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Article #1452 - REScheck Webcast Questions - Walls

This article is based on questions asked during both the May 6, 2005 REScheck Training Webcast and the July 28, 2005 as well as questions received through email following these events. The original language of the questions and answers has been modified to make them as generic as possible while retaining the applicable specific information.

For horizontal areas and linear lengths in wall areas, do you use the interior dimension or measure to the exterior face?

The area of the wall should be measured on the exterior face from the top of the floor to the bottom of the roof. The portion of the building envelope, including opaque area and fenestration, that is vertical or tilted at an angle of 60 degrees from horizontal or greater is considered a wall.

How do you enter unusual wall types in RES check ? Examples: Envelope - straw bale, adobe, rammed earth, SIPs.

For assembly types that do not fit into any of the assemblies shown in the drop down list in RES check , the user can choose " Other " as the assembly. If you use " Other " as the assembly type, you must enter an overall U-factor for the entire assembly (including air films). Be prepared to provide the building department with manufacturers'' literature or documentation of U-factor calculations.

What if the R-value of the insulation at the rim joist is different than the value in the wall?

Input the total square footage of the rim joist area as a separate wall entry. You can also edit the component definition by double clicking on the word "wall" in the component column and change it to "rim joist".

Can I calculate all of the walls in a two story home and then enter my windows under one wall section? Does this affect the compliance or should the first floor windows be entered under the first floor wall and the 2nd floor windows under the 2nd floor wall?

If the walls are all the same construction and are all above grade, you can add all the walls together and show all of your windows under the one wall entry in RES check . If you have different u-factors for any windows or doors, you should enter those components separately under the wall entry.

Should the area of windows and doors be taken out of the area of the walls?

No, enter the total gross area of the wall component in the Gross Area field. No, enter the total gross area of the wall component in the Gross Area field. The gross wall area includes the area of all windows and doors within the wall.

RES check automatically deducts the windows and any glass doors from the total square footage of the walls and will display the window wall ratio on the compliance certificate.

Does the flange on the exterior wall insulation need to be fastened over the studs to form a continuous vapor barrier?

The IECC does not define how the insulation shall be fastened, however, there are several articles and videos explaining the proper technique for fastening the insulation to the studs. For batt insulation with flanges, the optimal technique is to fasten the flange to the face of the stud. Fastening the flange to the face of the stud allows for the full depth of insulation to be maintained as well as providing a continuous vapor barrier.

The following links offer information on this subject:

• Wall Insulation
• The Importance of Vapor Retarders

If we have a brick veneered wall over a framed wall should we consider the R-value of the continuous air cavity between brick and frame wall as part of the wall?

The standard wall types available in the pull down menus include typical construction U-factors. The assemblies in RES check (except the assembly called "other") have assumed U-factors for the entire assembly except for the insulation value. For example, a wood frame wall will include the R-value of interior and exterior air films, plywood siding, sheathing, and wood studs. If you choose "other" as your assembly type, you must enter the entire U-factor of the assembly and be prepared to provide the backup specifications to the building official as to how the U-factor was derived.

Are the R-values for joists, studs, etc. included or is that a number you type in the "continuous" column? If they are already included, what is the "continuous" column used for?

The assemblies in RES check (except the assembly called "other") have assumed U-factors for the entire assembly except for the insulation value. For example, a wood frame wall will include the R-value of interior and exterior air films, plywood siding, sheathing, and wood studs. If you choose "other" as your assembly type, you must enter the entire U-factor of the assembly and be prepared to provide the backup specifications to the building official as to how the U-factor was derived.

The Continuous Column in RES check is used for insulation installed continuously over structural members and is typically rigid foam board or insulated sheathing.

The following link offers additional information on this subject:

Insulation Overview

Does the type of exterior sheathing need to be noted or play any role?

If no insulated sheathing is entered (entered in the software as continuous insulation), the sheathing is assumed to be plywood with an R-value of 0.83. If insulated sheathing is used, only 80% of the net wall is assumed to be covered by the insulating sheathing. The other 20% is assumed to be covered with plywood (R-value = 0.83).

If you choose "other" as the assembly type, you must enter the U-factor of the entire assembly and be prepared to provide the backup specifications to the building official as to how the U-factor was derived.

If you add thermal sheathing do you enter this to the continuous insulation next to the cavity selection?

Yes, you would enter the value in the continuous insulation column. Keep in mind, the assemblies listed in RES check already have a default value added for standard sheathing (depending on the assembly component). If no sheathing is indicated by the user, the sheathing is assumed to be plywood with an R-value of 0.83. If insulating sheathing is used, only 80% of the net wall is assumed to be covered by the insulating sheathing. The other 20% is assumed to be covered with plywood.

How can I get an R-value of 21 in a 2x4 cavity exterior wall?

The following three systems are equal to achieve an approximate R-21:

1. use 2x4 wall with R-11 batt and R-7 rigid board insulation
2. use 2x4 wall with R-13 batt and R-6 rigid board insulation
3. use 2x4 wall with R-15 batt and R-5 rigid board insulation

The following link offers additional information on this subject:

Insulation Options

Can one use cavity insulation in conjunction with continuous on ceiling and or walls?

Yes, insulation to be installed between furring or studs is entered in the Cavity R-Value column. If the installed wall insulation will be continuous (such as with exterior rigid insulation or interior "draped" insulation), the insulation R-value is entered in the Continuous R-Value column. A combination of insulation systems may be used. For example, installing R-13 batt insulation between the studs and R-5 rigid insulation on the exterior of the stud wall.

Cavity insulation is insulation installed in the cavities between structural members, such as wood studs, metal framing, and Z-clips. Continuous insulation runs continuously over structural members and is typically rigid foam board. Cavity insulation is affected by thermal bridging, while continuous insulation reduces thermal bridges.

Does RES check take thermal mass into account?

RES check does not perform load calculations; it computes an overall UA (u-factor x area) of the envelope components.

In most locations, the code allows walls having a heat capacity greater than or equal to 6 Btu/ft² °F to have a higher Uo factor than low-mass wood- or metal-frame walls (see Tables 502.1.2a-502.1.2c of the MEC; Tables 502.1.1(1)-502.1.1(3) of the 1998 IECC; and Tables 502.2.1.1.2(1)-502.2.1.1.2(3) of the 2000 and 2003 IECC). Masonry or concrete walls weighing at least 30 lb/ft² and solid-wood walls weighing at least 20 lb/ft² are eligible for this credit (the area to be considered is the exterior surface area of the mass wall). In the software, eligible mass wall components receive this credit as an increase in the code building UA (the mass wall required Uo factor is greater than the low-mass wall required Uo factor). Brick veneers or log walls constructed of logs less than 7 in. thick currently do not receive this credit.

The Uo factor for all mass walls except log walls is based on the R value of the insulation, the type of mass wall (solid concrete or block masonry), and the location of the insulation (exterior or interior). For log walls, the Uo factor is based on the thickness of the logs plus any additional insulation that might be used. (The area considered is the exterior surface area of the mass wall.) Section A.2.3 in Appendix A describes the computation for determining mass wall Uo factors. Refer to the document titled, " Methodology for Developing the REScheck Software through Version 3.6 ."

In a structure that has a brick ledge, when using rigid exterior insulation there is a transition area from where the brick ledge starts to the rim joist. How do you insulate this area? An example -- if I am using an R-10 2" Dow Board I do not have enough room behind the brick to continue the insulation.

The Cavitymate page of the Dow Chemical Company website has several diagrams with different installation techniques with Styrofoam.

Are there situations where you might enter both cavity and continuous insulation?

Yes.

Insulation to be installed between furring or studs is entered in the Cavity R-Value column. If the installed wall insulation will be continuous (such as with exterior rigid insulation or interior "draped" insulation), the insulation R-value is entered in the Continuous R-Value column. A combination of insulation systems may be used, for example, installing R-13 batt insulation between the studs and R-5 rigid insulation on the exterior of the stud wall.

Do I need to include rim joists and knee walls?

Yes, both components should be entered and insulated. See following article for additional information:

Rigid Board Insulation Installed as Draft Stop in Attic Kneewall - Code Notes

How do I enter a wall type that is not listed in RES check ?

You can choose "other" as your assembly type from the drop down list. You must also enter the U-factor of the entire assembly and be prepared to provide the backup specifications to the building official as to how the U-factor was derived.

What is the difference between cavity and continuous insulation?

Cavity insulation is insulation installed in the cavities between structural members, such as wood studs, metal framing, and Z-clips. Continuous insulation runs continuously over structural members and is typically rigid foam board. Cavity insulation is affected by thermal bridging, while continuous insulation reduces thermal bridges. The Continuous Insulation Column in RES check is used for insulation installed continuously over structural members and is typically rigid foam board or insulated sheathing.