ASHRAE Journal - September 2024 - 24

BUILDING SCIENCES
COLUMN
Joseph W. Lstiburek
Residential Low-Rise Construction
Insulated Concrete
Forms (ICFs)
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE
Insulated concrete forms (ICFs) are no longer the new kid on the block (we have
been here before, ASHRAE Journal, April 2009) and are making inroads in low-rise
residential construction. Back then I focused on high-rise multistory buildings and
referred to them as large Lego blocks made from polystyrene insulation filled with
concrete. The foam is a stay-in-place form for cast-in-place concrete (Photo 1).
The major concern with ICFs is their ability to control
rain entry. Additional concerns relate to insects and
groundwater. An ICF exterior wall can be classified
as a perfect barrier with the ability for the assembly
to act as a storage reservoir for limited quantities of
rain penetrating the exterior face. The plastic foam
ICF material and reinforced concrete are not generally
adversely affected by penetrating rain water.
However, most interior finish materials can be
adversely affected. As a result, permeable interior
finishes are recommended to facilitate drying to the
interior of any rain water that has penetrated the
exterior face. Gypsum board can be attached to wood
furring on the interior of the ICFs or adhered directly
to the interior of the ICFs. Similarly, interior plaster
finishes or renderings can be directly applied to the
interior of the ICFs.
Here is where I point out once again not to use
impermeable linings such as vinyl wall coverings
or sheet plastic vapor barriers. ICF assemblies do
not require interior vapor control layers, i.e., vapor
barriers or vapor retarders, anywhere humans
24
ASHRAE JOURNAL ashrae.org S E PTEM B ER 2024
typically live. Drying happens in both directions.
Stucco claddings are common with ICFs and are
directly applied to the ICFs. Claddings, other than
stucco, are attached to the exterior of the foam
blocks with furring strips. The back ventilation and
back drainage of these types of claddings on furring
strips-a " rain screen " control strategy-limits the
water load on the exterior face of the foam blocks
such that no treatment of the exterior face of the foam
blocks is necessary. The joints in the foam blocks with
respect to rain penetration can be ignored. However,
this is not the case with window and door openings as
we will see later.
ICFs can be used with the three typical residential
foundation approaches-slabs, basements and
crawlspaces.
Figure 1 shows the use of ICFs with a monolithic slab.
Rigid insulation is installed on the face of the slab
edge, and the ICF is " cantilevered " over the edge of
the slab edge to line up " flush " with the exterior slab
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in
Westford, Mass. Visit www.buildingscience.com.
http://www.buildingscience.com http://www.ashrae.org
ASHRAE Journal - September 2024

Table of Contents for the Digital Edition of ASHRAE Journal - September 2024
