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Vented Attic Insulation for Conditioned Space, Earth Built Home

EarthBuilt | Posted in General Questions on

Hello, Folks!

A few questions for experienced people. I have an earth built home. The basement is the “main” floor with walkout doors on front side and built into the hill on back side. The “second” floor is ground level with an (almost like an A frame) roof that comes close to the ground. That “second” floor, on ground level, is basically our attic, no doors but good windows. Make sense?

It’s vented with soffit vents all along two long sides to gable vents with attic fan. 

The second floor is ready for a finished heated and cooled room up there. Nothing drywalled yet. 8 foot ceilings. Basically a big square box inside the A shape of the roof under the trusses. How should this be insulated?? 

The soffit vents are basically at floor level for this space. Baffles or air chutes will be needed to connect the top part of the roof where gable vent is to soffit vents in about 20 inches spans. The ceiling between the trusses, seems easy to insulate with two layers of rockwool for at least r45. The vertical walls are 2x4s.

1.How would you get enough r value there between the vertical studs? Is Rockwool a good choice? We thought the sound barrier benefits and fire safety were attractive.

2.What vapor barrier would you use and where would you put it? Does it go drywall, vapor barrier, studs, rockwool between studs, then air from soffits? The soffit vents are at the floor, about 6 feet away from bottom of studs, and that unusable floor space currently has loose insulation. It’s blocked off to prevent clogging vents.

This is my first time, and I’ve read many threads here and learned so much.

(Not super interested in closing it off or making an unvented attic at this point.) We have the ability to add pex bathroom plumbing as well, if insulated, and it’s already wired for power. The space is about 17 feet by 50 feet. Zone 4A in the Midwest. 

Can’t wait to hear ideas! Thank you!

SM

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Replies

  1. user-5946022 | | #1

    I would be inclined to put some rigid spacers or something against the underside of the deck to create the vent, and perhaps a layer of rigid insulation under that protect the integrity of the vent (make sure it does not get crushed). Then rockwool however thick you need it. You may have to furr down the rafters to get your desired thickness. Then create the vapor barrier either with a drywall, or intello plus drywall. Either way I would be careful of penetrations through the vapor barrier - try to use sealed J box lights instead of can lights, etc.

    Are you doing the work yourself, or hiring it out? No one cares as much as you do, so if doing it yourself you may be able to use a more complicated detail. I would not trust subs to care enough to ensure complicated details are properly executed.

  2. EarthBuilt | | #2

    Thanks for the reply! Definitely going to need something to create the special vents to pull air from the low soffit to gable area, not sure what product or if we'll make it from rigid foam.

    The walls are at a 90 degree angle to the floor, come up several feet, then angled in, then a flat ceiling below top of attic. The roof creates triangles of space on the outside of the wall studs where the soffit vents are located. I think those vertical wall studs are too thin to hold much insulation, and not sure how to hold in 8 to 10 inches thickness of ventilation vertically. Is there a way to using fewer furring strips horizontally just to hold in the second layer of rockwool? Some other combination of rigid foam?

    We were hoping to do all the work ourselves, but don't want to miss something. (We'll also need a few access doors so we can adjust things down the road as needed.) I'll be sure to look into the lights. Nothing like working in the attic when it's well over 100 to inspire more insulation on the final space!

    1. charlie_sullivan | | #3

      That picture helps a lot. You have four different areas to insulate:

      1. The attic of the attic, on top of the new ceiling. That's easy. I'd blow in cellulose because you can go all the way to R-60 with little extra cost and it will fill in around the truss structure well.

      2. The 45 degree part where you are up against the roof. You want a good ventilation channel through there, and then insulation. You don't have room for that without bringing the new position of the drywall in a bit. If you add some 2x4s running from the truss to the uprights, you can make as much space as you want and are willing to give up. You could opt for some high R-per-inch insulation there, if you don't want to make space, or you could make enough space to get great insulation with cheaper insulation.

      3. The vertical walls--sometimes called knee walls but maybe these are shoulder walls? If you use something like rockwool, you'll need an air barrier on the side facing the vented space. And you'll need a way to hold the batts, which you presumably want much thicker than the 2x4s, in place. That could be done with foam boards, if they are thick enough to avoid any concerns of condensation on the inward facing surfaces. I'd consider, however, doing a sort of double-stud wall, with the outer, newly added studs being 2x3 or even even 2x4s ripped in half, and then netting between them to hold blown in cellulose.

      4. The gable end walls. It looks like maybe there's something there already?

  3. Expert Member
    AKOS TOTH | | #4

    Those look like bonus room trusses, not an A frame. Insulating these is not too bad except at the slope part where you have to fur down. I would insulate the rafters behind the knee walls to have the space for storage and ducting.

    The big problem with these is air sealing, these types of rooms can easily be responsible 3/4 to 1/2 of the air leakage of the whole house. You have a lot of lumber that will poke through your air barrier and that is not easy to seal up.

    You can read more about how to seal these up here:

    https://www.greenbuildingadvisor.com/article/bonus-room-problems
    https://www.finehomebuilding.com/2012/09/06/two-ways-to-insulate-attic-kneewalls

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