In two jobs of attic floor insulation, and again in the completion of a kitchen remodel in my home, I have made a thorough try of mineral wool batt insulation by Roxul, company now called Rockwool, and by Owens Corning, product called Thermafiber. One experience is summarized in this post. All are collected in blog posts search; Label: Mineral Wool Batts. I am late in preferring mineral wool batts to fiberglass because I work mainly in attics and crawl spaces, where mineral wool has seemed difficult and extravagant. In fact, real momentum in the deployment of mineral wool batts is since the 2013 acquisition of Thermafiber by Owens Corning.
Second Home With Attic Floor Mineral Wool Batt Insulation
This 1918 home has excellent 4 ft wide roof eaves sheltering windows and walls and giving an 18" tall attic perimeter. R49 attic floor insulation out to exterior walls is uniquely possible. Good elevation over exterior walls is an asset to running modern wiring down the exterior walls.
The attic has been barely accessible, dark and dangerous. Despite hostility, the large attic has household items preserved in storage. There is appearance of thick insulation, but many defects reduce effective value to a very poor R5.
After the attic floor has been framed to support an attic ladder, set a barricade to contain dust and plaster chunks of new attic access cuts conveniently aligned with a stairwell.
The found 2x6 attic floor joists will be strengthened to 2x12, with box beam methods. Build and install a ladder rough opening frame, using select 2x12 lumber. Bahco hand saw cuts through lath and plaster are guided true against the RO frame that is well fit to the ladder frame.
11 1/4" plywood rips serve as box beam webs. Beam top elements are longest-possible 2x4. Everywhere the found loose-fill mineral wool is only 2" deep; found batts draped over that had no value and has been in my way. I am happy to reset all found fiberglass batts within the box beams, packed intimately and with much elimination of framing thermal shorts. Found fiberglass batts are progressively shaken free of dust and roof debris upon sheets and into 55 gallon drum liners, piled and rationed as base fill within variable spaces below the level of flooring 2x4 nailers.
I started out knowing only of Rockwool batts, available as R15 and R30, and in unrated 2" thickness. Cut batts 23" wide, to lengths (shorter dimension) accurately fitting framing spaces, using a long commercial bread knife.
18" tall perimeter vertical framing is continuation of exterior walls below, always with closure of cavities in the attic floor (not balloon framing). Hard-covered insulation is 12" thick. I am able to go anywhere in the careful placement of wiring, lighting and new fans, then to be capped with a maximum depth of insulation.
In a final, nicely-accessible zone 54” wide at the North end of the attic, show learned exemplary use of mineral wool batt insulation. I ran out of found wimpy fiberglass for the base layer. At LHS, 2” of loose-fill mineral wool gives intimate bedding on lath and plaster roughness. Over that add R11 kraft-faced batts and R8 mineral wool batts, a total of R21. Crossing Thermafiber R30 batts 24” by 48”, 6” thick, bring the total in 11.5” depth, to true R49.
Over the master bath drywall at RHS, there was no found loose fill. begin with R11 unfaced batts for 1 1/2” fill to tops of on-flat 2x4s found.
Make note of more insulation stack-ups in 2x12 box beam caviies, exemplary of the use of mineral wool batt insulation, rarely disposing found insulation.
This is typical. I gave up on the use of housewrap strips after nearly hurting myself with kick of a circular saw dividing the plastic housewrap roll. Never again!
Note batt stack-up nearing attic completion, with little remaining found-fiberglass and no raised flooring. Total about R52. What else might one do to achieve serviceable insulation greater than R49? Top-layer R30 batts 24” x 48” are placed, and may be retrieved, from the flooring edge. A covered bath fan and lights will need service someday, then not destroying insulation. The stackup will vary with what materials are at hand, and with needs to fit objects buried.
This is the Fakro LTK 25/47 attic ladder leading to the completed attic now very energy-efficient, and useful.
Summarize the placement of 1/2" CDX plywood, 22.5 sheets reassembled from 24" rips.
Summarize the attic features.
Summarize the best energy-savings opportunity, closure of attic floor pits. I employ a concept of "equivalent bare area", to collect the savings value of closing air-tight and insulating over the pits.
Here are summary photos of the floor pits as found, and as closed. The opportunities are mostly associated with drop ceilings of closets, now closed or filled full.