R-Value Archives - Hunter Panels

Real World Factors Affecting Insulating Power

QUESTION:

What real world factors can affect a product’s insulating power?

ANSWER:

R-value is a reliable measurement of how materials will perform in a static test environment, however real world application will differ from laboratory circumstances. The insulation used in an exterior wall will be part of the overall building envelope, not used in isolation. Factors such as moisture, air infiltration, and convective flows within the wall all can play a part in the overall thermal efficiency of a wall system.

QUESTION:

How do products obtain their R-value?

ANSWER:

Air

Closed-cell foam insulations derive their R-value from the blowing agents/gases contained within the cells. The blowing agents for different types of foam plastics will vary, but all are selected for their ability to enhance the thermal performance of the foam. Because of their closedcell composition, all closed-cell foam products provide excellent resistance to moisture and air intrusion along with thermal performance.
Fibrous materials derive their R-value from air trapped within pockets between the fibers. Air and/or water movement through fibrous insulation products will reduce the insulating power, which is why fibrous insulations have traditionally been used within interior stud cavities with barriers (walls) which restrict air & water movement on all 6 sides of the cavity.

To illustrate how fibrous insulations work and how air or water can impact insulating power, think of a person who wears a sweater outside on a cool day to keep warm. On a calm, cool day the sweater does a nice job keeping them warm. If the wind picks up, the air penetrates the fibers and the sweater no longer insulates the way it did when the air was not moving, in fact they may put a wind breaker over the sweater to try to keep warm. If it rains, the sweater gets wet as water penetrates the fibers, again impacting the insulating ability. When the sweater dries out, it will insulate well again, but how long does it take to dry and how quickly will that person want to put it back on? What happens if the weather is consistently wet or there is a significant amount of wind? Would a sweater be your first choice to keep you warm in such conditions?

Moisture

Both bulk water and water vapor should be considered when making choices for continuous insulation or “ci”. There are a number of tests utilized to measure water resistance of various building materials, and it is important to understand what they measure and how those measurements apply to various product applications.

The ASTM C209 and ASTM C272 are most frequently utilized for products used in an exterior environment where the presence of water is likely. ASTM C272 has been predominantly used when focusing on below-grade application. ASTM C209 has been more closely associated with above grade exterior walls due to the inclusion of a drainage period. ASTM C1104 is a test created specifically for mineral fiber insulations that does not include the presence of bulk water.

Independent tests performed on Hunter Xci polyiso and a leading dual density mineral fiber board product yielded telling results. Both materials were subjected to identical test procedures at accredited independent facilities. R-value measurements used were taken at both 75°F and 40°F mean reference temperatures

In the dry state, the measured average R-values were in line with published R-values for both the 2″ ISO (R-13) and 3″ dual-density mineral fiber (R-12.9). In the wet state, test averages resulted in the polyiso product performing within 3% of published R-values. In the same wet state, test averages resulted in the mineral fiber product R-values being diminished by 70% from published Rvalue.

In the dry state, the measured average R-values were in line with published R-values for both the 2″ ISO (R-13) and 3″ Dual-Density Mineral Fiber (R-12.9). In the wet state, test averages again resulted in the polyiso product performing within percentage points of published R-values. In the same wet state, test averages resulted in the mineral fiber product R-values being diminished by 62% from published R-value.

Closed cell foam insulation products have excellent moisture resistance, while fibrous materials do not. Tech bulletins or FAQ from fibrous insulation manufacturers state that when product is wet, R-value will be reduced, but they do not typically reference how much reduction will be seen. The above results show the R-value reduction to be substantial, even when drainage opportunity is provided.

CONCLUSION:

Polyiso insulation performs better in retaining R-value in wet environments, such as exterior wall cavities, than mineral fiber. Consideration should be given to such factors when selecting insulation materials.

Complying with the International Energy Conservation Code (IECC)

PRESCRIPTIVE R-VALUE VS. PRESCRIPTIVE U-VALUE FOR ABOVE-GRADE EXTERIOR WALLS

To comply with the International Energy Conservation Code (IECC) requirements for insulating above-grade exterior walls, building professionals can follow one of two commonly used prescriptive methods: the R-value path or the U-value path, as set forth in the opaque thermal envelope requirements of the IECC.

R-VALUE PRESCRIPTIVE PATH

Complying with the R-value method is straightforward – simply use products with R-values that meet or exceed the values shown in the IECC for the appropriate climate zone and wall type. For example, the prescriptive R-value for metal framed walls in most climate zones (Zone 3 and above) is R-13 + 7.5 ci. This means the wall must incorporate insulation of R-13 or greater within the stud cavity, and insulation of R-7.5 or greater as continuous insulation, as shown in the following illustration:

U-VALUE PRESCRIPTIVE PATH

The U-value method is different. It takes the thermal resistance of all the components of the wall assembly into consideration, not just the insulation. The R-values of the wall assembly components are added together. Since U-value is the reciprocal of R-value, the U-value of the assembly is determined by dividing 1 by the total R-value. There are tables within the code documents that assign R-values to certain components of wall assemblies such as cavity air spaces and interior air film.

The U-value calculation will be slightly different for framed walls depending on whether the framing members are steel or wood. Since steel conducts heat, there is a reduction in effectiveness of the stud cavity insulation within a steel framed wall assembly — in other words, the effective R-value of the wall assembly is less than the insulation’s stated R-value. This is not any fault of the insulation, rather it is because the steel framing is a thermal short circuit in the wall. Therefore, the energy efficiency of the stud cavity insulation is reduced by a “framing factor” percentage found in the IECC. In the below example, the reduction is 54% for steel framed walls 16″ oc, meaning the R-value of the insulation is multiplied by .46 (1 – 0.54).

Wood framing provides some measure of insulating value, so it is treated differently than steel framing when calculating U-value for the wall assembly. The most common way to calculate wood framing U-value is the “parallel path” method, whereby the U-values for the framing path and the cavity path are calculated and then added. So, if the wood studs are 16″ oc, the code calls for using a framing factor of 0.25 (meaning the studs take up 25% of the wall area). The U-value of the framing path is then multiplied by 0.25 and the U-value of the cavity path is multiplied by 0.75 to account for the remainder of the wall assembly. The U-value of the total wall is the sum of the framing path U-value and the cavity path U-value.

Testing for R-Value

Question:

How does the test method work for determining R-value of a building product?

Answer:

R-value is the most commonly used measurement tool used in the building and construction industry to evaluate resistance to heat flow. The higher the product’s R-value measurement, the greater the insulating power. Measuring R-value, or resistance to heat flow, is accomplished by placing the material to be tested between two plates of different temperatures. ASTM C518 utilizes both hot and cold plates that are air-impermeable, directly in contact with the sample being tested. Heat flow is then measured across the sample as temperatures of the plates are maintained. One plate would be considered the “cold side” and the other would be considered the “warm side”. The mean temperature is the average between the temperatures on both sides of the test mechanism (Temp of cold side + Temp of warm side/2).

Question:

Temperature and R-values, what are the important facts to understand?

Answer:

Thermal resistance of materials can vary with temperature, so the FTC has established published requirements for R-value to be measured at 75 degree mean temperature creating a uniform standard across materials. Attempts to redirect conversation toward R-values at different mean temperatures are often misleading or confusing, which is why the FTC established the 75 degree mean temperature requirement. It is important to note, mean temperature is not synonymous with ambient temperature, or outdoor temperature. Mean temperature in terms of R-value would be the average between the temperature on the cold side of the wall and the temperature on the warm side of the wall.

Assuming the indoor temperature of a building to be 68 degrees year round, that temperature could serve as either the warm side or the cold side. Assuming the indoor temperature of a building to be 68 degrees year round, that temperature could serve as either the warm side or the cold side.

Evaluating the information nationally, the average mean low temperatures for major metropolitan areas across the US are above 50 degrees mean, and the average mean high temperatures range from above 60 degrees mean into the upper 70’s depending on geographic region. Thusly, it is important to understand the definition of mean temperature in terms of R-value calculation. Redirection of conversation away from FTC mandated R-value measurement is a deliberate attempt to mislead and confuse which could result in improperly insulated buildings.

How Much R-Value Do You Need?

It can be confusing to keep track of your climate zone code requirements due to the fact that the minimum insulation requirements have increased 66 – 100% over the last 9 years. It is important to remember that a building owner only gets one chance to increase the insulation in the roof system approximately every 20 years. When determining the desired R-Value, you should consider future energy requirements and associated energy costs over the entire lifespan of the installed roof system. The current codes reflect the MINIMUM requirements, we recommend when planning for the future to exceed the code by an additional R-10. (Paybacks typically can be seen in 6 – 10 years*)

This guide provides assistance when designing the minimum R-Value in commercial buildings for energy efficiency for thermal insulation above the deck. The respective R-Values are based on the climate for each stated region and only a suggestion. State codes may differ and should be considered. Current code information for each state is available on the websites for the Building Codes Assistance Project and the US Department of Energy Building Energy Codes Program.

Building Codes Assistance Project: http://bcapcodes.org/code-status/state/
US DOE: https://www.energycodes.gov/status-state-energy-code-adoption

The chart below shows the progression of the R-Value changes by climate zones since ASHRAE 90.1 2004.

Determining Minimum R-Values on Tapered Insulation Systems

According to the 2021 International Energy Conservation Code (IECC), under Insulation Requirements, the building thermal envelope shall meet the requirements of Tables C 402.2 based on the climate zones shown below:

Question:

How does this affect R-Values on a Tapered Insulation System?

Answer:

The R-Values shown above are the minimum R-Values required to meet Code requirements. Since a Tapered Insulation system will increase thickness away from the low points, the overall system R-Value will exceed the Prescriptive R-Value requirements.

Question:

Does an Average R-Value that meets the Prescriptive requirement satisfy Code standards?

Answer:

No. The terminology “Average R-Value” for tapered systems is an outdated term and not recognized by the IECC.

Question:

Does building code allow for a change in R-value when using sumps?

Answer:

There is an exception provided that allows the minimum thickness to be up to 1″ less than the prescriptive R-Value (at ¼” per foot slope) as long as the system R-Value meets or exceeds that Prescriptive R-Value.

Example:

Zone 6 requires an R=30 minimum, which is 5.2″ thick. This exception will allow the minimum thickness to be 4.2″ at the low point, or 5.2″, 4′ away from the drain on a ¼” per foot tapered system.

IECC 2021, SEC. C402.2 ROOF ASSEMBLY
Exception 2 Allows A 1-Inch Insulation Thickness Variation