What are SIPs (Structural Insulated Panels)?

by Erik North on November 25, 2011

SIPS, structural insulated panels

 

What are SIPs? That’d be structural insulated panels…but travel more than 10 feet outside the energy and construction industries and the phrase ‘structured insulated panels’ will draw some serious blank looks.

So what are SIPs and what are their relation to building and energy use? They’re an integrated approach to building construction that combines a building’s thermal, vapor and air controls into a single panel.

What Are SIPs – Your Boring Wiki Style Definition

Structural insulated panels are composite construction panels made of foam insulation sandwiched between two layers of a sheathing material.

The foam insulation is usually EPS, XPS or a similar polyurethane foam. The sheathing material can be a wide variety as well, like plywood, fiberboard or aluminum but OSB is most common.


 
The third component in a SIP wall construction is the spline or panel connector. This is often a wood member but the resultant thermal bridging kinda defeats the purpose of building an entire wall from insulation. Subsequently, splines are often built from composite material or insulated materials.

What Are SIPs – Advantages

There are several advantages of SIPs over standard stick construction. I love how since I always start with the advantages on these ‘What are SIPs’-type articles, it sounds like I’m just gushing over these things!

Speed of Assembly – Anyway, structural insulated panels can be assembled much more quickly than standard construction with an experienced crew (ah, there’s the rub). When properly planned, the onsite assembly of full wall panels is faster than the old bottom plate, wall studs, top plate, brace, nail, nail, nail, nail, nail.

Materials – A common sight during new home construction is a dumpster chock full of wood scrap. Every angle or wood member less than 8 feet means a cut and a wood scrap. Even though the industry has gotten much better at recycling post construction waste, SIPs produce less waste to begin with. SIPs are designed and pre-fit and involve considerably less on-site carpentry.

R-value – The sandwiched foam creates a continual layer of insulation with obvious benefits. Embedding XPS or EPS foam board into the wall panel gives the assembly a higher effective R-value than a comparable wall with fiberglass or cellulose.


 
Thermal Bridging – Normal platform construction (with top and bottom plates, three stud corners and 2×6 spaced 16 inch on center) have considerable thermal bridging. This compromises and lowers the whole wall R-value. Structural insulated panels, provided they’ve been properly sealed and insulated at the seams, have very little thermal bridging and much more uniform R-value across the wall.

Air Leakage – The building envelope, barring being encased in solid lucite, always leaks some air. There are well known air leakage paths like chimney framing or the smaller leaks that come from having thousands of microscopic seams, cracks and joists through the building.

Structural insulated panels, being single continuous panels, have far fewer seams and make much less leaky buildings. So much so that many SIPs contractors include mechanical ventilation from the design stage. If you strongly suspect the building will be very tight, mechanical vents can ensure sufficient fresh air (OK, that’s a slight negative because of the added cost).

Mechanical Systems – Lastly, because of the improved insulation and greatly reduced air leakage, the heating and cooling loads are correspondingly reduced. The heating and air conditioning systems can be designed smaller, matching these lower heating and cooling loads.

What Are SIPs – Disadvantages

There have to be some disadvantages, right? I mean otherwise every house in North America would be built this way. And they are two doozies and one lesser doozy.

Labor – First, it is much harder to find skilled contractors with experience building with SIPs than finding one experienced with standard platform construction. Experienced, local companies are harder to find and line up for this style construction.

Cost – SIPs are, yes, generally more expensive. There are quality arguments to be made (you pay more for a Lexus but get a better car) and the smaller mechanical systems in part offset this price.

Flexibility – SIPs are pre-measured to exact specifications. In urban development and infill lots, any size variations can make the SIPs much more difficult to install.

Moisture – Lastly, in cold, high humidity environments like Juneau, Alaska (OK, not ‘like’ Juneau…exactly Juneau) there have been reported roof failures. Juneau, Alaska is an extraordinarily nasty climate for buildings: There is precipitation in some form 300 days a year with relative humidity averaging around 75% (and regularly reaching 100%). The causes and solutions for the roof failure problems are detailed in this PDF.

So what are SIPs? SIPs are an integrated approach to construction, combining the insulation, air and vapor control layers into a single panel. Properly installed, they can make a very tight and well insulated home.

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