Structural Insulated Panels (SIPs): Passive House Walls

Structural Insulated Panels (SIPs): Passive House Walls 

Structural Insulated Panels (SIPs) Exterior Shell Design

During the planning stages of the home, we selected structural insulated panels (SIPs) as our most exterior wall system above grade (we constructed a double wall system).

Exterior View of Structural Insulated Panels (SIPs)

SIPs are comprised of dense foam insulated structures sandwiched between OSB on both sides.  They can come in varying dimensions including eight, 10, 12 inch widths with further customized size configurations available.  At widths of 12 inches or greater, costs per SF get disproportionally larger.  This cost differential was yet another justification for building the exterior shell with a smaller SIP that would be combined with another interior wall system for the additional insulating value we had targeted.

There were several different insulating materials of structural insulated panels (SIPS) to consider including: expanded polystyrene (XPS), extruded polystyrene (XPS) , polyurethane (PUR) and carbon impregnated EPS.  These different materials ultimately determine the insulating R-value of the wall system.  While XPS and PUR materials provide the highest R-value per inch, we found their incremental costs to be excessive relative to the lower performing insulating materials, such as EPS.  In practice this means one has to increase the wall thickness to achieve equivalent R-values similar to the XPS SIPs.

Typical insulating R-values of various SIP configurations are:

• expanded polystyrene SIP (EPS) wall - 3.8 per inch
• polyurethane SIP (PUR) wall - 6.4 per inch
• extruded polystyrene SIP (XPS) wall - 4.6 per inch

Murus, of north central Pennsylvania, claims the following insulating performance of their SIP walls:

EPS SIPs Thickness R-value 4-1/2" 16 6-1/2" 23 8-1/4" 30 10-1/4" 38 12-1/4" 45

PUR SIPs Thickness R-value 4-1/2" 26 5-1/2" 33 6-1/2" 40

XPS SIPs Thickness R-value 4-1/2" 19 6-1/2" 29 8-1/4" 37 10-1/4" 47 12-1/4" 57

Suretight claims a 20% performance increase for their Neopor

	4 ½” SIP	6 ½” SIP	8 ¼” SIP	10 ¼” SIP	12 ¼” SIP
EPS Foam Core	15 - 16	23 - 25	30 - 32	37 - 40	45 - 49
Neopor® EPS Foam Core	18 - 19	27 - 29	35 - 38	43 - 47	53 - 57

SIPs Side View: Carbon Impregnated EPS

We believed we found a good balance in price versus performance, per inch.  Suretight of western Pennsylvania, manufactures a carbon impregnated EPS system which offers higher levels of performance than a conventional EPS insulated wall.

Interior view of SIPs

Since we decided to build a double shell wall system, we opted for the 8.25" SIP wall that provided an insulating value of R38 and we opted to not have chases cut into them as one would normally need to do to allow for wires and the like to be run through the walls.  This allowed us to maintain the highest consistency in insulation performance while at the same time, minimizing the potential for additional air-leakage that could occur from making (even partial) penetrations into the wall.

Why Fiberglass Batts are a Poor Insulation Choice for Passive Houses

• Moisture content compromises insulating performance - merely a 1% moisture content, reduces R-values by 50%!
• Air penetrations (leakage from outside) substantially decrease insulating performance - merely a 2% air leakage also results in R-value reductions of 50%!
• Air cavities are not as fire resistant - even if the material itself is fire rated, air pockets provide food for fire spreading inside the wall
• Fiberglass can off-gas and reduce interior air quality - this can be especially bad for a tight home
• Fiberglass provides no air tightness function - requiring a spray and batt approach to achieve some level of air tightness which may not be as robust
• SIPs provide exceptionally quite interiors, especially when  coupled with triple pane glazings

SIPs: A quick and easy way to frame plumb exterior walls

Like the below grade foundation Superior wall system, the SIP wall was manufactured to precise tolerances which made for very plumb and level walls. All door and window openings were precisely cut in size, allowing for especially tight fittings of all of our windows and doors.  Like the superior wall system, SIP construction takes place in an extremely quick fashion--a matter of a day or two.  The panels are placed by a small crew with a crane.

SIPs as Especially Strong, Stable, and Durable walls

Typical characteristics of SIPs are:

• Strength - In all physical dimensions & high load bearing capability
• Thermal stability - with minimal thermal drifting over time
• High wind (hurricane) resistant - can withstand winds in excess of 120mph
• Fire resistant - Class 1 fire rating, also in part to no air in wall cavity, especially without chases
• Earthquake resistant
• Low water vapor penetration

SIPs as the Air-tight Barrier

As there are not that may seams between sections of SIP walls and the panels themselves are solid core, we treated them as air-tight barriers.  All we needed to do is to thoroughly make certain these seams are air-tight (more on this later).

Front door & window cutouts

Additional References:

• Structual Insulated Panel Association