single term, how real insulation systems perform as installed in real buildings. Unfortunately, real buildings are complicated. Temperatures (both inside and out) are very seldom steady enough to permit accurate heat flow measurements. Solar gains, air infiltration, lighting loads, and occupancy effects complicate the situation further. For metal buildings, the thermal performance is further complicated due the abundance of highly conductive metal components. Thermal short circuits due to fasteners and compression of insulation over purlins and girts contribute to the complexity of the thermal system. The NAIMA approach was to develop U-factors for metal building insulation systems based on a combination of hot box testing and three-dimensional finite element analysis of metal building insulation systems.

Finite Element Analysis

Finite Element Analysis (FEA) is a numerical method whereby a complicated geometry can be mathematically modeled by dividing the region of interest into many small "elements". This essentially turns a complicated problem into many simple problems that can be solved by iteration using a computer. FEA software (such as the ANSYS package) is used routinely in many industries to solve a variety of complex thermal problems. Using FEA, it is possible to properly account for the thermal complexities of metal building envelopes and to develop accurate U-factor estimates. A validated FEA model can be used to investigate many more cases than would be possible with hot box testing alone.

The FEA model was validated using hot box testing to verify the assumptions involved. In these tests (performed per ASTM Test Method C 976) a representative section of a metal building roof was constructed in an opening between a hot "metering chamber" and a cold "climatic" chamber. The metering section size was 8' x 8'. Steady temperatures were maintained on both the hot and cold side of the box and energy flow into the hot side was monitored accurately. The method has proven accurate, but these tests are costly, time consuming, and require considerable skill to produce accurate results, particularly for highly insulated or geometrically complex constructions.

Metal Building Roofs

The validated finite element model was used to calculate U-factors for a number of metal building wall and roof configurations insulated with traditional faced mineral fiber insulation. Table 1 gives selected results for roofs:

(*) U-Values in Table 1 are taken from the ASHRAE Table A-2

As in any modeling effort, the results will vary depending on the assumptions. Key assumptions that apply to these results are listed below:

• NAIMA 202 insulation certified to Certified Faced Insulation Standard.
• 8 in. tall by 3 in. wide purlin and girts (0.0625" steel)
• 5 ft purlin spacing
• 0.026" roof sheet
• For Screw Down Roofs
• #12 steel screws
• Fastener spacing 12" o.c.
• Insulation compressed to 1/8" between purlin and roof sheet
• For Standing Seam Roofs
• 24" clip spacing
• 1/4" steel screws with rubber washers
• Insulation compressed to 3/4" between purlin and thermal block
• Insulation compressed to 1/8" under clips
• 1"x3" extruded polystyrene thermal blocks
  

These assumptions were felt to apply to typical metal building roof constructions.

One key assumption is that NIA Certified faced insulation is used. The Certified Faced Standard is a standard product specification for flexible faced fiberglass metal building insulation. It was developed by the MBI Laminators Committee of the National Insulation Association and it covers the composition and physical properties of faced insulation intended for use in the walls and roofs of metal buildings. The standard requires that the thermal performance of the laminated insulation product meet the rated R-value out of package.

Discussion

Table 1 includes a column that gives the percentage reduction in heat flow through the roof compared to the uninsulated case. As we know, insulation systems follow the law of diminishing returns. The first increment of insulation produces the largest return, with additional increments reducing heat flow further, but by smaller amounts. This is clearly shown in the table. Determining the "optimal" insulation level becomes an engineering economics problem, which attempts to balance the incremental installed cost with the incremental dollar savings generated over the life of the project. Standard 90.1 utilizes this approach in setting the U-factor criteria for the various climatic zones.

For screw down roofs (SDR), the compression of the insulation between the purlins and the roof sheet has a major impact on the thermal performance of the system. In addition, metal fasteners are a significant short circuit for heat transfer through the envelope. The FEA model properly accounts for these details. Even with these defects, reductions in heat flow of roughly 90% (compared to the uninsulated case) are possible.

For standing seam roofs (SSR), the U-factors are significantly better (lower) than for the SDR roof with the same nominal insulation value. This is primarily due to the inclusion of the 1"x3" thermal spacer block placed over the purlins where the insulation is compressed. The block is an effective fix for the thermal short circuit at the purlins. Thermally, the SSR with the thermal spacer block is a much more efficient system that should be considered in colder climates. Using double layers of insulation, U-factors down to 0.046 Btu/(hr•ft²•°F) are achievable (a 96% reduction compared to the uninsulated baseline).

Finally, note the results for filled cavity insulation systems. For most metal buildings, the vapor retarder facing and the insulation layers are installed from the top of the roof prior to installation of the roof sheeting. This method leaves the purlins exposed to the indoor conditions below. Other installation approaches use banding to support the insulation from below the purlins. These systems can be used on new buildings, and may be adaptable to re-insulation work in existing buildings. The approach has the additional advantage of allowing the insulation to completely fill the cavity between purlins. While the thermal short circuit of the steel purlins is still present, its effect is reduced.

A typical "filled cavity" system was analyzed using the FEA approach. In this case, R-19 insulation (supported from below by banding attached to the bottom of the purlins) is installed to completely fill the cavity. R-10 insulation is installed over the purlins with a thermal block. This system has a U-factor of 0.041 Btu/(hr•ft²•°F), which translates to a 97% reduction in heat transfer compared to the uninsulated case.

Metal Building Walls

Table 2 summaries the FEA results for selected metal building walls installed in the conventional manner. These calculations assumed a girt spacing of 7 ft.

As expected, the results are somewhat better than the U-factors for screw down roofs, reflecting primarily the wider girt spacing assumed.

Summary

The revised ASHRAE Standard 90.1-99, for the first time, treats metal building walls and roofs as distinct envelope elements. Maximum U-factor criteria are set based on life cycle economics. The NAIMA three-dimensional finite element model was used to estimate the thermal performance of typical metal building envelope elements taking into account compression of insulation at the purlins and girts as well as thermal shorts due to clips and fasteners. The resulting U-factors are published in Appendix A of the Standard and may be used to demonstrate compliance.

There are many metal building insulation systems available in the industry. Performance claims generally cannot be compared directly simply because of the various methods and assumptions made. FEA offers a cost-effective way of standardizing the evaluation, allowing contractors, architects or owners to make "apples to apples" comparisons. The cost of each system can then be evaluated to determine the real economic value of alternative systems.