The exterior shell of a residential structure separates the outside environment with the interior environment. The purpose of the basement, walls and roof has several goals. The legal building code minimum is to provide a safe, economical structure that protects the inhabitants from the external enviroment. The structure should not fall down, it should resist fire for at least 15 minutes, and should not be too expensive. The new and improved building code is also starting to address energy efficiency, mostly by air tightness. In the wood frame building world this is some times referred to as the ‘plastic bag effect’ and there is a very legitimate concern that by sealing a chip board and stick house in plastic that mold, rot and significant indoor air quality issues will arise.Logging in the Comox Valley (1890)In Canada wood is cheap and plentiful (although it seems that the better quality wood is exported). A wood structure can last more than 80 years if it is well built for it’s climate, and properly maintained. But there are other options. A bit of research (how did we survive before the internet?) will turn up three popular residential shell systems.
Platform or stick framing Wood studs before sheathing
This system starts from the outside in with protective siding (stucco, vinyl, cement based board (Hardi Plank), wood, etc). The siding protects the moisture barrier from mechanical damage (tearing, puncture, etc), direct water and solar (UV) damage – the most common and least expensive weather barrier is tar paper that will resist water penetration for up to 30 minutes. Next is the sheathing layer- plywood or chipboard (OSB), this is primarily to stop the wood stud structure from ‘racking or raking’, that is being pushed sideways (, is the engineering term). A code built wood house can withstand winds of up to 100 miles per hour. The sheathing is also used to attach the siding. Next, the wood stud system, most commonly 2×6 wood studs provides vertical strength and a space for the insulation. Then a plastic layer is attached to the studs to stop air and moisture from entering the house, and treated air (warmed or cooled) from leaving the house. And finally a layer of dry wall, this provides the required 15 minutes of fire resistance, protects the plastic and wiring and provides additional structural strength. Various sizes of nails are used to fasten the system together. There are variations of this system, and different materials can be used.
Structural Insulated Panels (SIPs)
Pre-made walls assembled
This system provides outstanding insulation value (from r21 to as high as r45) and fairly good air tightness. The SIPs are made of a layer of chipboard (aka OSB) glued to a layer of rigid foam (4 to 12 inches thick), usually expanded polystyrene (EPS) or polyurethane and then another glued on layer of chipboard sheathing. As with stick frame, the exterior is tar papered or house wrapped which protects the wood layer from moisture and the siding is attached to the sheathing layer. On the interior surface a plastic air/vapour barrier may not be required if the edges of the panel are properly sealed. The required drywall layer provides the minimum 15 minute fire-rating as fifteen minutes is recognized as the time needed for the occupants to exit at the outbreak of a fire. The structural strength relies on the combination of the wood panels and the rigid foam. SIPs are modular and custom made for on site assembly. The panels can be easy to assemble (glued and nailed) but may be difficult to wire or run plumbing. This panel system can also be used as a roofing system.
Insulated Concrete Forms (ICFs)
Any siding installs over exterior wallsThis is the system that Nutmeg Homes prefers. ICFs are basically interlocking blocks often about 4 feet long and 16 inches high, made of panels of stiff strong insulating foam and the panels are separated/held together by plastic ‘webs’. The insulating foam is nearly always made of expanded polystyrene (EPS)- this is the same class of foam that food containers are made from, stiff, inert (no off gassing), fantastic insulating properties (R-20 to R-27, not including the thermal mass effect of the concrete) and self extinguishing. The blocks interlock with various tab systems, just like Leggo blocks. The integrated webs, often made from recycled plastic, serve several purposes, they hold the block apart (ie. 6 in., 8 in. or even 12 in. ) to create the void that will be filled with concrete. The webs also create vertical nail/screw strips, usually every 8 inches on interior and exterior of the walls. Finally the webs will have a rebar placing system that lock in the horizontal rebar. The vertical rebar drops in the interior void, typically held in the center of the block by the horizontal steel. At Nutmeg Homes we generally end up with a 16 inch by 16 inch grid of rebar from the footings to the roof! As the layers are placed, the door and wind openings are framed and finally the blocks are filled with concrete (using a pumper truck). The exterior siding is attached (to the plastic webs) with screws or nails and protects the EPS from damage and UV radiation (sunlight). We have seen an owner built house stand for more than eight years before siding was attached with no UV or other damage, and have heard of a windy waterfront ICF home being occupied for a year before siding was attached. There is no weather barrier (tar paper or house wrap) or rain screen required, the siding (any type) can be attached directly to the ICF wall. The same with the interior wall, dry wall is attached directly to the EPS and webs. (Some building inspectors may want tarpaper or plastic, but it is not required). Interior walls do not need to be drywalled as the ICF block provides 2 hours fire rating (and that is at 2,000 degrees F.) but the EPS and the wiring and plumbing needs to be protected and drywall is nearly always used. There are many brands of ICF blocks, there are 3 manufacturers in BC alone, and 60 to 80 brands in North America.Flared window detail The shell also holds up the roof and provides the needed strength and the attachment system for the doors and windows. The ICF system allows for a variety of opening options. Since the walls are typically 12 inches thick window and door opening can become architectural and design features. There are no ‘headers’ (lamimated layers of wood) required over window and door openings and so there is more flexibility in placing openings (extra rebar reinforcement is usually need though) and openings can be quite large (ie 16 feet wide, or wider). Concrete, because it is so strong, give designers and architects lots of room to be creative.
How long will an ICF wall last? The answer is in centuries, or millennium. Why do we still build and live in houses that are disposable?