Insulation Below Attic Platforms

Scope Images
Install insulation under platforms constructed in the attic for storage or equipment.
Install insulation under platforms constructed in the attic for storage or equipment.
Scope

Install insulation under walkways or platforms installed for storage or equipment constructed in the attic.

  • Add framing and blocking to raise the height of the platform to allow for the full height of code-required insulation under the platform.
  • Install insulation without misalignments, compressions, gaps, or voids underneath all attic platforms. 
  • Install insulation so that it is in contact with the air barrier (e.g., drywall ceiling).

See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.

Description

Attics are often well insulated, but there is one space that can be overlooked: below attic platforms. Usually platforms are used for walkways, storage, or to hold HVAC equipment. The platforms are typically created by laying plywood sheets on braced 2x6 framing running perpendicular to attic floor framing/ceiling joists. Insulation needs to be added prior to attaching the plywood sheets and completing the platform (Neuhauser 2012). Making sure the space below is adequately insulated, without compression, is critical to avoiding thermal bridging and the associated energy and financial costs.

There are three keys to correctly adding insulation below attic platforms:

  1. Increase the height of the storage or HVAC platform in the attic to allow for proper depth of the insulation beneath the platform without compressing the insulation.
  2. Install insulation without misalignments, compressions, gaps, or voids underneath all attic platforms.
  3. Install insulation so that it is in contact with the air barrier (e.g., drywall ceiling).

The amount of insulation needed will depend on the climate zone. In climate zones 1 to 5, the insulation must be greater than or equal to R-21. In zones 6 to 8, the insulation must be greater than or equal to R-30. Be sure to check local codes for current requirements.

A Note about Air Sealing: Insulating a home is important, but it will not be very effective if the home is not air sealed. The same is true for the space below the attic platform. Be sure the space is properly sealed before installing the insulation. This includes ceiling drywall intersections with interior walls. Ceiling drywall acts as an air barrier, but must be taped and mudded or caulked to be effective (PNNL 2010). 

Increase the Height of the Platform

To ensure adequate space for the insulation below the attic platform, the platform must be raised. This helps prevent the insulation from being compressed, which limits its effectiveness.

  1. Determine the necessary height to avoid compressing the insulation. The actual height will depend on the type of insulation used and the R-value needed.
  2. Raise the height of the joists by adding 2x4s, 2x6s, or other materials to the top edge of the joists as needed to accommodate the required space.

Install the Insulation

The type of insulation used may vary. Be sure to check with the manufacturer’s specifications as to density, etc., to make sure the installation will reach the minimum R value for the climate zone.

  1. Lay out, blow in, or otherwise install the insulation per the manufacturer’s requirements.
  2. When applicable, check the depth of insulation to make sure the correct R-value is reached.
  3. Inspect the insulation and the space to make sure there are no misalignments, compressions, gaps, or voids.
Measure the Depth of Insulation
Figure 1 - Measure the Depth of Insulation. It is important to compare the manufacturer’s instructions to the actual depth installed to make sure the installed insulation meets the necessary depth required to achieve the R-value required for the climate.  

Install Insulation to Be Fully Aligned with the Air Barrier

Making sure the insulation is in contact with the air barrier between the attic and the living space below will help to further eliminate any chance of thermal bridging. Typically, the air barrier is the drywall of the ceiling of the space below.

  1. Lay out, blow in, or otherwise install the insulation so that it is in contact with the air barrier.

  2. Inspect the insulation, visually or otherwise, to make sure continuous contact with the air barrier is made.

Ensuring Success

There are two keys to ensuring the success of insulating below attic platforms. First, make sure to allow enough space for the insulation so that the platform does not compress it, reducing its effectiveness. Second, make sure the insulation is in full contact with the air barrier below. Typically this is the drywall ceiling of the space below the attic. Any gaps in contact could still allow thermal bridging, defeating the purpose of insulating the space.

Climate

ENERGY STAR Certified Homes, Version 3/3.1 (Rev. 09)

Rater Field Checklist

Thermal Enclosure System. 
3. Reduced Thermal Bridging. 
3.3 Insulation beneath attic platforms (e.g., HVAC platforms, walkways) ≥ R-21 in CZ 1-5; ≥ R-30 in CZ 6-8.

The map in Figure 1 shows the climate zones for states that have adopted energy codes equivalent to the International Energy Conservation Code (IECC) 2009, 12, 15, and 18. The map in Figure 2 shows the climate zones for states that have adopted energy codes equivalent to the IECC 2021. Climate zone-specific requirements specified in the IECC are shown in the Compliance Tab of this guide. 

Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18.
Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18. (Source: 2012 IECC)

 

Climate Zone Map from IECC 2021.
Figure 2. Climate Zone Map from IECC 2021. (Source: 2021 IECC)

 

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Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

ENERGY STAR Certified Homes, Version 3/3.1 (Rev. 09)

Rater Field Checklist

Thermal Enclosure System. 
3. Reduced Thermal Bridging. 
3.3 Insulation beneath attic platforms (e.g., HVAC platforms, walkways) ≥ R-21 in CZ 1-5; ≥ R-30 in CZ 6-8.

Please see the ENERGY STAR Certified Homes Implementation Timeline for the program version and revision currently applicable in in your state.

DOE Zero Energy Ready Home (Revision 07)

Exhibit 1 Mandatory Requirements.
Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.
Exhibit 2, Item 2) Ceiling, wall, floor, and slab insulation shall meet or exceed 2015 IECC levels and achieve Grade 1 installation, per RESNET standards.

2009 International Energy Conservation Code (IECC)

Insulation below attic platforms is not specifically addressed in the 2009 IECC. Access hatches and doors. Access doors separating conditioned from unconditioned space are weather-stripped and insulated (without insulation compression or damage) to at least the level of insulation on the surrounding surfaces.

2012, 2015, and 2018 IECC

Insulation below attic platforms is not specifically addressed in the 2012 IECC. Table R402.4.1.1 Air Barrier and Insulation Installation, Ceiling/attic: Access openings, drop down stairs or knee wall doors to unconditioned attic spaces are insulated and sealed.

Retrofit: 

2009, 2012, 2015, 2018, and 2021 IECC

Section R101.4.3 (Section R501.1.1 in 2015, 2018, and 2021 IECC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

2009 International Residential Code (IRC)

Insulation below attic platforms is not specifically addressed in the 2009 IRC. Section N1102.2.3 Access hatches and doors. Access doors separating conditioned from unconditioned space are weather-stripped and insulated (without insulation compression or damage) to at least the level of insulation on the surrounding surfaces.

2012, 2015, and 2018 IRC

Insulation below attic platforms is not specifically addressed in the 2012 IRC. Table N1102.4.2 (N1102.4.1.1 in 2015 and 2018 IRC) Air Barrier and Insulation Installation, Ceiling/attic: Access openings, drop down stairs or knee wall doors to unconditioned attic spaces are insulated and sealed.

Retrofit: 

2009, 2012, 20152018, and 2021 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018, N1109.1 in 2021 IRC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

References and Resources*
Author(s)
U.S. Environmental Protection Agency
Organization(s)
EPA
Publication Date
Description
Guide describing details that serve as a visual reference for each of the line items in the Thermal Enclosure System Rater Checklist.
Author(s)
Neuhauser
Organization(s)
Building Science Corporation,
National Renewable Energy Laboratory
Publication Date
Description
Report about a project that examines implementation of advanced retrofit measures in the context of a large-scale weatherization program and the archetypal Chicago, Illinois, brick bungalow.
Author(s)
Baechler,
Gilbride,
Hefty,
Cole,
Williamson,
Love
Organization(s)
Pacific Northwest National Laboratory,
Oak Ridge National Laboratory
Publication Date
Description
Report identifying the steps to take, with the help of a qualified home performance contractor, to seal unwanted air leaks while ensuring healthy levels of ventilation and avoiding sources of indoor air pollution.
*For non-dated media, such as websites, the date listed is the date accessed.
Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Building Science Measures
Building Science-to-Sales Translator

High-R Attic Insulation = High-Efficiency or Ultra-Efficient Attic Insulation

Image(s)
Technical Description

There are two levels of attic insulation: high-efficiency insulation, which meets the 2015 International Energy Conservation Code, and ultra-efficient insulation, which is 25% more efficient than this national code. Using high-efficiency and ultra-efficient insulation along with professional installation (e.g., no gaps, voids, compression, or misalignment with air barriers;  complete air barriers; and minimal thermal bridging) creates conditioned spaces that require very little heating  and cooling, along with even comfort and quiet throughout the house.

High-Efficiency or Ultra-Efficient Attic Insulation
Sales Message

High-efficiency attic insulation helps provide added thermal protection. What this means to you is less wasted energy along with enhanced comfort and quiet. Knowing there is one opportunity to optimize performance during construction, wouldn’t you agree it’s a great opportunity to meet or exceed future codes?

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