Basement Insulation Code
Basement Insulation Requirements in Lehigh County, Pennsylvania
The answer is 15/19, and there are three legal ways to build it. Here is the number, the table it comes from, the date it took effect, and the reason you have probably been given three different answers.
- Climate zone
- 5A
- Basement wall
- 15 / 19
- Code in force
- 2021 I-Codes, PA amended
- Effective
- January 1, 2026
- Source
- 34 Pa. Code 403.21
The requirement, and where it comes from
Lehigh County is in Climate Zone 5A. The Pennsylvania-adopted energy table, Table R402.1.3, gives the basement wall requirement for Climate Zone 5 as 15/19. That table is reprinted verbatim in 34 Pa. Code section 403.21(a)(9)(vi)(D), so it is readable in Pennsylvania's own regulation rather than only in the model code.
Footnote c to that table defines it:
"15/19" means R-15 continuous insulation on the interior or exterior of the home or R-19 cavity insulation on the interior of the basement wall. Alternatively, compliance with "15/19" shall be R-13 cavity insulation on the interior of the basement wall plus R-5 continuous insulation on the interior or exterior of the home.
Pennsylvania moved to the 2021 International Codes on January 1, 2026, under 34 Pa. Code section 403.21 as amended November 7, 2025 at 55 Pa.B. 7701. Before that date the state was on the 2018 codes. Any answer that cited the 2021 code as being in force during 2025 was early, and any guide still citing the 2018 codes today is a cycle behind.
Three legal ways to build the wall
All three comply. Which one you use is decided by how much headroom and floor width you can afford to give up.
R-15 continuous
Rigid foam over the full face of the foundation wall, interior or exterior, with the framing in front of it. Roughly three inches of XPS. Check the product's published R-value per inch rather than assuming.
R-19 cavity
Cavity insulation on the interior of the basement wall. In practice that means 2x6 framing, because R-19 does not fit in a 2x4 bay.
R-13 cavity plus R-5 continuous
A 2x4 wall with R-13 batt, built in front of one inch of rigid foam against the concrete. This is the assembly that fits a low basement ceiling, and it is the one most Lehigh County basements get built with.
The insulation runs the full height of the wall. Section N1102.2.8.1 requires it to be installed from the top of the basement wall down to 10 feet below grade or to the basement floor, whichever is less. Floor to sill, not just the exposed band at the top.
Why you have been given three different answers
The requirement is set by climate zone, and the climate zone is assigned by county. Cross a county line and the number changes. Berks County moved from 5A to 4A under IECC 2021, which means a job in Kutztown has a different requirement than a job in Allentown.
| County | Zone | Basement wall |
|---|---|---|
| Lehigh | 5A | 15 / 19 |
| Northampton | 5A | 15 / 19 |
| Carbon | 5A | 15 / 19 |
| Monroe | 5A | 15 / 19 |
| Schuylkill | 5A | 15 / 19 |
| Berks | 4A | 10 / 13 |
| Bucks | 4A | 10 / 13 |
| Montgomery | 4A | 10 / 13 |
| Philadelphia | 4A | 10 / 13 |
County climate zone assignments are from the Department of Energy and PNNL county climate zone dataset published with the Guide to Determining Climate Zone by County, Building America and IECC 2021 Updates.
The mold question, answered properly
This part is building science rather than a code citation, and it is the part that decides whether the wall is still sound in ten years.
Mold needs three things: water, something organic to eat, and time. A basement wall built with fiberglass batts pressed against bare concrete supplies all three. The concrete is cold and permeable. Warm humid interior air passes straight through fiberglass, reaches that cold face, hits its dew point and condenses. The paper facing and the drywall are the food. Nobody opens the wall for ten years, so there is the time.
Rigid foam against the concrete solves it by moving the cold surface to the outside of the insulation, so there is no surface inside the wall cold enough for water to form on. That is the entire reason the code offers a continuous insulation option, and it is why option C, one inch of foam plus an R-13 batt, is the assembly we build most often.
Do not add polyethylene sheeting on the interior face. A below-grade wall needs to dry inward. Plastic behind the drywall traps whatever moisture reaches the cavity with no escape in either direction, and it causes the exact failure it was installed to prevent.
And none of this works over standing water. If the wall is actively wet, the water gets fixed first. Insulating over an active moisture problem means the drywall, the insulation and the flooring all fail together and you pay for the basement twice.
Rim joists are the leak nobody looks at
The rim joist, the band of wood sitting on top of the foundation wall around the whole perimeter, is usually the largest single air leak in the house and the coldest wood in the basement. It is also where most basements get their humidity.
It gets air sealed, not just stuffed. Rigid foam cut to fit each bay and sealed around all four edges works. Closed cell spray foam works. Fiberglass on its own does not, because fiberglass does not stop air movement, so humid interior air travels through it and condenses on the cold rim board behind it where you cannot see it happening.
If there is existing fiberglass in your rim joist bays, it is not doing the job it was put there to do. That is not a code failure. It is the difference between a basement that smells right in August and one that does not.
The rest of the Zone 5 numbers
These are the values that go on the energy compliance sheet with the permit set.
| Assembly | Climate Zone 5 |
|---|---|
| Basement wall | 15 / 19 |
| Ceiling | R-49 |
| Wood frame wall | R-23, or 13+7.5, or 20+3.8 |
| Floor | R-30 |
| Slab | R-10 at 4 ft, or R-15 at 3 ft |
| Crawl space wall | 15 / 19 |
| Fenestration | U-0.30 |
| Skylight | U-0.55 |
| Basement wall U-factor alternative | U-0.050 |
Compliance is demonstrated with REScheck or the Pennsylvania Alternative Residential Energy Provisions, both named in 34 Pa. Code section 403.21(e)(1). Pennsylvania excludes the ERI and performance path provisions from the adopted code, so stay on the prescriptive path or REScheck.
Common questions
What R-value does code say I need for a basement wall?
In Lehigh County the requirement is 15/19. That is Pennsylvania-adopted Table R402.1.3 for Climate Zone 5, reprinted verbatim in 34 Pa. Code section 403.21(a)(9)(vi)(D).
Footnote c to that table defines what 15/19 means: R-15 continuous insulation on the interior or exterior of the home, or R-19 cavity insulation on the interior of the basement wall. It then adds a third option: R-13 cavity insulation on the interior of the basement wall plus R-5 continuous insulation on the interior or exterior.
Is that the updated code for sure in Lehigh County?
Yes, as of January 1, 2026. Pennsylvania moved to the 2021 International Codes on that date, under 34 Pa. Code section 403.21 as amended November 7, 2025 at 55 Pa.B. 7701.
Before January 1, 2026 Pennsylvania was on the 2018 codes, effective February 14, 2022. If you got an answer in late 2025 that cited the 2021 code as already in force, it was a week or more early. If you are reading a guide that cites the 2018 codes today, it is a cycle behind. This is the single most common way the wrong number gets repeated.
Someone told me R-20. Where does that number come from?
R-20 is an above-grade wood frame wall value from the 2018 energy code, not a basement wall value. The two assemblies have different requirements and they get mixed up constantly.
Under the Pennsylvania-adopted 2021 table, the Climate Zone 5 above-grade wood frame wall is R-23, or 13 plus 7.5, or 20 plus 3.8. The basement wall is 15/19. If someone quotes you R-20 for a basement wall, ask which table they are reading from.
Does the requirement change depending on which town I am in?
Not within Lehigh County. It changes when you cross a county line, because the number is set by climate zone and the climate zone is assigned by county.
Allentown, Macungie, Coopersburg and every other Lehigh County municipality are all Climate Zone 5A at 15/19. Berks County moved from 5A to 4A under IECC 2021, so a job in Kutztown is 10/13. Bucks, Montgomery and Philadelphia are also 4A. Carbon, Monroe, Northampton and Schuylkill are 5A.
Is one inch of foam board enough?
One inch of rigid foam on its own is not enough. One inch of rigid foam plus an R-13 batt in a 2x4 wall in front of it is a compliant assembly, because R-5 continuous plus R-13 cavity satisfies 15/19.
That is option C in the footnote, and it is the one that works in a basement with limited headroom and limited floor width. Confirm the rigid board's published R-value. Not all one inch products are R-5.
Do I have to insulate the whole wall or just the part above grade?
The whole wall. Section N1102.2.8.1 states that where basement walls are insulated, the insulation shall be installed from the top of the basement wall down to 10 feet below grade or to the basement floor, whichever is less.
Floor to sill, not just the exposed band at the top. Stopping the insulation partway down is one of the more common failures found at inspection, and it also creates a cold stripe at the bottom of the wall where condensation forms.
Can I put fiberglass batts straight against the foundation wall?
You can physically, and it is the fastest way to grow mold in a basement. This is building science rather than a code citation, but it is not controversial.
A foundation wall is cold and it is permeable. Warm humid interior air that reaches the face of that concrete hits its dew point and condenses, and with a paper-faced batt sitting against it there is now water, an organic food source and no drying path. Rigid foam against the concrete solves it by keeping the cold surface on the outside of the insulation, which is exactly why the code offers a continuous insulation option at all.
Should I put plastic sheeting over the insulation?
Not against the concrete, and generally not on the interior face of a basement wall either. A below-grade wall needs to be able to dry inward.
Polyethylene on the interior face of a basement stud wall traps whatever moisture gets into that cavity between the plastic and the concrete, with no way out in either direction. If you are using rigid foam against the wall, the foam is already the vapor control layer and a second one behind the drywall is what causes the problem it was meant to prevent.
How should the rim joists be handled?
The rim joist is usually the largest single air leak in a house and it is the coldest surface in most basements. It gets air sealed and insulated with a material that does not let humid air reach the wood.
Rigid foam cut to fit each bay and sealed around all four edges, or closed cell spray foam, both work. Fiberglass alone does not, because fiberglass does not stop air movement, so humid interior air travels straight through it and condenses on the cold rim board behind it. If there is existing fiberglass in your rim joist bays, it is not doing the job it was put there to do.
What about fire blocking? I was told every 8 feet.
The code says 10 feet, not 8. Section R302.11 requires fireblocking in concealed spaces of stud walls and partitions, including furred spaces, vertically at the ceiling and floor levels and horizontally at intervals not exceeding 10 feet.
It is also required at interconnections between concealed vertical and horizontal spaces, such as soffits and dropped ceilings, which is where basements usually get caught. Acceptable materials under R302.11.1 include 2 inch nominal lumber, half inch gypsum board, and batts or blankets of mineral wool or glass fiber securely retained in place. Unfaced mineral wool stuffed into a furred bay is legal fireblocking and it is the fastest way to close one. Spray foam is not on that list unless it is part of an approved tested assembly. Fireblocking is the single most cited item on a finished basement inspection.
If I make the basement airtight, do I need an ERV so I do not grow mold?
An energy recovery ventilator exchanges stale indoor air for outdoor air while recovering most of the heat. It is a whole house ventilation strategy, not a basement moisture fix.
In a finished basement the humidity problem is usually solved more directly and more cheaply: keep bulk water out, keep the cold surfaces on the outside of the insulation so nothing inside the wall reaches dew point, exhaust the bathroom to the outside, and run a dehumidifier. If a whole house ventilation strategy is on the table it is a conversation about the whole house, not about the basement wall.
How do I prove compliance on the permit application?
Pennsylvania accepts REScheck or the Pennsylvania Alternative Residential Energy Provisions. Both are named explicitly in 34 Pa. Code section 403.21(e)(1).
Stay on the prescriptive path or REScheck. Pennsylvania excludes the ERI and performance path machinery from the adopted code, so a compliance route built on an energy rating index is not available here. We produce the energy sheet as part of the permit set.
If a bedroom is part of the plan, read basement egress windows next. That is the other requirement that changes the price of a basement.
Where these numbers come from
Every number on this page links to the text it came from. Open any of them and check us.
- 0134 Pa. Code 403.21, Uniform Construction Code
Pennsylvania's own regulation. Names the adopted code edition, reprints the amended energy tables and carries every Pennsylvania amendment quoted on this page. Amended 7 November 2025 at 55 Pa.B. 7701, effective 1 January 2026.
- 02Department of Energy and PNNL, county climate zone data files
The dataset that puts Lehigh County in Climate Zone 5A and moved Berks County to 4A under IECC 2021. Published with the Guide to Determining Climate Zone by County.
- 03IRC 2021 R302.11, Fireblocking
Vertically at ceiling and floor levels, horizontally at intervals not exceeding 10 feet, plus the list of acceptable materials.
- 04Pennsylvania Attorney General, contractor registration search
Check PA225448, or any other contractor's registration, against the state's own record before you sign anything.
R-values and section numbers are quoted from the 2021 International Codes as adopted by Pennsylvania under 34 Pa. Code section 403.21, effective January 1, 2026, including the Pennsylvania amendments. Table R402.1.3 and its footnote c are reprinted verbatim at 34 Pa. Code section 403.21(a)(9)(vi)(D). Fireblocking is IRC 2021 section R302.11. County climate zone assignments are from the Department of Energy and PNNL county climate zone dataset.
The moisture, rim joist and mold sections are building science and our own standard, not code requirements, and they are labeled that way on purpose. We would rather tell you which is which than let a good practice pass as a legal obligation.
Lehigh Valley Builders Group LLC is a Pennsylvania Registered Home Improvement Contractor, PA225448. We draw our own permit sets, produce the energy compliance sheet, pull the permit and stand at the inspections.
Ask us the code question before you buy the insulation.
It costs you nothing and it is the cheapest point in the project to get it right.