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spray foam vs mineral wool, or ?? for vented attic.

cs55 | Posted in Energy Efficiency and Durability on

currently live in a single story 1500 sqft house located in the southern us.  last year we replaced the 20 year old 12 seer 3 ton central ac with a 1 ton and 1.5 ton minisplit. the central ac was in a vented attic. they got us through 105f summers and 5f winter days. but the house could be more comfortable.

the attic has soffit vents but very few baffles, so thats something to fix. theres no air sealing so thats also something to fix. but i really dislike blown in insulation and theres a mix of cellulose and fiberglass covered 2×8 joists.

i would like to remove to air seal, but not sure what to go with. 

spray foam on the attic floor would give continuous insulation, but at the cost of a lot of annoyance for any future electrical work. 

could air seal and an r30 mineral wool batt would fit between the joists, but i still have several knee walls from a mix of 8 and 11′ ceilings. theres also a very large catwalk for the existing central ac. 

if im wanting to avoid blown in insulation, would r30 mineral wool batts between joists and then another laid perpendicular be very efficient? then perhaps have spray foam along the soffit baffles/top plate, knee walls, and any other hard to get places. 

or, instead of more mineral wool laid perpendicular,  would foam board laid on top be beneficial at all? 

sorry for long post 🥲

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  1. Expert Member
    BILL WICHERS | | #1

    If you can get in at the eaves enough to put in baffles, you might be able to air seal too. If you really can’t get in there though, then spray foam is a good way to insulate and air seal the hard to reach spots. I would avoid using spray foam over the entire attic floor though, it’s expensive and doesn’t really offer any other advantages in this application since you can air seal the old fashioned way once you’ve removed the old insulation. You would have to remove the old insulation prior to spray foaming the attic floor.

    Blown in insulation is the easiest and cheapest way to insulate, and it gives you continuous insulation too since it’s normally installed deep enough to “bury” the framing on the attic floor. I would use blown cellulose here. If you want to go with batts, I would use fiberglass instead of mineral wool to save money, and use either batts or rolls between the attic floor joists. I would then use unfaced fiberglass rolls over the top of those joists, run perpendicular to them. Ideally you want to build up to at least about R49 worth of total insulation on your attic floor.

    Mineral wool batts over the tops of the joists are extra cost with little benefit. If you want to use mineral wool between the joists you can, but over the tops just use unfaced fiberglass roll insulation. Mineral wool has two main advantages over fiberglass: it has a little more R per inch, and it’s easier to install well in cavities between framing members. In an attic floor application, more R per inch doesn’t matter much since it’s easy to just pile on a thicker layer of insulation, and the ease of installation concern is only an issue between the joists, not over the top of them. What I have done in the past to save money is to use high density fiberglass between joists and regular R19 or thicker roll insulation over the tops.


    1. cs55 | | #2

      i think i have enough access to add baffles to every single stud bay for one side of the house -- house is pretty square, and some for about 1/4 of another side. i really just dislike blown in insulation -- almost to an irrational point. spray foam will cause issues with cabling/electrical work in the future. while blown in makes it annoying but at least doesn't involve hacking away at closed cell foam.

      the spray foam was more of a sledgehammer approach at air sealing.

      the mineral wool is mainly because an r30 batt would fit neatly inbetween 2 2x8s and allow for a perpendicular roll of fiber glass or more mineral wool on top.

      thank you for the advice.

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