What is a SIP?

A structural insulated panel (SIP) is a prefabricated building material composed of a foam insulating core laminated between two structural skins. Using SIPs is the most commonly used method of insulating and enclosing timber frames. The SIPs we use to insulate our timber frames are constructed either of an expanded polystyrene (EPS) or extruded polystyrene (XPS) core, and oriented strand board (OSB) skins. SIPs vary in thickness depending on the type of foam core and the core thickness, both of which dictate R-value or resistance to heat flow of the panel. The greater the R-value the greater the resistance to heat flow. EPS and XPS have R-values of 4.1/inch and 5.1/inch respectively. Typical R-values and panel thicknesses for EPS wall and roof panels in New England are R-24, 6.5"and R-32, 8.25" respectively. Typical R-values and panel thicknesses for XPS wall and roof panels in New England are R-19, 4.5"and R-29, 6.5" respectively. SIPs used for insulating timber frames are commonly manufactured in widths of 4' and lengths of up to 24'.

Both EPS and XPS foam cores are stable and inert materials, both have properties where R-values actually increase as the outside temperature decreases, and because they are formaldehyde free, they do not "off-gas." Of the two, we prefer to use EPS because it is more environmentally friendly as it is blown with an agent that contains no CFC or HCFC ozone depleting gases, is manufactured with only small amounts of petroleum, and it can be recycled. EPS also offers a lower cost per inch of thickness, and has a relatively low melting point which helps with on-site panel modifications. EPS panels are standard within the industry and produced by most SIP manufacturers.

We commonly use SIPs to create walls and roofs in portions of a structure devoid of timber framing. This is called "hybrid"construction and is common in projects where customers want to see timber in some of their rooms but not in others. Using SIPs in this manner also simplifies construction by maintaining the same insulation system throughout the house. Dormers, mudrooms and bump-outs are often constructed in this manner.

We also apply SIPs to the walls of reclaimed antique barn frames that we have restored for houses or out-buildings, and that may need some additional structural integrity. These wall SIPs can then support any number of different roof systems provided they incorporate a structural ridge.

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