Open Vs. Closed Cell Foams

Introduction

We’re often asked if our foam cores are closed-cell vs. open-cell. Technically, plastic foams commonly used for insulation are all open-cell to a certain degree. There are some materials advertised as “closed-cell”, but it’s a question of degree, rather than simply open vs. closed. The feature that defines an insulation foam is gas trapped within solid plastic. The actual geometry of how the gas is trapped is the focus of this bulletin.
A closed-cell foam is one where distinct bubbles of gas are trapped individually in the plastic. See Picture 1. An open-cell foam is one where there are windows between adjacent bubbles which often create serpentine passages through the foam. See Picture 2. In the case of common insulating foams, none are perfectly closed cell, some just have many fewer windows and passages. In most cases, foam made from the same plastic can be either open-cell or nominally closed-cell, depending on the foam’s density. The higher the density, the fewer windows and passages within the foam.

Open Cell Foam

Picture 2: Open Cell Foam

Closed Cell Foam

Picture 1: Closed Cell Foam

Naming Convention

Best we can tell, the naming convention of open-cell vs. closed-cell originated in the spray-in-place polyurethane foam industry. For polyurethane, foam densities >= 2.0 lb/cuft are called “closed-cell” and densities < 1.5 lb/cuft are described as open-cell.
Typically, the open-cell or closed-cell foam specification is used as a proxy for other properties such as permeance, water absorption, stiffness, and compressive strength. Unfortunately, the “open-” vs. “closed-” convention becomes strained when it’s applied across different plastics and manufacturing methods. In essence, a closed-cell version of one plastic foam may be more permeable than the open-celled version of another because the number of windows in the cell walls isn’t the only factor in a foam’s properties. The size of the bubbles in the foam, size distribution, size of the windows, and the shape of the passages from cell to cell also matter.
Because of all of this, Foard doesn’t use the terms “open-cell” and “closed-cell” because it’s too incomplete. In an effort to get to the root performance issues, we prefer to use the measurable property values, because they work across all foam types, densities, and manufacturing methods.

Material Properties

Typically, the property people are most concerned about is permeance (the rate at which water vapor diffuses through the foam). The most common target value for the allowable permeance of a wall or roof assembly is 1.0 perm or less. Permeance below 1.0 perm is generally considered to be a “vapor barrier”, or, more correctly, an adequate water vapor control layer, by the building codes.
The combination of the foam’s properties and the thickness of foam used determine the permeance of the wall/roof assembly. Focusing on the general permeance of the foam (measured in perm-inches) alone is an incomplete representation. Table 1 lists the fundamental properties of the foams Foard Panel uses. Table 2 lists the properties of whole panels for some of our panel choices, including the contribution of the OSB skins.
As you can see, all of our panels can easily function as the “vapor barrier” in any wall/roof assembly. This is why we often point out that additional vapor retarding materials aren’t necessary. Also, it’s clear that even the most permeable, most “open-cell”, of our foams easily meets code requirements and most all building science recommendations.

Table 1: Foam Properties

Foam Type Typical Density in SIPs (lb/cuft) 1” Thick Permeance (perm) 3.63” Thick Permeance(perm)
EPS 1.0 5.0 1.4
Neopor 1.2 3.5 0.97
XPS 1.6 2.0 0.55
Polyisocyanurate 2.0 1.0 0.28

Table 2: Panel Permeance (perm)

Panel Type / Thickness 4.5” 6.5” 8.25” 10.25” 12.25”
EPS SIP 0.58 0.47 0.40 0.35 0.31
Neopor SIP 0.49 0.38 0.32 0.27 0.24
XPS SIP 0.36 0.26 0.21 0.18 0.15
Polyisocyanurate SIP 0.22 0.15 0.12 0.10 0.08

Practical Considerations

Our long-term durability investigations have shown that exterior finishing details have a vastly larger influence on long term building durability than any difference in core choice or difference in panel permeance. For example, so long as the panel’s permeance is less than 1.0, we have not found an example of long term building damage where a lower permeance panel would have helped. Over the same number of investigations, we’ve seen dozens of cases where long term building damage was caused by using synthetic building wrap or by poorly flashed windows.

Recommendation

Our recommendation is to base the choice of the core type and panel thickness on the building’s desired thermal performance, budget, and allowable wall/roof thickness. The differences in any of the foam’s structure has a very minor impact on the performance and durability of our SIPs.