Connecticut REScheck Requirements (2026)

Key facts:

• 2021 IECC with CT Amendments — Effective Oct. 1, 2022

• Statewide Zone 5A

• R-60 Ceilings — Stricter Than Base IECC

• Blower Door: 3 ACH50 Mandatory

 

Connecticut commonly requires REScheck for new residential construction, residential additions and alterations, conditioned detached structures, and barndominiums and ADUs. Connecticut enforces one of the strictest residential energy codes in the United States: R-60 ceilings, R-30 walls, U-0.30 windows, and mandatory 3 ACH50 blower door testing apply statewide to all 169 towns. No local amendments are permitted. A 2026 code update based on the 2024 IECC was expected July 1, 2026 but is delayed pending legislative approval.

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Connecticut jurisdictions require REScheck and energy compliance documentation for residential permit applications under the 2022 Connecticut State Building Code, which incorporates the 2021 International Energy Conservation Code with Connecticut-specific amendments, effective October 1, 2022. Connecticut's residential energy code is among the strictest in the nation — the entire state is in IECC Climate Zone 5A, and Connecticut significantly exceeded the base 2021 IECC by requiring R-60 ceiling insulation (vs. R-49 in the base code), R-30 walls (vs. R-20), and U-0.30 maximum windows (vs. U-0.32). Blower door testing at 3 ACH50 is mandatory for detached homes. Connecticut law requires the State Building Inspector to adopt the latest IECC edition within 18 months of publication, making Connecticut one of the most current energy code states in the country.

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⚠ 2026 Connecticut State Building Code Update Is Delayed: The next Connecticut State Building Code — incorporating the 2024 IRC, 2024 IECC, and related codes — was anticipated to take effect July 1, 2026. However, the State Building Inspector has confirmed the effective date is delayed, pending approval by the Legislative Regulation Review Committee of the Connecticut General Assembly. As of June 2026, the 2022 Connecticut State Building Code (based on the 2021 IECC) remains the enforceable standard. Monitor the Connecticut DAS Office of State Building Inspector website for updates. When the 2026 code takes effect, it will include 2024 IECC requirements that are significantly stricter than the current 2021 IECC baseline.

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Statewide Code Structure

Connecticut's 2022 State Building Code is administered by the Department of Administrative Services (DAS), Office of State Building Inspector, under Connecticut General Statute § 29-252. The code applies uniformly to all 169 Connecticut towns, cities, and boroughs — and to all state agencies. No local jurisdiction may adopt amendments that weaken the state code. Connecticut law requires the State Building Inspector to adopt the latest IECC within 18 months of publication, making Connecticut one of the most current energy code states in the country.

• 2022 Connecticut State Building Code — 2021 IECC with CT amendments — effective October 1, 2022

• Mandatory for all 169 Connecticut towns; no local amendments permitted

• State Building Inspector required by CGS 29-252 to adopt latest IECC within 18 months of publication

• Connecticut's code lacks weakening amendments — state has not reduced base IECC requirements; it has increased them

• 2026 code update (based on 2024 IECC) expected but delayed — 2022 code remains current

• Code applies to new construction, additions, alterations, and changes of occupancy

 

Common Permit Requirements

• REScheck compliance report (prescriptive or UA trade-off path)

• Manual J HVAC load calculations

• Manual S equipment selection documentation

• Window and door schedule (U-0.30 maximum — stricter than base IECC)

• Insulation R-value specifications and installer certification

• Blower door test results — 3 ACH50 mandatory for detached homes

• Duct leakage test results (post-construction 4 CFM25/100 sq ft)

• Foamed-in-place insulation certification per CGS 29-277 (when applicable)

• Energy certificate posted at electrical panel at project completion

• Mechanical ventilation documentation when home tests below 3 ACH50

 

Connecticut's Three Compliance Pathways

Prescriptive Path (Table R402.1.2)

• Meet all R-values and U-factors from Connecticut's amended Table R402.1.2 for Zone 5

• Ceiling R-60, walls R-30 or R-20+5ci or R-13+10ci or R-20ci, basement R-19 or R-15ci or R-13+5ci, floor R-30, slab R-15 at 4 ft, window U-0.30

• Most common path for standard new construction

• All mandatory requirements (blower door, duct testing) apply regardless of path

 

UA Trade-Off / REScheck Path (R402.1.5)

• Total building UA ≤ reference design UA

• REScheck accepted as documentation tool for this path

• Allows component trade-offs — enhanced windows offset less insulation in one assembly

• SHGC requirements must still be met

• Popular for custom homes and projects with architectural trade-offs

• UA maximum fenestration U-factor weighted average cap applies

 

ERI / Performance Path (R405 / R406)

• Energy Rating Index maximum: 55 for Zone 5

• Connecticut amendment: ERI compliance must be demonstrated without on-site renewable energy

• R401.2.5 Additional Energy Efficiency requirements not required when using ERI path

• HERS Rater required for ERI certification

• Total Building Performance (R405) also available using approved simulation software

• All mandatory requirements still apply on all paths

 

Connecticut's ERI Path Has a Key Amendment: Under the base 2021 IECC, builders can use on-site renewable energy (solar panels) to reduce the Energy Rating Index score toward the maximum threshold. Connecticut removed this option — ERI compliance must be demonstrated based on building envelope and mechanical system efficiency alone, without credit for on-site renewables. This makes Connecticut's ERI path stricter than the base IECC and ensures that buildings meet the energy standard through actual construction efficiency, not solar offsets.

 

Connecticut's Zone 5A Insulation Requirements

Connecticut's entire state is in IECC Climate Zone 5A (Cool-Humid). There is no zone boundary confusion — every project in every one of Connecticut's 169 towns uses Zone 5A values. However, Connecticut significantly amended the base 2021 IECC Zone 5 requirements upward. The table below shows Connecticut's actual enforceable values alongside the base 2021 IECC values for comparison.

Assembly  ·  Connecticut 2022 Code  ·  Base 2021 IECC Zone 5  ·  CT vs. Base IECC

• Ceiling / Attic:  R-60 / U-0.024   —   R-49 / U-0.026   —   CT requires R-60 — 22% stricter than base

• Wood Frame Walls:  R-30 cavity OR R-20+5ci OR R-13+10ci OR R-20ci / U-0.045   —   R-20+5ci OR R-13+10ci / U-0.057   —   CT adds R-30 cavity-only option; stricter U-factor

• Floor:  R-30 / U-0.033   —   R-30 / U-0.033   —   Same as base IECC

• Basement Walls:  R-19 or R-15ci or R-13+5ci / U-0.050   —   R-15/19 / U-0.050   —   Additional R-13+5ci option added; same U-factor

• Crawlspace Walls:  R-19 or R-15ci or R-13+5ci / U-0.055   —   R-15/19 / U-0.055   —   Additional R-13+5ci option added; same U-factor

• Slab Edge (unheated):  R-15 at 4 ft depth   —   R-10 at 2 ft depth   —   CT requires significantly deeper and more insulation

• Windows / Fenestration:  Maximum U-0.30   —   Maximum U-0.32   —   CT tightened window requirement

• Blower Door:  3 ACH50 (detached); 5 ACH50 (attached >850 sq ft); 6.5 ACH50 (attached ≤850 sq ft)   —   3 ACH50 all types   —   CT relaxed for attached units; same for detached

 

R-60 Ceilings — The Most Impactful Connecticut Amendment: Connecticut's requirement for R-60 ceiling insulation is the single most significant departure from the base 2021 IECC and from most other states. The base IECC requires R-49 in Zone 5. Connecticut requires R-60. Where full-height uncompressed R-49 insulation extends over the wall top plate at the eaves, R-49 satisfies the R-60 requirement — but this requires an energy heel truss or rafter design that provides sufficient depth at the eave. Without this exception, R-60 must be installed throughout the entire ceiling plane. This is the most common Connecticut REScheck failure for builders coming from neighboring states like Massachusetts (R-49) or Rhode Island.

 

Blower Door and Air Leakage Requirements

Connecticut's 2022 State Building Code requires mandatory blower door testing with an important Connecticut-specific amendment that differentiates between detached and attached residential units.

Air Leakage Thresholds by Building Type

• Detached one- and two-family dwellings: Maximum 3 ACH50 — standard base 2021 IECC requirement; mandatory testing; no visual inspection alternative

• Low-rise attached dwellings greater than 850 sq ft: Maximum 5 ACH50 — Connecticut-specific amendment relaxing the base 3 ACH50 for larger attached units

• Low-rise attached dwellings 850 sq ft or smaller: Maximum 6.5 ACH50 — Connecticut-specific amendment for small attached units

• Testing per RESNET/ICC 380, ASTM E779, or ASTM E1827

• Written report provided to building official

• Whole-house mechanical ventilation required when result is below 3 ACH50 — ERV or HRV commonly used in Connecticut's cold climate

 

Duct Leakage Requirements

• Post-construction total leakage: Maximum 4 CFM25 per 100 sq ft of conditioned floor area

• Rough-in (with air handler installed): Maximum 4 CFM25 per 100 sq ft

• Rough-in (without air handler): Maximum 3 CFM25 per 100 sq ft

• Duct testing required when any portion of distribution system is outside thermal envelope

• Connecticut amendment — HVAC addition exception: Duct leakage testing not required when an existing HVAC system is extended to serve an addition and the addition requires less than 40 linear feet of new duct in unconditioned space

• All duct systems in unconditioned spaces must be insulated to R-8 minimum (R-6 allowed for ducts under 3 inches)

 

Connecticut-Specific Code Requirements

Foamed-In-Place Insulation Restriction

Connecticut has a unique statewide restriction on foamed-in-place insulating materials under Connecticut General Statute § 29-277. Since May 28, 2013, foamed-in-place insulating materials — except urethane foam insulation and styrene foam insulation — may not be sold or installed in Connecticut unless the manufacturer or supplier has certified to the State Building Inspector that the material complies with CGS 29-277.

In practice, this means that spray polyurethane foam (SPF) — both open-cell and closed-cell — and polystyrene (EPS and XPS) foam insulation are the acceptable foamed-in-place products in Connecticut. Other foamed-in-place materials require specific certification. Builders using spray foam should verify their product meets the CGS 29-277 exemptions before installation.

 

Other Connecticut-Specific Requirements

• ERI path — no on-site renewables credit: Connecticut removed the ability to use on-site renewable energy (solar) to reduce the ERI score. ERI compliance must be demonstrated based on building envelope and mechanical efficiency alone (ERI maximum 55 for Zone 5)

• R401.2.5 Additional Efficiency not required on ERI path: Unlike some other states, Connecticut does not require an additional efficiency package when using the ERI compliance path

• Slab edge insulation depth: R-15 at 4 ft depth for unheated slabs — significantly deeper than the base 2021 IECC's R-10 at 2 ft depth

• Mechanical ventilation: Required whenever blower door result is below 3 ACH50 — cold Connecticut winters make HRV systems particularly beneficial for heat recovery

• High-efficacy lighting: All permanently installed lighting must use high-efficacy lamps

• HVAC sizing: Equipment must be sized per ACCA Manual J or equivalent method

 

Is REScheck Required in Connecticut?

Projects Requiring Energy Compliance

• New single-family homes

• Duplexes and townhomes

• Low-rise residential additions and alterations

• Finished basement conversions to conditioned space

• Garage conversions to conditioned space

• Conditioned sunrooms and bonus rooms

• Conditioned barndominiums

• Accessory dwelling units (ADUs)

 

Older homes: Projects like additions, basement finishing, or wall stripping require compliance with the current code for the new work. Basic retrofits (attic insulation only) focus on safe installation rather than full code compliance — verify with the local building official.

 

Connecticut-Specific Notes

• Connecticut's 2022 State Building Code applies uniformly to all 169 Connecticut towns — no local amendments or local variations are permitted

• REScheck is the accepted documentation tool for prescriptive and UA trade-off compliance — widely accepted at all Connecticut building departments

• Blower door testing is mandatory — no visual inspection alternative is available under Connecticut's 2022 code

• The Connecticut code includes a specific foamed-in-place insulation certification requirement under CGS 29-277 — verify product compliance before installation

• Connecticut is among the first states to adopt each new IECC edition due to the 18-month statutory requirement — expect stricter requirements when the 2026 code update takes effect

• Energy Star and NGBS certifications may be recognized as meeting or exceeding code — verify with the local building official

 

Common REScheck Failure Reasons in Connecticut

Most Common Errors

• Entering R-49 ceiling insulation instead of Connecticut's required R-60 — the most common Connecticut REScheck error; builders familiar with neighboring Massachusetts (R-49) or Rhode Island frequently use R-49 for Connecticut projects without realizing CT requires R-60

• Window U-factor above 0.30 — Connecticut requires U-0.30 maximum; the base 2021 IECC allows U-0.32; standard windows specified for neighboring states may not meet Connecticut's stricter limit

• Slab-edge insulation depth entered at 2 ft instead of Connecticut's required 4 ft — the base 2021 IECC requires R-10 at 2 ft; Connecticut requires R-15 at 4 ft; this is a significant specification difference frequently missed by builders coming from other states

• Slab-edge R-value entered at R-10 instead of Connecticut's required R-15

• Wall assembly specified without meeting Connecticut's stricter options — cavity-only R-20 walls are NOT permitted on the prescriptive path in Zone 5 (Connecticut or base IECC); R-30 cavity, R-20+5ci, R-13+10ci, or R-20ci are the Connecticut options

• Using ERI path with on-site renewable energy credit — Connecticut removed this provision; ERI must be demonstrated without solar credit

 

Mechanical & Envelope Issues

• Blower door failures at 3 ACH50 — Connecticut winters are cold; homes with inadequate air sealing at rim joists, top-plate bypasses, electrical penetrations, recessed lighting, and window rough openings frequently exceed the 3 ACH50 threshold

• R-60 ceiling eave condition — R-49 satisfies R-60 only where full-height uncompressed R-49 insulation extends over the wall top plate at the eave; homes without an adequate energy heel truss cannot claim the eave exception and must install R-60 throughout

• Duct leakage failures — 4 CFM25/100 sq ft post-construction; unconditioned attic and basement duct systems with poor mastic sealing at boots and connections are common failure points

• Foamed-in-place insulation not CGS 29-277 compliant — using non-certified foamed-in-place products triggers enforcement action; verify product compliance before installation, especially for products other than standard spray polyurethane foam or polystyrene

• Missing mechanical ventilation documentation — when blower door result falls below 3 ACH50, whole-house ventilation must be installed and documented; ERV or HRV required; missing this documentation is a common final inspection failure

• Duct insulation below R-8 in unconditioned spaces — particularly in uninsulated basements and attics where flex duct with inadequate R-value is specified from non-CT project templates

 

Major Cities and Jurisdictions in Connecticut

Hartford & Central Connecticut (Zone 5A)

Hartford, New Britain, Bristol, and surrounding central Connecticut cities are in Climate Zone 5A. As the state capital and insurance industry hub, Hartford has significant commercial and residential construction activity. Connecticut's statewide uniform code applies equally in Hartford as in any other town. Design winter temperatures in Hartford are approximately 11°F, making the full Zone 5A requirements — including R-60 ceilings, continuous wall insulation, and 3 ACH50 blower door — well-suited to the climate. Hartford's construction market includes significant urban infill, which creates unique air sealing challenges around party walls and mixed-use structures.

 

New Haven & Shoreline (Zone 5A)

New Haven, Bridgeport, Milford, and the Long Island Sound shoreline communities are in Climate Zone 5A. The coastal location creates milder winters than inland Connecticut but also introduces elevated humidity and summer latent loads. Yale University drives significant residential and mixed-use construction in New Haven. The 2022 code applies uniformly in coastal communities — there are no coastal exceptions or SHGC requirements that differ from inland areas, since Zone 5A has no SHGC maximum restriction.

 

Stamford, Greenwich & Fairfield County (Zone 5A)

Stamford, Greenwich, Norwalk, Westport, Fairfield, and Danbury — the Fairfield County communities closest to New York City — are all in Climate Zone 5A. This is one of Connecticut's most active and highest-value residential construction markets, driven by New York City commuters and corporate relocations. Builders and architects working in this market frequently work on projects in both Connecticut and New York — and must be aware that Connecticut's R-60 ceiling, R-30 walls, and U-0.30 windows are significantly stricter than New York State's requirements.

 

Waterbury, Meriden & Naugatuck Valley (Zone 5A)

Waterbury, Meriden, Naugatuck, and the Naugatuck Valley communities are in Climate Zone 5A. These inland Connecticut cities experience colder winters than the shoreline, with design temperatures that justify the full Zone 5A code requirements. Active residential construction in these communities follows the same 2022 Connecticut State Building Code as everywhere else in the state. Blower door testing at 3 ACH50 and R-60 ceiling insulation are standard permit requirements.

 

Frequently Asked Questions About REScheck in Connecticut

What energy code does Connecticut currently use?

Connecticut's current residential energy code is the 2022 Connecticut State Building Code, which incorporates the 2021 International Energy Conservation Code (IECC) with Connecticut-specific amendments, effective October 1, 2022. Connecticut significantly exceeded the base 2021 IECC by requiring R-60 ceiling insulation (vs. R-49), R-30 walls (vs. R-20+5ci in base IECC), U-0.30 maximum windows (vs. U-0.32), and R-15 slab insulation at 4 ft depth (vs. R-10 at 2 ft). A 2026 code update based on the 2024 IECC was expected July 1, 2026 but is delayed pending Connecticut General Assembly approval. Connecticut law (CGS 29-252) requires adoption of the latest IECC within 18 months of publication.

 

What climate zone is Connecticut in?

Connecticut is entirely in IECC Climate Zone 5A (Cool-Humid). The entire state — all 169 towns from Greenwich to Putnam, from Greenwich to Stonington — is in Zone 5A. There is no zone boundary within Connecticut. This single-zone structure means the same insulation values apply statewide: R-60 ceilings, R-30 or continuous wall insulation options, R-30 floors, R-19 or R-15ci basement walls, U-0.30 windows, and R-15 slab at 4 ft depth.

 

Why does Connecticut require R-60 ceiling insulation instead of R-49?

Connecticut amended the base 2021 IECC Zone 5 ceiling requirement upward from R-49 to R-60. This reflects Connecticut's commitment to aggressive energy efficiency — the state law requires adoption of the latest IECC within 18 months, and Connecticut consistently adds requirements that exceed the base national model code rather than reducing them. R-60 ceiling insulation is among the highest residential ceiling requirements of any state in the country. R-49 installed over 100% of ceiling area satisfies the R-60 requirement where full-height uncompressed R-49 extends over the wall top plate at the eave — requiring an energy heel truss or rafter design.

 

Is blower door testing required in Connecticut?

Yes. Blower door testing is mandatory for all new residential construction in Connecticut — there is no visual inspection alternative. For detached one- and two-family dwellings, the maximum air leakage is 3 ACH50. For attached dwellings greater than 850 sq ft, Connecticut amended the threshold to 5 ACH50. For attached dwellings 850 sq ft or smaller, the threshold is 6.5 ACH50. Whole-house mechanical ventilation — typically an ERV or HRV — is required when a home tests below 3 ACH50.

 

What wall insulation options are available in Connecticut?

Connecticut's 2022 State Building Code allows four wall assembly options for wood frame construction in Zone 5A: (1) R-30 cavity insulation only; (2) R-20 cavity plus R-5 continuous insulation (R-20+5ci); (3) R-13 cavity plus R-10 continuous insulation (R-13+10ci); or (4) R-20 continuous insulation alone (R-20ci). Cavity-only R-20 without continuous insulation is NOT permitted on the prescriptive path in Zone 5 — this is a key difference from Zone 4A states where cavity-only R-20 walls are sometimes allowed.

 

Can local towns in Connecticut adopt stricter or different energy codes?

No. Connecticut's state building code applies uniformly to all 169 towns, cities, and boroughs under Connecticut General Statute § 29-252a and § 29-253. No local jurisdiction may adopt amendments that differ from the state code. This means every Connecticut permit office — from Greenwich to Putnam — enforces exactly the same energy requirements. Connecticut's uniformity is similar to New Jersey and Wisconsin in this regard.

 

Does Connecticut require REScheck for additions?

Yes. Connecticut's 2022 State Building Code requires energy code compliance for residential additions, alterations, and changes of occupancy. New work must comply with current code requirements without requiring the unaltered portion of the existing building to be brought into compliance. REScheck is accepted for the UA trade-off compliance path. The blower door requirement and duct testing requirements apply to new addition construction where applicable.

 

What is the foamed-in-place insulation restriction in Connecticut?

Under Connecticut General Statute § 29-277, foamed-in-place insulating materials — except urethane foam insulation (spray polyurethane foam) and styrene foam insulation (EPS/XPS) — may not be sold or installed in Connecticut unless the manufacturer has certified to the State Building Inspector that the material complies with the statute. In practice, standard spray polyurethane foam (both open-cell and closed-cell) and polystyrene foam products are the acceptable foamed-in-place insulation types. Other foamed-in-place products require specific certification. Builders should verify product compliance before installation.

 

When will Connecticut adopt the 2024 IECC?

The 2026 Connecticut State Building Code — incorporating the 2024 IRC, 2024 IECC, and related codes — was anticipated to take effect July 1, 2026. However, the Connecticut DAS Office of State Building Inspector confirmed in 2026 that the effective date is delayed, pending approval by the Legislative Regulation Review Committee of the Connecticut General Assembly. The 2022 State Building Code (2021 IECC with CT amendments) remains the enforceable standard. When the 2026 code takes effect, it will incorporate 2024 IECC requirements that are stricter than the current standard. Monitor portal.ct.gov/das for updates.