top of page

Insulation - 2015 IRC Code Highlights

Your Home's Thermal Envelope

Residential Insulation Considerations

Building Thermal Envelope: The basement walls, exterior walls, floor, roof, and any other building elements that enclose conditioned space or provide a boundary between conditioned space and exempt or unconditioned space.

Things to remember with regards to the newly adopted 2015 IRC by Pennsylvania:


PA302.1 Ceilings with attic spaces (N1102.2.1 (R402.2.1)): The 2015 IRC requires R-49 insulation for ceilings with attic spaces. However, there is an exception. R-38 shall be deemed to satisfy the requirement for R-49 wherever the full height of uncompressed R-38 insulation extends over the wall top plate at the eaves.

PA302.2 Ceilings without attic spaces (N1102.2.2 (R402.2.2)): Where the design of the roof/ceiling assembly does not allow sufficient space for the required insulation, such as cathedral ceilings, the minimum required insulation for such roof/ceiling assemblies shall be R-30.

Note: If designing your building envelope based on the IRC, this reduction of insulation from the requirements of Section N1102.1.2 shall be limited to 500 square feet (46 m2) or 20 percent of the total insulated ceiling area, whichever is less. If designing to the PA Alternative Energy Code, there is no area limit on cathedral ceilings.


2015 code requires walls to be insulated with a minimum of R-20 or R-13 + 5 (“13 + 5” means R-13 cavity insulation plus R-5 insulated sheathing. If structural sheathing covers 40% or less of the exterior, continuous insulation R-value shall be permitted to be reduced by no more than R-3 in the locations where structural sheathing is used – to maintain a consistent total sheathing thickness.).

Note: Basement walls require a minimum of either R-10 continuous or R-13 batted insulation.

Hot Water Pipes

Concerning hot water pipe insulation

IRC: Any piping equal to or greater than ¾” diameter, or piping serving more than one dwelling unit, or piping located outside the conditioned space, or piping from a hot water heater to a distribution manifold, or under a floor slab, or buried, must be insulated with R-3

PA-Alt: Mechanical system piping capable of carrying fluids above 105॰F (40॰C) or below 55॰F (13॰C) shall be insulated to a minimum of R-3, and all insulation exposed to the weather must be protected from damage or degradation of insulation material.


(No significant changes to note)

Also note:

An HVAC duct leakage test is required (N1103.3.3) Total leakage should be less than or equal to 4 cubic feet per minute per 100 square feet of conditioned floor space where the air handler is installed at time of test. Where the air handler is not installed at time of test, the total leakage shall be less than 3 cubic feet per minute per 100 square feet of conditioned area.

Ductwork not completely enclosed within the building thermal envelope must be insulated as follows:

A blower door test is required (N1102.4.1.2). Per PA UCC, the air exchange is required to be 5 ACH or less @ 0.2 water gauge (50 Pascals)

Whole house mechanical ventilation is required (R303.4) Per Table M1507.3, The ventilation rate is based upon the square footage of the dwelling and number of bedrooms within the dwelling. (Rates may vary depending on the use of either continuous or intermittent ventilation). The whole-house mechanical ventilation system shall be provided with controls that enable manual override.


11,091 views2 comments

Recent Posts

See All
bottom of page