Article No: 89

2006-05-02 07:06:25
R-value
By: Carole McMichael


Illustration courtesy of E-MAXX

About 15 years ago, Dave Shepherd, then program manager of residential technology with the Portland Cement Association, touted the energy efficiency of an ICF home he had built to potential buyers. They were more interested in whether or not it had a Jacuzzi. Those buyers are still out there, but there is no doubt that the public is becoming more concerned about what it will cost to heat or cool a house, especially after the recent hard winter and continually rising fuel bills.

One of the long-standing measures in figuring energy efficiency is R-value. "R-value is a measurement of the resistance of the flow of heat through a substance," Shepherd said. "Basically, almost everything has an R-value. Metal and stone tend to transmit the heat easily and therefore have a low R-value. Materials that are light and fluffy, but trap air tend to slow the movement of heat. The slower a material transmits heat, the better its resistance and the higher the R-value."

The 'R's' of insulation
In building with concrete, it is not the concrete, but the insulation used with it that beefs up the R-value for concrete systems. If a builder wants to be able to answer a client's queries, he needs to compare the various insulation ratings. Blown in cellulose and fiber glass became very popular in the 1960s for wood frame buildings and are still used in the attic and roof areas in many concrete homes. R-value improves with the thickness of the batt or area filled. Polystyrene, most commonly used in concrete systems, comes in a variety of molded block or panel forms, (EPS, expanded polystyrene, or XPS, extruded polystyrene). Polyisocyanurate (polyiso) boards and open-cell or closed-cell foams, which expand to about three inches after being sprayed in, are also used.

EPS forms generally have a rating of R-5 per inch. XPS is a slightly denser, more tightly compacted product that has a slightly higher R-value and is more expensive. It is sometimes used for block insulation in roofs, molded with a tapered shape to create a drainage plane. It is also used on foundation walls prior to backfilling. Sometimes the polyiso foam board is used in small areas where the design calls for especially high R-value or in lieu of layering other insulation materials that would take up too much space.

Figuring it out
Same-type insulation doesn't automatically produce the same R-value; so, each concrete system should be evaluated separately. Currently, R-value is calculated with a Guarded Hot Box Test. This test involves building a wall in the middle of a box. One side of the wall is warm and the other cool. The R-value is measured according to how fast the heat flow moves to change the temperature 1 degree. Although test results are considered accurate for the material, they do not reflect real-life conditions.

"In the static test, there is no air movement to affect results," Shepherd said. "But there are drafts and air leakage in real houses. Houses are under positive or negative pressure because you have mechanical things functioning. For example, the dryer turns on, taking air from your house and vents it outside. You have to have air to leak in somewhere to replace that loss. A number of appliances are regularly sucking air out of the house; then add to that opened doors. Air can seep back in through walls, around foundations, wall junctures, and gaps in sheet rock and windows. That air moves through insulation at some point. If the insulation has air seeping through it, the R-value diminishes, sometimes tremendously.

"Manufacturers may have tests done as part of quality control," Shepherd said. "I've been to one that does density testing. They can interpolate the R-values from these tests and then give builders ratings per inch. For this reason, builders often don't look at R-value. They just think in terms of how many inches of insulation they need."

When builders go to the building board for permits, they probably will find that the insulation values of polystyrene are accepted. In the case of ICFs, it is not difficult to demonstrate the inches of foam and concrete. Most of the bigger form manufacturers will have needed data on their product available. If a board is still not satisfied, there are independent third-party certified construction material labs that could do testing.

Focusing on the wall
The R-value of the product is just the starting point. Next, the U-value, or wall R-value, must be determined. The following list of methods gets progressively more complex by figuring in more factors that affect heat loss in a total wall. The pure insulation R-value is the least accurate. The nominal wall value is based on the R-value of other layers as well as the simple material figure. The Parallel Path method also figures in insulation breaks. Isothermal plane ratings account for lateral heat movement as well. The dynamic version of the Hot Box test allows for manufacturing and assembly variations on top of all the previously mentioned factors.

Obviously, from house to house, even building with the same ICF or other concrete systems, no formula could compensate for all the differences. What the builder needs is a fairly reliable approximation. He may turn to a heating engineer for a heat-loss analysis to help correctly size the HVAC unit. Money saved up front in buying a smaller unit and thereafter, in lower energy bills can be a big selling point for any concrete home.

In the computer age, many builders may be looking for a software program that will simplify wall-value calculations. The few programs already out there are highly technical and expensive, and not what the builder is looking for.

"The Portland Cement Association is currently having a computer program developed by the Construction Technology Laboratories, an independent for-profit subsidiary of PCA," Shepherd said. "Funded by HUD, the program will help size HVAC." It should be available by the end of the year.

Codes
Codes, which are independent of the actual building material, usually require a minimum of R-13, but there is no national standard that all parts of the country must meet. Even more significant, some codes don't consider how the wall R-value is calculated, so many houses that appear to meet code actually may not.

In some areas, codes are beginning to reflect the effect of thermal mass on the wall R-value, allowing a builder to use less insulation in conjunction with a thermal mass credit. This is referred to as the mass-corrected R-value.

Concrete's bonus
"Thermal mass determines a material's ability to store heat energy," Shepherd said. "The more mass you have, the more efficient the material is. In comparison, wood has a mass of about 30- or 40-pound per cubic foot; concrete is about 150 pounds per cubic foot--four or five times greater. Thermal mass is a bonus for the concrete world because it moderates inner temperature swings, delaying energy transfers.

"Jon Gajda, senior engineer for the Construction Technology Laboratories, authored a thermal mass study, comparing 11 different walls: steel studs, wood studs, concrete block, precast, several different kinds of ICFs and concrete walls that just used sheets of polystyrene on outside face, on the inside face and in the middle. The study is officially titled "Thermal Mass Comparisons of Wall Systems" (Product No. CD026, published 6/2001). The software he used in this study was a U.S. Department of Energy program (DOE 2.1E ).

"The study took readings every hour of every day for 25 different zones across the country. Then, it redid the test, rotating the houses. How a builder sites a house is really critical to how it performs. For developers, this can affect the layout of streets in subdivisions because there is an optimal way for streets to run to have more solar gain. In the study, placement for solar gain accounted for a 10 percent energy swing. Unfortunately, choosing the optimal site placement is not always an option because most people want to site their house for the best view.

"Climate does not affect R-value because that is a lab measurement; but it will affect thermal mass. Where thermal mass has the greater benefit is where the temperature swings are the greatest. In Miami, where it is 80 degrees in the day and 70 degrees at night, it does not make a great difference. However, in Colorado, where there is a 35-degree swing, the extra mass from concrete systems makes a big improvement.

"There is a movement that is still in its infancy called Whole System Design. Instead of just designing a wall to work really well, designers and builders are looking at all the pieces that work well together. You could build a house with great thermal performance and buy cheap, poor quality windows that leak like a sieve or fill a ceiling with all kinds of can lights that punch holes in the heat envelope."

For the most part, energy efficiency is about saving money, though energy conservation is a desired side effect. Still, builders and buyers have to weigh the cost-benefit to see that the cost of materials doesn't exceed the payback in the cost of the energy saved.

Concrete systems are usually well positioned to provide better than required wall R-value, especially when corrected for thermal mass, but builders need to be aware of the various factors that work against its advantages. Where climate swings are extreme, that is where insulation does its work; and that takes them back to the starting point - R-value.