March 1, 2009

Abstract: 

This report is available from the Canadian Urethane Foam Contractors Association. It is reproduced here for convenience. A common question encountered by SPF applicators, building designers, and code officials is the need for an additional vapor barrier or retarder. Experience by many contractors and some consultants suggest that special low permeance layers such as polyethylene are rarely needed in many types of walls. Theory indicates that closed cell foam is sufficiently vapor impermeable to control diffusion condensation and that low-density open-cell foam applications may require additional vapor diffusion control in some extreme environments. However, the need for, and type of additional vapor control layers remains unanswered to many.

Executive Summary

Spray polyurethane foam (SPF) is an airtight foam plastic insulating product installed in-situ by spray application. The product is used in the walls, floors, and roofs, of both commercial and residential construction. There are two broad classes of SPUF; low-density 8 kg/m3 (0.5 pcf), open-cell and quite flexible foam, and a high-density 32 kg/m3 (2 pcf) closed-cell rigid foam. Both product classes are studied here.

A common question encountered by SPF applicators, building designers, and code officials is the need for an additional vapour barrier or retarder. Experience by many contractors and some consultants suggest that special low permeance layers such as polyethylene are rarely needed in many types of walls. Theory indicates that closed cell foam is sufficiently vapour impermeable to control diffusion condensation and that low-density open-cell foam applications may require additional vapor diffusion control in some extreme environments. However, the need for, and type of additional vapour control layers remains unanswered to many.

A research project was initiated to help answer these questions. The objective of the project was to provide recommendations, based on sound scientific evidence, of the need for additional vapour control for both classes of SPF installed in framed walls of a wide range of building occupancy types and cold climates. A combination of full-scale natural exposure field tests, climate chamber measurements, and hygrothermal computer modeling was applied.

The National Building Code of Canada specifies that vapour barriers are not required when "it can be shown that uncontrolled vapour diffusion will not adversely affect any of, (a) health or safety of building users, (b) the intended use of the building, or (c) the operation of the building services. The research demonstrated the ability of typical framed walls using spray polyurethane foam insulation, with and without additional vapor barrier layers, to meet these requirements.

More specifically, the research concluded* that:

  • Closed-cell (about 2 pounds per cubic foot density or more) spray foam applied in thicknesses of over 2" (50 mm) will control vapor diffusion to safe levels in all climates up to 10000 HDD and interior winter-time relative humidities of up to an over 50%RH. As thickness increases the level of diffusion control increases. The diffusion control is equivalent to walls with the traditional fiberglass batt and polyethylene.
  • Open cell (1/2 pound per cubic foot density) foam can control diffusion in climates that are not too cold (eg under 4500 HDD) and when the interior winter RH level is controlled by appropriate ventilation to below about 40%. Open cell foam does not have sufficient vapor control for use in very cold climates (4500 HDD to 5000 HDD) unless the interior winter-time RH is strictly controlled (to below about 30%RH).
  • For either type of foam, the wood framing provides sufficient inherent vapor resistance to maintain the moisture content within the safe range even in very cold exterior climates (10 000 HDD) and very humid interior conditions (50%RH in winter).

As for all walls made of all materials, a functional air barrier assembly must be provided, as well as rain control, fire control, structural sufficiency, etc.

The one-D WUFI Pro 3.3 hygrothermal modeling program was validated as an effective and accurate tool for predicting the moisture content of the sheathing in the field tests. It can be used to predict the performance of other wall assemblies in other climates if care is taken to define the material properties and boundary conditions.

Climate chamber vapor diffusion tests on a range of different products were conducted under a temperature gradient. These tests confirmed the performance noted in the field tests and demonstrated that different commercial products of the same class (closed-cell or open-cell) performed in a very similar manner.

An interesting observation noted in the materials sub-system climate chamber tests is that the HCFC-245 blown foam behaved essentially the same as the legacy HCFC-141b products. The vapour permeance of the new generation appears to be slightly less than the previous one.

Summary Results of Vapor Barrier Requirements

1 Introduction

Spray polyurethane foam (SPF) is an airtight foam plastic insulating product installed in-situ by spray application. The product is used in the walls, floors, and roofs, of both commercial and residential construction. There are two broad classes of SPF; low-density 8 kg/m3 (0.5 pcf), open cell and quite flexible foam, and a high-density 32 kg/m3 (2 pcf) closed cell rigid foam. Both product classes are studied in the research reported here.

A common question encountered by SPF applicators, building designers, and code officials is the need for an additional vapour barrier or retarder. Experience by many contractors and some consultants suggest that special low permeance layers such as polyethylene are rarely needed in many types of walls. Theory indicates that closed cell foam is sufficiently vapour impermeable to control diffusion condensation and that low-density open-cell foam applications may require additional vapor diffusion control in some extreme environments. However, the need for, and type of additional vapour control layers remains unanswered to many builders, designers, and code officials.

The objective of this research project is to provide recommendations, based on sound scientific evidence, of the need for additional vapour control for both classes of SPF installed in framed walls of a wide range of building occupancy types and cold climates.

1.1 Background

It is well understood in the construction industry that increasing insulation is a cost-effective means to reducing energy consumption over the life of the structure and thereby reducing the environmental and economic impact of operating energy consumption. Not as well understood, however, is that the amount of energy savings depends on the choice of insulation, how is it installed and where it is located in the building enclosure assembly. Poor design and workmanship can reduce the effectiveness of the insulation and produce an enclosure that transfers much more heat than the theoretical value of the insulation would indicate. In addition, if enclosure weaknesses such as thermal bridging are not properly addressed, the heat transfer will short circuit around the insulation, making the heat control layer less effective overall.

The most commonly available insulating materials are fibreglass, rock wool, cellulose, and foam plastics. Each class of product has different characteristics, such as fire resistance, costs, vapor permeability, ease of installation, etc. One of the most often listed characteristic is that of the resistance of heat flow per unit thickness.

Figure 1.1: Average RSI values of common insulation types (Straube & Burnett 2005)

Some insulation materials have the added benefit of providing significant resistance to air leakage or vapour diffusion or both. For example, some types of foam plastic have a high resistance to flow of heat, air and vapour and therefore have the potential to function as the heat, air and moisture control layers. At the other end of the spectrum, a material like fibreglass batt performs well as a heat control layer only. In an enclosure using fibreglass as the heat control layer, the air and moisture control layers must be designed and provided separately with other materials.

Spray polyurethane foam (SPF) is one type of foam plastic that is of great interest in building enclosure design because it can perform very well as multiple control layers. SPF provides one the highest heat resistances of any commonly available insulation products. The foam is created and applied on-site from a two-component liquid that mixes as it is being sprayed from a pressurized gun. The two liquids react chemically, bubbles form, the product expands, and the liquid is transformed into cellular plastic. The advantage of the on-site application process is that the liquid foam enters cracks, gaps and irregular cavities and fills them up as it expands. Once it cures, SPF creates a seamless, semi-rigid thermal and air barrier layer.

Medium and high density spray polyurethane foams also provide considerably more vapour resistance than traditional insulation materials. As a result, there will be applications in which medium and high density SPF can serve as the vapour control layer. Unfortunately, there is often confusion on the part of designers, builders and code enforcement officials about if and when these cases exist. If the cases could be identified and codified, the construction industry could benefit from eliminating a time consuming and costly step in construction.

1.2 Vapor Barriers and Air Barriers

Air has a limited capacity to hold water vapor: this maximum capacity drops significantly as the temperature drops. Condensation occurs when the air’s capacity at a surface is exceeded and water vapor reverts to a liquid. Water vapor moves to potential condensation surfaces by two mechanisms:

  1. vapor diffusion, the flow of vapor only from regions of high vapor content to regions of low vapor content and
  2. convection (typically called air leakage), the flow air from regions of high pressure to regions of low pressure carrying water vapor along with it.

Vapor barriers or vapor diffusion retarders address the flow of vapor by diffusion only. Air barrier systems control the flow of vapor by air flow.

Airflow transports much more vapor than diffusion in most cases. Air barrier systems are always required in buildings (and required by Canadian building codes), and are often provided by sealed, continuous and supported 6 mil poly, sealed and continuous drywall, or sealed and continuous housewrap products, etc. Air barriers also ensure good thermal performance, reduce sound transmission, and help ensure good indoor air quality.

Spray polyurethane foam of both types can be part of an air barrier system. Continuity must be provided whenever the SPF is not fully adhered to an air impermeable substrate. Foam sprayed between studs provides an excellent air barrier. However, wood-to-wood joints between double studs, at sill plates to floor sheathing, and joints around windows require sealing to provide a continuous air barrier.

Vapor diffusion can transport sufficient quantities of vapor to result in condensation in some cases. To control the amount of vapor transported by diffusion, vapor barriers (e.g., 6 mil poly. The National Building Code of Canada specifies that vapour barriers are not required when:

“it can be shown that uncontrolled vapour diffusion will not adversely affect any of, (a) health or safety of building users, (b) the intended use of the building, or (c) the operation of the building services.”

The research reported here investigated the ability of typical framed walls using spray polyurethane foam insulation, with and without additional vapor barrier layers, to meet these requirements. In all cases, a functional air barrier system was provided (in the form of sealed drywall or a continuous chain of SPF and wood), as this is required in all buildings.

1.3 Experimental Program

The research consisted of there phases:

  • Field measurements of performance of SPF and fiberglass batt insulated walls in a real wall exposed to the environment of South-western Ontario. The computer model was validated in this phase.
  • Laboratory measurements of vapor diffusion wetting in a climate chamber under steady-state conditions. Different brands of SPF were investigated in this phase.
  • Computer modeling of performance under a wide range of Canadian climate conditions, interior occupancies, and materials.

Each of the research phases is described in the separate chapters that follow.

2 Field Measurements

2.1 Introduction

This chapter presents the setup and results of a full-scale field investigation of the need for additional vapour retarding layers in both types of SPF in framed walls. Eight test walls were constructed and installed in the University of Waterloo’s BEGHut test facility, maintained at a high (50%RH) interior humidity level. The moisture content of the exterior wood sheathing and wood studs were monitored for a period of over two years and the results used to assess performance.

Hygrothermal modeling was then performed and compared to the observed results to validate the model. Using the validated hygrothermal model, recommendations for the use of additional vapour control layers as a function of SPF type, wall assembly, and climate (interior and exterior) are discussed.

2.2 Experimental Setup

2.2.1 Test Facility Description

The University of Waterloo’s BEGHut, located in Waterloo, Ontario is designed to investigate the performance of full-scale wall assemblies under natural exposure in this climate. This facility is maintained at a constant 20°C and 50% RH year-round. This is a high level for an office or residential building in cold climates, but is representative of museums, hospitals, and swimming pools. Interior relative humidity levels for houses in this climate zone typically range from 30-40% during the winter and 50-60% during the summer months.

2.2.2 Test Walls

The four assembly types (north and south duplicates; eight 2’ wide test walls total) were installed November 2005 in the University of Waterloo’s test hut. . .

Download complete report here.

Footnotes:

* for walls with exterior layers of sheathing, membranes, cladding and other layers with a permeance of more than about 60 ng/Pa s m2