Building Science and Structural Insulated Panels

Buildings are environmental separators, creating an indoor environment protected from the external climate. They allow the regulation of temperature, air movement, humidity, rain, snow, light, dust, odours, noise, vibrations, insects and vermin. And, they must accomplish this in a safe, healthy and durable manner. 

Building scientists are making the houses we live in, work in, or play in safer, healthier, economical and sustainable. In the last fifty years there have been three important changes to the way homes are built: 

  • the development of tighter building enclosures
  • the introduction of thermal insulation
  • the advent of forced air heating and cooling systems 

Each of these changes has made homes more comfortable, but has made houses less durable and more unpredictable in performance.  These changes interact with each other and are complicated by the effects of climate and the occupants. These factors have led to problems with health, safety durability, comfort and affordability concerns and to warranty issues.

Building science looks at a house as a system of the building enclosure, with a foundation assembly, a wall assembly and a roof assembly. Specifically, the building enclosure must: 

  • hold the building up
  • keep the rain water out
  • keep the ground water out
  • keep the wind out
  • keep the water vapour out
  • let the water and vapour out if they get in
  • keep the soil gas out
  • keep the heat in during winter
  • keep the heat out during the summer
  • keep the noise out

So how does building science support the use of structural insulated panels (SIPs) in achieving these functions in a modern home? A Structural Insulated Panel building has to meet the same requirements that all buildings meet, but SIPs buildings meet many of these requirements more easily than typically constructed buildings. Especially in the following areas.

Air Barriers

Air barriers are systems of materials designed and constructed to control airflow and separate indoor (conditioned) air and outdoor (unconditioned) air. In SIP buildings the SIP themselves are the air barrier system. Each panel meets both the air barrier material requirement and the air barrier assembly requirement. Sealing panels at their joints is the only additional requirement to limit air flow and vapour borne moisture.  

Formance Wall3

SIP Wall and Roof Properties

SIP assemblies have several unique properties. The cores are solid, homogenous and air impermeable so convective air flows and condensation due to air leakage are not possible.

SIPs have three distinct layers (inner layer, outer layer and the core) of equal vapour resistance. The typical and most common 115 mm thick panel is uniformly vapour semi-impermeable and resistant to vapour flow. The advantage is the panel can be used successfully  in any climate zone.

Energy Efficient Building Envelope

Rain is the single most important factor to control for a durable structure. Rain penetration is governed by capillary momentum, surface tension, gravity and wind (air pressure) forces. Capillary forces draw rain water into pores and tiny cracks, while the remaining forces direct rain water into larger openings. The physics of rain have been studied  since the 1950’s and we know how to design to keep rain out of a building and how to allow it to flow to the exterior when it does.

The unique properties of a SIP building enclosure become apparent in providing an energy efficient home because of the air barrier system and the properties of the SIP walls and roof in providing an air tight and vapour resistant enclosure. A common saving of energy in a SIP home can be 50%.

Other Benefits of a SIP Home

Studies have also been carried out on SIP homes in the following topics: 

  • Resilience to earthquakes, hurricanes and climate change. SIP houses have borne the brunt of these natural disasters essentially intact and still able to provide shelter for the occupants.
  • Healthy buildings.  A SIP home will be quiet, warm in winter, cool in summer and have a low relative humidity. Studies have shown the temperatures between 18-22 degrees c with a relative humidity of less than 65% are easily achieved.
  • Sustainability building with SIP. Structural insulated panels are one of the most airtight and well insulated building systems available, making them an inherently green product. An airtight building will use less energy to heat and cool, allow for better control over indoor environmental conditions, and reduce construction waste.

Increasingly home owners, potential home owners, designers, builders, local and central government are interested in homes which are warm, dry, energy efficient, healthy, sustainable and affordable. There are grading, measuring and rating systems and information available, for example: Homestar, Green Star, Passive House, Zero Net Energy.  Structural Insulated Panels comprehensively meet the science requirements and decades of practical experience demonstrate that we must build better homes.    

Building scientists take a fact, evidence and systems based view and, like all occupations, building scientists have a language which has a specific technical meaning.  Below are some of the terms used in the Building Science world:

  

Glossary:

Air Barrier

The air barrier system is the primary air enclosure boundary that separates indoor (conditioned) air and outdoor (unconditioned) air.

Air Barrier System

Is an assembly of materials that together are designed, installed or act to control the flow of air across the building enclosure.

Building Enclosure

Is an environmental separator, separating the interior environment  from the exterior environment. It controls heat flow, air flow, water vapour flow, rain, Ground water, light and solar radiation, noise and vibrations, contaminants, environmental hazards and odours, insects, rodents and vermin and fire. A building enclosure provides strength and rigidity and must be durable, and economical.

Cold Roof

A vented roof.

Conditioned Space

The part of the building designed to be thermally conditioned for the comfort of occupants or for other reasons.

Drainage Plane

Drainage planes are water repellent materials (building papers, building wraps, foam insulations, which are designed to and constructed to drain water to the exterior of the building,

Hot Roof

An unvented roof.

Indoor Air

Air in a conditioned space.

Low-E

A coating for high performance windows. The “e” stands for emissivity or radiated heat flow by reducing heat flow from a warmer air space to a colder glazing surface.

Occupiable  Space

An enclosed pace inside the pressure boundary, intended for human activities.

Outdoor Air

Air outside the building.

Orientated Strandboard (SB)

Is manufactured from waterproof heat cured adhesives and rectangularly shaped wood strands that are arranged in cross-oriented layers, similar to plywood. Results in a structural engineered wood panel that shares many of the strength and performance characteristics  of plywood.

Permeance.

The physical property that measures water molecules diffusing through a material. It is to vapour diffusion what conductance is to heat transfer.

Pressure Boundary

The basic requirement for buildings is to create an indoor environment that is different from the outdoors. That is; buildings are environmental separators. They allow the regulation of temperature, air movement, humidity, rain, snow, light, dust, odours, noise, vibrations, insects and vermin. And, all must be accomplished in a safe, healthy, durable and affordable way.

R-value

Quantitive measure of resistance to heat flow or conductivity. Many in the  industry consider R-value to be the primary indicator of energy efficiency, it only deals with it only deals with one of three modes of heat flow – the other two being convection and radiation. To place R-value in context, 25% to 40% of a typical home’s use can be attributed to air infiltration.

SIP Core

Rigid insulation material (mostly expanded polystyrene or EPS for short) sandwiched between structural insulated panel (SIP).

SIP Spline

Connection system used to connect two panels together at vertical, in-plane joints. Timber is commonly used but structural considerations will determine the type of spline.

Structural Insulated Panel (SIP)

Structural Insulated Panels (SIPs) are sandwich panels and/or stress skin panels that contain a structural core insulation bonded to structural panel exterior and interior skins. The foam core in a SIP performs a structural, insulating and air- sealing function in wall, roof and floor systems.

Thermal Boundary

The layer in a building enclosure that controls the transfer of heat (energy) between the interior and the exterior. It is a component of the building enclosure and it may, but does not have to align with the pressure boundary.

U-factor

Building scientists will use R-values for measures of resistance to heat flow for individual building materials, U-factor is always used as a summary measure for the conductive energy measure of building enclosures.

Vapour Barrier

Is a material that is vapour impermeable, often rubber membranes, polyethylene film, glass, aluminum foil.

Vapour Control Layer (or Layers)

The element (or elements) that is used (or are) designed and installed in an assembly to control the movement of water by vapour diffusion.

Vapour Permeable

Materials like building wraps and building papers.

Water Resistive Barrier

Also referred to as a drainage plane.

Wind-Washing

Air movement that occurs due to wind entering building enclosures, typically at the outside corners and roof eaves of buildings. Can have a significant impact on thermal and moisture movement and thermal and moisture performance of exterior wall assembles.

Zero Energy House

Any house that averages out to net zero energy consumption by supplying more than it needs during peak demand, typically using one or more solar energy strategies, energy storage and or net metering. SIPS provide efficiencies in the building enclosure that go a long way to achieving a zero energy home.   

Reference: ‘Builders Guide To Structural Insulated Panels’ by Joseph Lstiburek

Builders Guide to SIPs2

Interested in finding out more; purchase a copy of the book for yourself. 

300-page building science manual for SIP construction. Includes HVAC strategies, drainage plans, air sealing and more.

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