Does a System Built, Pre-Fabricated, Kit Home Improve Quality? Part 3
By Stewart Elliott, Co-Founder Right Home Company
If we are to address improving quality and managing costs, we need to make better use of materials and labor. Pre-fabrication in a factory takes the guess work and on-site human error out of the equation while making better use of labor and materials. Recycling and waste are much more easily managed in a factory than on site. The result is a better built home in less time, with less material, less on site waste, and for less cost in an ever increasing cost market.
There are countless examples of manufactured Structural Composite Lumber (SCL) aka ‘engineered’ wood products, that improve the quality of building materials and reduce the impact on our natural resources. For example, industry manufactures a product equal to a 2x10 joist called an ‘I’ joist. It is made with ~2x2 tops and bottoms attached to a web of OSB with the same overall 9-1/4” depth as a 2x10[1] is supposed to be. The ‘I’ Joist is consistently straight, dimensionally predictable and it is stronger/stiffer resulting in a less ‘bouncy’ floor. The board footage of a 9-1/4” ‘I’ joist is .95 bf/lf vs. a 2” x 10” @ 1.67bf/lf; over 50% savings in lumber! AND it is made of smaller ‘harvest’ trees farmed like corn, grown in rows, cut, de-limbed, ground to the proper strand size, and loaded into a large semi-truck to be taken to the OSB manufacturing plant.
There are LVLs (Laminated Veneer Lumber) PSL (Parallel Strand Lumber), Glue Laminated beams, Micro-Lams, and others. SCLs all have qualities that are unmatched with sawn lumber including consistency, predictability, and competitively priced. Part of the improved performance is that these SCL materials can be more precisely calculated due to their known characteristics.
Engineering is a precise science being applied to nature’s inconsistency in the ‘natural’ sawn material[2]. This forces a ‘safety factor’ forcing larger members than the ‘engineered’ size applied to a consistent and predictable product. A 15% safety factor is not unusual which would take a 2x10 to a 2x12.
Another improvement may be to reduce the number of parts, thus reduce the number of potential problems, the KISS principle – “the simplest answer is often the right answer.”
A site framed 1-1/2-story conventional 1,800 sf bungalow style home has about 2,000 sf of wall area and ~15,000 bf of 2x6 studs, headers, top and bottom plates, blocking, and outside layer of OSB sheathing, resulting in some 450 studs, plates, bracing, blocking & headers, 70 sheets of OSB, a total of ~520 pieces of lumber that must be handled, culled, cut to size, and properly secured together.
What if we replace the exterior ‘stud wall’ with a different assembly such as a Structural Insulating Panel (SIP), eliminating the crooked 2x6 studs to begin with? Structural Insulating Panels (SIPs) are pre-fabricated in a factory or plant. They do not rely on 2x lumber, rather they are a lamination of an outer and inner skin (typically OSB) glued to high R-value rigid foam (typically EPS).
The same 2,000 sf of wall using SIPs, plates, splines, opening bucks, corners, and both layers of OSB is less than 3,500 bf – one fifth the amount lumber of a 2x6 conventional wall. And there will be about 30 SIP panels and 170 miscellaneous lumber pieces; a total of 200 pieces, all Ready To Assemble!!!
Another observation where pre-fabricated building components improve quality, safety, and resource management.
[1] Sawn 2x10’s are supposed to be 10-1/4” but are inconsistent and vary from <10-1/4” to >10-5/8” which is very problematic in constructing a flat and level floor.
[2] This includes such characteristics as varying grain slope, number, size and quality of knots, varying density
