The Impact Of The Envelope

April 21, 2005
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Say you go to bid on a customer's replacement air conditioner. You know that the owners have made some home improvements since their last unit was installed - they added new windows, say, or a new roof and insulation, or better flooring. Which of these improvements will have the most impact on their system's cooling load?

Glenn Hourahan, P.E., vice president of research and technology for the Air Conditioning Contractors of America (ACCA), wanted to find out. He evaluated residential building envelopes in various climates, and considered their effects on the HVAC load.

"Building envelopes significantly impact the heating and cooling requirements of a home," he reported. "Some envelope elements have a much greater impact than other elements. Recognizing this will position contractors to recommend upgrades that will reduce the size and cost of the heating-cooling systems while providing for healthy, comfortable indoor environments."

Conversely, ignoring this fact - replacing an old system with the same-size system, without paying attention to the home's new actual heating or cooling load requirement - could actually degrade equipment performance and cause homeowners to be dissatisfied with the temperature and moisture control that the new systems provide.

Envelope Basics

The envelope is "the entire skin of the home," Hourahan said. "It includes the materials, components, and subassemblies that make up the home - the floors, ceilings, and walls of the conditioned space - and includes openings such as windows, doors, and vents."

The purpose of the envelope is to maintain occupant comfort; control heat flow and air movement; and control moisture by keeping rain and ground water out and letting water and water vapor out if it gets inside.

The benefits of good envelopes, Hourahan said, include improved thermal comfort, better indoor air quality (IAQ), smaller heating-cooling systems, and lower energy use. Nationally, this translates to reduced reliance on fossil fuels and reduced greenhouse gas emissions.

"The correct design, proper installation, and watchful maintenance of good building envelopes provides for improved thermal comfort and better IAQ, all with the use of smaller heating and cooling plants," he said.

Obviously, contractors who rigorously observe ACCA Manual J procedures will obtain more accurate load estimates and, when smaller equipment is called for, will be able to offer cost-effective options to their customers.

The question Hourahan posed for himself was, which elements of the building envelope have the most effect on HVAC sizing and energy consumption?

Table 1. Baseline load simulations for heating and cooling in Miami and Chicago.

Base Simulation

Hourahan crafted similar scenarios in two distinct climates: Miami for the cooling load and Chicago for heating. His case home was a single-zone, 2,400-square-foot structure with 75 degrees F, 50 percent relative humidity (rh) for internal cooling; 70 degrees, 30 percent rh for heating.

The R-15 walls were filled-core block, brick veneer, and wood frame. Windows were operable, two-pane clear glass. The ceilings were under a vented attic, no radiant barrier, white roofing tile, eight-foot ceilings, and R-19 insulation.

Floors were carpeted over a tightly enclosed crawlspace with R-11 blanket insulation and R-4 crawlspace wall insulation. The simulation varied insulation levels, window types, roof colors, degree of internal shading, etc. to ascertain the relative impacts the changes had as compared to the baseline load.

Chicago's baseline heating load was 55,849 Btuh, and its cooling load was 23,597; Miami's heating load was 23,586 Btuh, while the cooling load was 27,877.

"Due to outdoor design conditions in the winter," Hourahan pointed out, "the peak heating requirement for Chicago is twice that of Miami. The summer cooling requirement, however, is much more similar at a nominal 2 tons." (See Table 1.)

Figure 1. Envelope attributes and the Miami cooling load.

Impact Of Floor Insulation In Miami

When focusing on the Miami cooling load, the level of floor insulation, surprisingly, "has very little effect on the overall cooling load for this example situation," Hourahan said. He pointed out that "improving the level of insulation in the floor has relatively little impact on the overall building load." (See Figure 2.)

He noted the overall floor contribution ranged from about 3 percent to 5 percent of the total cooling requirement even if the floor insulation is varied from R-30 to R-0. However, the crawlspace wall insulation has an interesting effect on the overall floor load contribution.

"The levels of insulation on the walls of the crawlspace have a much greater impact, causing the floor loads to vary from 6.7 percent of the total building load (i.e., when there is no insulation on the crawlspace wall) to a floor load contribution of just under 2 percent when R-19 insulation is installed on the crawlspace wall.

"On the other hand, the level of ceiling insulation and the quality of the windows not only have a significant load, but the variations are quite large." (See Figure 1.) In short, the biggest Btuh losses for the Miami cooling load are going up and out through ceilings.

"The greatest benefit, on a very modest investment, is adding insulation to the attic," he continued. For instance, in a Miami home with no ceiling insulation, "just going from R-0 to R-7 reduces the cooling requirement by 14,000 Btuh - half the base load of the simulation example." Going from R-7 to R-19 reduces the total cooling requirement by another 11 percent as compared to the baseline. "Adding ever greater quantities of insulation will have diminishing returns," Hourahan said.

Figure 2. Crawlspace wall insulation can have more of an impact on the floor load than floor insulation does.

Impact Of Windows

After ceiling insulation, the window quality level has the next greatest impact on the Miami cooling load. There is value in updating single-pane to double- or triple-pane versions. In the baseline simulation, single-pane windows contribute about 38 percent of the overall building load.

Yet, window contribution can be reduced to just 18 percent of the overall load by going to high-quality, low-emissivity, double-pane windows. The impact that window shading has on the overall load is also in the range of 20-percentage points difference.

Therefore, when a contractor does his heat loss/heat gain load calculations, the use of window shading will have a significant impact on the ultimate sizing and selection of the air conditioning unit. Regardless of what windows are in place, closing blinds or curtains to the afternoon sun will help maintain indoor temperatures.

Heating In Chicago

"Like the Miami cooling example, the floor contribution also has a rather insignificant impact on the total building load," he said. "However, due to the much greater temperature differences for heating (71 degrees in Chicago) vs. cooling (15 degrees in Miami), the ceiling contributions under the ventilated attic, and the infiltration contributions are considerably greater portions of the total building load."

In addition, wall insulation becomes more important in the Chicago heating load example; still not as important as ceilings, but more important than, say, floors.

"Attics and walls with little or no insulation will greatly improve from the addition of insulation," he said. The greatest bang for the buck comes from adding insulation to the attic, especially if you are going from R-0 to a minimum of R-19.

According to Hourahan, new windows can also have a major impact, especially if the home upgrades from single-pane windows to double- or triple-pane windows. These window improvements offer additional benefits of fewer drafts and higher inside-surface temperatures, thereby reducing winter condensation problems.

Summary

"Whether designed in or added later, improvements to the building envelope significantly reduce home heating and cooling loads," he summarized. Elements to focus on include:

  • During summer cooling - ceiling insulation, windows, and infiltration.

  • During winter heating - ceiling insulation, infiltration, and window/wall insulation.

    Contractors need to be aware of these factors, particularly in the replacement market.

    "For contractors, this means that if you have homes with dissimilar insulation levels or construction attributes, you should expect the heating and cooling loads to be different."

    Doing a ballpark load estimate will not be effective; ceiling/attic insulation and window construction must be considered for any meaningful accuracy.

    If a home has undergone extensive insulation upgrades since the last HVAC system was installed, this needs to be factored in so that the subsequent replacement system is properly sized.

    "If the contractor doesn't want to listen, or says the upgrades don't make any difference, [the homeowner] should look to securing the services of a different contractor," he said.

    Publication date: 04/25/2005

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