TradesHow this works
Enter the R-value you want and the depth of the cavity you have to fill. The tool lines up five common materials by their R-value per inch, shows the total R each reaches in your depth, and marks which ones actually meet the target. Then it recommends by your priority: the cheapest material that fits, or the most R per inch when the cavity is tight.
R per inch is a planning figure and real products vary with density, so treat the result as a starting point. Where depth is tight, high R-per-inch foam wins. Where you have room, the cheaper materials reach the same total R for less money.
Common questions
What R-value can I get in a 2x4 or 2x6 wall?
A 2x4 cavity is about 3.5 inches deep and a 2x6 about 5.5 inches. Fiberglass or cellulose at roughly 3.5 per inch gives around R-13 in a 2x4 and R-19 in a 2x6. Mineral wool reaches about R-15 and R-23. Closed-cell foam packs the most, around R-22 in a 2x4.
Which insulation has the highest R-value per inch?
Closed-cell spray foam, at about 6.5 per inch, nearly double fiberglass. That makes it the pick when you are short on depth, but it is the most expensive and usually needs a professional installer. Where you have room, cheaper materials reach the same total R.
Is more R-value per inch always better?
Only when depth is tight. If your cavity is deep enough, a cheap material like fiberglass or cellulose reaches the same total R for less money. Paying for high R per inch makes sense in a shallow cavity, a retrofit, or where you cannot build the wall thicker.
What R-value do I need?
It depends on your climate zone and the assembly. Colder zones and roofs or ceilings need more than walls in a mild zone. This tool takes your target and finds the material that reaches it. Check your local energy code for the minimum that applies to your project.
What if nothing reaches my target in the cavity?
The cavity is too shallow for that R-value. Your options are to deepen the cavity, add continuous rigid insulation on the exterior, which also cuts heat loss through the studs, or accept a lower cavity R. The tool flags this and shows the depth the best material would need.
Sources: R per inch: fiberglass batt ~3.5, cellulose ~3.5, mineral wool ~4.2, open-cell ~3.6, closed-cell ~6.5 (DOE/LearnMetrics, Rockwool) · Cavity depths: 2x4 = 3.5 in, 2x6 = 5.5 in; cross-checks to ~R-13 fiberglass / R-15 mineral wool in 2x4. Engine version 1.0.0. Risk tier 1.