How to use the framing factor effective r-value calculator
- Enter the cavity R-value (the insulation between the studs) and the framing R-value (the wood or metal across the wall).
- Enter the framing share: the part of the wall area taken up by studs, plates, headers and corners.
- Add the other layers in series: the total R-value of sheathing, siding, drywall and air films that cover the whole wall. Use 0 to see the framed section alone.
- Read the effective R-value of the wall, its U-factor and how much the framing costs compared with an all-cavity wall.
Formula
U (framed section) = share / R_framing + (1 - share) / R_cavity
R (framed section) = 1 / U
R (wall) = R (framed section) + R (other layers)
Loss from framing = 1 - R (wall) / (R_cavity + R_other)
This is the parallel path method: heat is assumed to flow straight through the cavity and the framing side by side, so their conductances (1/R) are added in proportion to area. The R-value of layers that cover the whole wall adds in series.
A 2x4 wall with R-13 batts
With a cavity R-value of 13, framing R-value of 4.4 and a framing share of 25 percent, the U-factor of the framed section is 0.25 / 4.4 + 0.75 / 13 = 0.05682 + 0.05769 = 0.11451, so the framed section is R-8.73, not R-13.
Adding 2.5 for sheathing, siding, drywall and air films gives a wall of R-11.23 against R-15.50 if the whole wall were cavity, a loss of 27.5 percent. At a framing share of 15 percent the framed section is R-10.05, and at 35 percent it is R-7.72.
Why a batch label overstates the wall
Insulation is labeled by what it does in an unobstructed cavity. In a built wall, wood or metal studs carry more heat than insulation does, and they take up a real share of the area. The effective R-value of the whole assembly is therefore lower than the R-value on the batt. Metal studs conduct far more than wood and can lower the result a lot, so use the real R-value of the framing material.
To add up layers that run across the whole wall, use the R-value calculator, which treats layers in series and does not include framing. For how much insulation to buy, see the insulation calculator.
What this method leaves out
The parallel path method gives an upper bound on heat loss for walls with a continuous layer on one or both sides, and real walls lose a bit less because heat spreads sideways through sheathing and drywall. Air leaks, gaps in the insulation, compression of batts, thermal bridges at window edges and moisture are not modeled. No code minimum or energy rating is applied. If you plan building work, ask a qualified professional for the numbers that apply where you live. The effect on bills can be explored with the heating fuel cost calculator.
Assumptions and limits
- Heat flows straight through each path and does not move sideways between the cavity and the framing.
- The framing share is the same through the whole thickness of the wall.
- The R-values you enter are example values until you replace them with those of your products. Wood framing is about the thickness of the cavity times the R-value per inch of the species.
- Layers in series are added with no air gaps between them. The R-value of air films and any cladding is part of that total.
Frequently asked questions
What framing share should I use?
Measure a wall drawing and count the studs, plates, headers, sills and corners. A figure of about 25 percent is a common assumption for conventional framing with studs 16 inches apart, and advanced framing uses less. Use the share that matches your plans.
Why does a higher framing share lower the result so much?
The framing path conducts about three times as much heat as the cavity at these example values. Each added percent of area on that path raises the combined conductance, so the effective R-value drops quickly.
Can I use SI values (RSI)?
Yes. The method is the same if every R-value you enter uses the same system. The results then come out in RSI and the U-factor in watts per square meter kelvin.
Does continuous insulation outside the studs change the result?
Yes, and in a good way. Put its R-value in the other layers field. It adds in series, so it raises the whole wall by its full value instead of being reduced by the framing share.