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Question DetailsAsked on 3/18/2017

Using vent rafters too, is it ok to stuff insulation in garbage bags at the end of the above the soffit vent?

Finishing off an attic, trying figure best way to insulate behind knee walls. We installed the rafter vents that go from the soffit vents up to the ridge vent. So as to block any further airflow I stuffed non-faced batting in garbage bags, rolled it up and stuffed at the and of the eaves - this does not block the airflow going into the baffles, it is just stopping in from coming in anywhere else around the baffles. We will be putting faced batting in the rafters and behind knee walls. My brother-in-law, who is in construction, advised NOT doing this because he said we still need to let it "breathe"...of course my wife agrees with him because he's the "expert". I've put this project on hiatus for several months now because of this. Need advice/assistance.

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Sorry - your scheme gets a fail from me too and a good example of how "not to" do it. It does sound like the BIL is more of an expert than you - sorry. I would do a bit more research at reputable sites about insulating an attic room good videos at brandname insulation manufacturer websites, some university and arctic building science websites, This Old House, Green Star program, Cold Climate Housing Research Center, Canadian Building Council, etc - but steer away from the 'miracle" insulation blanket or foil blanket come-ons. [I listed the two arctic building centers because they obviously concentrate on insulation and attic moisture issues more than lower-48 researchers - though obviously some of their insulation recommendations will be extrem for lower-48 construction.]

Your brother in law is right - if it does not have ventilation and breathe (circulation), you run a very high risk of moisture problems down the road - and you WILL get moisture (condensed, roof or roof vents leaking, from leaking/disconnected fan vents, etc. in any attic.

Since you are talking insulating the attic room, I presume this will be "conditioned" space - connected to the rest of the house airspace, not an "attic airspace" storage room only.

Assuming you are in an area with cool/cold winters or even a cool rainy season - so the attic (in the unconditioned space outside the attic room) will be cold and damp at least part of the year - you want vapor barrier under the drywall on the inside face of the room walls/ceiling (and floor if not conditioned space underneath), with UNFACED insulation in the room joist/stud cavities. [If attic room is already framed and inside surface is drywalled/floored already, then second-best would be foil-faced insulation batts with the foil against the drywall/flooring. And I would get the retrofit type with the over-width foil facing which can be caulked and stapled to the studs at each side to give something resembling a vapor barrier, though of course the studs themselves cannot have a vapor barrier at the drywall contact in that case]. Then as good a ventilation as you can get not only through the rafter bays from eave to ridge vent, but also through the crawlspace (which should be accessible through airtight hatches from the room for inspection and maintenance) and around the endwalls of the room (unless built right up to the outside wall of the house)

UNFACED insulation because in most areas, the attic will be getting moist in either the winter or rainy season, or during summer humid nights as the attic cools down with humid air in it (and commonly both over the course of the seasons) so you always get at least some tendency toward condensation in there. Normally it dissipates all right by itself - as long as you have ventilation and insulation which can release the moisture - meaning unfaced in this case, because you have potential moisture sources both on the inside of the wall and from the outside and maybe from the house ceiling and the roof also at during different times.

Blocking off any airflow from the eave vents, as you propose with the insulation bags, will cause the crawlspace behind the kneewall to become a dead air space - not only for any moisture coming in from outside, but you always get at least some and usually a lot of moist warm air infiltration from below if you have not deliberately sealed off the airflow through the attic floor/underlying ceiling and wall penetrations, and the possibility of moisture/water from the roof. You want the eaves to flow air freely into the crawlspace "behind" the kneewall - though you will likely need an eave baffle at the eave itself so the attic "floor" insulation (right above the ceiling below, up against the outside house wall) does not block the eave vent and so the insulation (if blown-in type) cannot move in airflow and block the vent - and yes Virginia, blown-in insulation does blow around and even form drifts in attics - my about 6 inch (settled thickness) layer over batt has drifted as much as 2 feet high in about 90 mph winds in the past - even peeling a bat or two back at the eave vent in one case.

BTW - if you have not put in the subfloor for the room yet - it should generally be insulated underneath and ventilated underneath from eave to eave too if possible - otherwise the underside of the floor should be made part of the conditioned space and provision made for ventilation into the attic room via air-permeable flooring (like carpet weith air-permeable padding) or deliberate venting, so you do not get moisture buildup and stagnant air under the flooring. Commonly with a ventilated floor this is done with a diagonal 2x4 firring joist layer between the attic floor/underlying ceiling joists (insulated space between those joists) and the insulated attic room subfloor joists. Otherwise, if the underfloor area is "conditioned space" then the vapor barrier in the knee and end walls has to be tied into the vapor barrier sitting above the underlying ceiling drywall, making a rectangular flat box-shaped conditioned space under the attic room floor and walls. (In that case there would normally be wood blocking between the floor joists as well, under the room walls, to retain the insulation and fasten the vapor barrier to). Otherwise the attic room can become a significant source of moisture to the unconditioned attic space, commonly causing frosting/condensation and mildew/mold and sometimes even rot in the attic floor joists under and outside the kneewalls - and sometimes getting into the bottom of the kneewalls as well.

You do NOT want eave vent baffles continuous from the eave to the ridge vent - true that will remove moist/hot air from right under the roof, but eliminates (unless they are the type with many continuous air slots in the bottom) any circulation in the unconditioned part of the attic - a bad thing. But even if that type, at the eave itself the baffle should be interrupted so good airflow can get into the crawlspace from the eave right where the baffle rises above the "floor" insulation level - so commonly about 6-12 inches inside the outside wall. Ditto above the kneewall. This will allow for ventilation at the "floor" level to the kneewall, which will then (hopefully) with a fair amount of vortices and circulation) flow up the kneewall to the baffles leading over the edge of the kneewall, then back out to open rafter bay to freely circulate through the attic area above the room (unless it has a cathedrall ceiling to the peak) as it moves on to the ridge vents.

Then assuming the kneewall goes basically all the way up to the rafters and has a flat ceiling, an eave baffle typically again over the top of the kneewall, to prevent the room "ceiling" insulation from blocking the airflow through the rafter bays, then open airflow again to the ridge vent to allow circulation above the room.

You say you will be putting faced (discusssed that issue already) insulation in the rafters - if you mean the ceiling joists that form the ceiling of the room, fine assuming it does not reach up into the roof rafters. If you means insulation between the roof rafters, up under the roof sheathing - again, bad idea with an unconditioned airspace attic. Not only will this block the airflow over the kneewall, but the moisture coming from the roof sheathing (and you always have some after a roof ages a few years or so) will then have nowhere to go, so it will promote roof sheathing rot - even more so if you get an actual leak and the insulation traps it and holds it against the framing and sheathing (promoting rot) rather than giving it a chance to be detected and to evaporate and exit at the ridge vent.

BTW - you should have, unless the room is cathedral ceiling, access to the space above the room also, through an airtight insulated hatch or if room is not full attic length, from the end-of-room wall, to inspect for leaks on underside of roof and do any repairs in that area. Also make sure any ducts and wiring and pipes and such in the attic are accessible for replacement/repair - and remember ALL junction boxes up there have to be accessible - as should any can-type light or fan or similar mounting boxes which are fastened in with reinforced or external brackets from above. Underside of roof should also be accessible at all duct/pipe penetrations as well if as all possible.

Below are a few related issue links - you can find a lot more discussions on attic ventilation andn insulation in the Home > Insulation (and also to a lesser extend in Home > Roofing) links under Browse Projects, at lower left. I also linked some Inspectopedia articles on attic ventilation and insulation FYI.

Answered 3 years ago by LCD

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