Pitched Roof Insulation

PRI

Pitched Roof Insulation (PRI) – PAS 2035 Retrofit Measure Overview

Description of Measure

Pitched Roof Insulation (PRI) involves insulating the internal slope of an unconverted pitched roof to reduce heat loss and improve the overall thermal efficiency of a dwelling. It is typically installed between and/or below rafters, making it a non-invasive internal retrofit that preserves the existing roof structure and coverings.

PAS 2035

Design Intent Under PAS 2035

Under PAS 2035, PRI is a fabric-first measure aimed at:

Reducing heat loss through the roof (typically 25% of total heat loss).
Improving internal thermal comfort in the upper floor.
Minimising thermal bridging and moisture risk in roof spaces.
Complying with U-value targets (typically ≤ 0.16 W/m²K).

The measure must be specified by a Retrofit Designer following a whole-dwelling assessment, with installation risk-managed by a Retrofit Coordinator.

INSTALL

Typical Installation Components

Between-rafter insulation

Rigid PIR boards (e.g. Celotex or Kingspan) fitted tightly between rafters.

Under-rafter insulation (optional)

Additional PIR boards installed across the rafters to improve thermal performance and reduce cold bridging.

Vapour control layer or breathable membrane

To manage interstitial condensation and maintain moisture balance, depending on existing roof construction.

Ventilation strategy

ensuring airflow is retained in eaves spaces or additional ventilation is introduced if required.

Fire and thermal performance

All materials used must meet relevant BS EN standards and be suitable for use under PAS 2030:2023.

COMPLIANCE

PAS 2035 Compliance Requirements

This measure must adhere to the following PAS 2035 principles:

Retrofit Assessment

Full property condition survey, occupancy assessment, and ventilation audit.
Identification of moisture risks and interaction with other measures.

Retrofit Design

Customised design based on property type, roof construction, and existing insulation.
Detail junctions, fixings, and condensation control strategies.

PAS 2030 Installation Standards

Only TrustMark-registered, PAS 2030-certified installers may carry out the work.
Installers must follow manufacturer guidance and standard installation practices.

Ventilation and Moisture Risk Management

Eaves and ridge ventilation retained or improved.
Vapour control specified to reduce interstitial condensation risk, based on BS 5250 guidance.

Retrofit Coordination and Sign-off

Oversight and quality assurance by a Retrofit Coordinator.
Post-installation monitoring or air testing may be advised in high-risk properties.

PROS

Benefits of Pitched Roof Insulation

Improves thermal performance of the dwelling and helps meet EPC improvement targets.

Reduces energy bills by minimising heat loss through uninsulated roof structures.

Minimally disruptive – installed internally, with no need to remove tiles or use scaffolding.

Supports government funding under ECO4, GBIS, and other eligible retrofit schemes.

Long-term savings and comfort benefits for the occupant.

CONS

Limitations / Risk Considerations

Not suitable for converted lofts – Room in Roof insulation applies in those cases.

Headroom may be reduced slightly if under-rafter boards are used in low loft spaces.

Complex roof shapes or restricted access may require specialist solutions.

Existing condensation issues must be resolved before insulation is added.

Stored items must be cleared for safe and complete installation.

ALIGN

Property Suitability Indicators

This measure may be suitable where:

The roof is pitched and unconverted.
There is no current rafter-level insulation or insulation is insufficient.
Roof coverings are in good condition and not due for replacement.
Eaves ventilation exists or can be reasonably introduced/maintained.

The space meets the minimum requirement to achieve the target U-value.

Interaction with Other Measures

Compatible with loft floor insulation where accessible.

Must be coordinated with ventilation upgrades (e.g. MEV, trickle vents) if whole-house airtightness is increased.

May be combined with internal wall insulation in top-floor rooms for continuous thermal envelope.

SUMMARY

Conclusion

Pitched Roof Insulation is a key fabric-first measure under PAS 2035 for improving energy performance in homes with unconverted lofts. It offers a cost-effective, low-disruption solution that aligns with U-value targets and contributes significantly to heat loss reduction, especially in older properties. When designed and installed correctly, it supports durable, moisture-safe, and grant-eligible retrofits that deliver real value to homeowners.