Insulation for Pretty Good House Goals
New construction, zone 2, about 50 miles south of Houston, 10 miles from the gulf coast. 98% of the homes in this area are built and insulated the same way they were 30 years ago (or longer) when my previous home was built. Blown rockwool or fiberglass batts in the walls and above ceilings. Vented attic (typically ridge vents). Little if any consideration is given to a tight building envelope. HVAC ducts in the attics, which can reach 140+* in the summer. Slab on grade foundation. OSB wall sheathing with Tyvek. No additional sealing. One ton of AC for every 600 square feet.
My goal is to build a “pretty good” house (not shooting for any awards or certifications). Attached is a cross-section of the house. Front of the house is on right side where exterior stairs are shown. House will have a full concrete foundation (top of foundation about 18″ above natural ground) with a crawl space built on the foundation. Top of foundation to finished floor is 48″. Crawl space under conditioned living area will be closed/unvented. Crawl space under front veranda and rear screened porch will be vented. Crawl space has a continuous CMU perimeter stem wall, including between the porches and conditioned living area.
As the cross-section shows, the current plan is to spray 2″ of closed cell foam on the inside of the conditioned crawl space CMU stem walls. Exterior house walls will have open cell spray foam. Underside of roof will also get open cell spray foam. All exterior walls are 2×6’s, and wall sheathing will be 7/16″ zip (not zip-R). Roof will be fiberglass shingles over OSB.
Because a lot of the HVAC ductwork will be in the attic areas above the porches, the plan is to encapsulate the entire attic, so there will be some type of insulation (currently OC spray foam) above the porch and veranda ceilings. You can’t tell from the cross-section, but the porches are each about 50′ long, so it’s a large area.
Spray foam hasn’t “caught on” here yet. Maybe a few percent of the new homes use it, and those probably don’t use it correctly. It’s also pretty expensive here. I’m pretty sold on the closed cell foam in the crawl space. I’m looking for alternative ideas for the insulation, including for example, using a combination of materials (e.g., spray foam in crawl space and under roof and blown rock wool in walls and over porch ceilings), or maybe a solution that doesn’t use any spray foam except in crawl space. However, I need to keep it pretty basic, i.e., no leading/bleeding edge solutions. One thing I don’t want to do is use Zip-R or insulation outside of the wall or roof sheathing.
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