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Places where I should not put batt insulation

derekr | Posted in General Questions on

Hey everyone I know I’ve asked a lot of questions on here but this should be my last, once I finish this insulation myself I’m paying people to finish the rest, thanks for all your help

i know that I can’t put batt insulation on the rim joist in my crawl space, with that said are there any other areas that I shouldn’t put batt insulation?

2 particular spots I have in question were these pictures, the top of my knee wall and a 2×12 on my second floor where the rest of my roof is secured to. Any danger of moisture build up around these 2 spots like the rim joist if I stuff insulation around them? There will also be a gap behind my insulation all the way to the top of the roof 1.25 inches – 1.5 inches for air flow

thanks again for all the informantion

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  1. Expert Member
    BILL WICHERS | | #1

    If there is a vent channel between the roof sheathing and the batts, you can use batts there. Code minimum vent channel is 1", I like to see a bit more space than that, especially for any low slope areas. The easiest way to maintain that space is with one of the foam or plastic vent baffles, which you appear to be using. You can also build your own on site.

    What is that piece of vent/duct I see? Sometimes those need to be insulated seperately to avoid condensation within the duct/vent.

    Areas you shouldn't use batts (or other air permeable insulation) are areas where moisture can migrate through the insulation and then condense on something. This can be rim joists, unvented roof assemblies, and directly against basement walls. There are probably some other places I'm not thinking of, but those three are the "big ones".


    1. derekr | | #2

      The vent is for bathroom exhaust

    2. derekr | | #3

      What about the 2nd floor of the house? The upstairs floor isn’t vented what stops moisture from collecting in it?

  2. jollygreenshortguy | | #4

    "i know that I can’t put batt insulation on the rim joist in my crawl space"
    I don't know the details of your situation. But batt insulation at the rim joist is fine IF you've got continuous exterior insulation. The mistake people make is to think that batt insulation is going to give them an air barrier. It won't. But with exterior insulation that issue goes away.

    1. derekr | | #5

      No exterior insulation, just some osb and my siding

      I’m most concerned about the the 2x12 on the second floor my roof is connected to, if that rotted out in 50 years my roof would fall in above lol, I wasn’t sure if batt insulation should have direct contact with that from below and above it

      Here’s a picture of it on the second floor, the other picture was below

  3. maine_tyler | | #6

    Maybe it's just too early for me, but I can't figure out where exactly you are referring to from your pics. Can you describe the location, as in rim joist, top plate, header, etc.

    The basic premise is that insulation in a wall is behind drywall and sometimes a vapor retarder. If the location you are describing is OPEN TO THE INTERIOR CONDITIONED SPACE, that's when there may be concern (depending somewhat on what's on the exterior side as well).

    1. derekr | | #7

      It won’t be open, that’s the roof and ceiling, drywall will be going on it

      It’s a steep 60 degree roof, that one picture is the top of a 7 foot wall on the first floor touching the roof rafters, ceiling height on the first floor is a little over 8 feet though

      I just realized now though that drywall won’t be completely covering that 2x12 on the top floor where the roof is connected to because the osb floor is in the way, wonder if that could cause an issue

      Maybe I should get a can of spray foam and put it where the osb and 2x12 are touching?

      I’m not going to have drywall on the roof in the small space behind my knee wall though, that won’t be a living space so not sure if that matters for moisture, maybe I should put drywall there too?

  4. Expert Member
    BILL WICHERS | | #8

    Do you have vent space between the sheathing and the edge of that 2x12? In one of your earlier pics, it looked like you did. Your vent channels need to be unobstructed from soffit to ridge (assuming a ridge vent). If the vent channels stop part way up, you need roof vents at the tops of the vent channels. Vent channels are a system, carrying air moving by way of convection currents, from the low vent to the high vent on the roof.

    As long as you have a vent channel you're ok with batts. If you don't have a vent channel, that's where issues can arise, because moisture could potentially accumulate and cause problems.

    BTW, you are supposed to insulate exhaust ducts for bathroom exhaust fans. Usually this is done with the duct that has an insulating sock around it.


    1. derekr | | #9

      Yes some of these pics are old and it’s hard to see because of the angle but a 1.5 inch slot was cut where the 2x12 and roof sheathing meet

      I’m going to send a pic later today of my plans and show where my drywall will be and won’t be and see if that makes a difference for the insulation

    2. derekr | | #10

      Ok tried to do this the best I could, hopefully it helps and gives an idea of the pictures too

      The only place I wasn’t putting drywall was behind the knee wall, wasn’t sure if that could cause an issue or change things

  5. maine_tyler | | #11

    There's a bit going on here. I may have missed some context. This is an A frame?

    Have you read this article about insulating cathedral ceilings (what you have)?

    In any case , you will want your interior air barrier, created perhaps mostly from the drywall, to be as continuous as possible.

    1. derekr | | #12

      Yes, it’s all going to be drywall in the heated space, the place behind the knee wall isn’t going to be conditioned, I’m using that space for duct work and wiring so I wasn’t going to finish that area

      That space behind the wall is like having an attic on the first floor of the house instead of at the top, that was my thinking anyway

  6. maine_tyler | | #13

    Is the plane of your insulation along the slope?
    I don't want to be negative, but it seems you will have a high probability of air leakage if you don't treat that sloping ceiling plane as a monolithic area to seal. Too many turns and joints. Unless you've really thought through all these connections.
    You say the space behind the knee wall isn't conditioned, but if your insulation is on the slope, it WILL be in the conditioned envelope.
    Best would be to have the entirety of your sloping sealing well air sealed.
    Are you installing baffles to keep the vent channel in tact?

    I should have linked to this article which better emphasizes the vented approach. Note the emphasis on air sealing:

    1. maine_tyler | | #14

      Could also try searching how to insulate a cape, which shares the knee wall conundrum that you have. The reccomendation is generally to insulate and seal along the slope, not the knee wall.

      1. derekr | | #15

        Yes I have baffles, are you saying it would be better to have the insulation on the knee wall itself instead of on the roof behind it if that areas not going to be conditioned?

        I think about attics that have insulation in the rafters without drywall, I don’t see how it would be any different from that, my soffit vents start down there behind that knee wall I need the insulation to start there

        I mean I’m going to have duct work and electrical in there, I don’t think the drywall people can work around it

        I’m also in zone 3 if that makes a difference

        I have to put rigid insulation on the inside of my whole roof plus drywall on top of it in zone 3?

  7. derekr | | #16

    This air sealing you’re talking about, is it more for energy efficiency or for moisture? I thought my 1.5 inch baffles under my insulation stopped moisture from condensing?

  8. derekr | | #17

    Yea, there’s no way for me to completely air seal that area, the hvac people took some of their duct work between the 2x12 roof rafters to reach my 2nd floor from the first floor

    If I put insulation in the roof behind the knee wall there and I air seal the heated space but I don’t air seal the roof behind the knee wall what would happen, the roof would be air sealed every where else except behind the knee wall

    It appears my only option in this case might be to insulate my knee wall and don’t insulate the roof in that location

    1. maine_tyler | | #18

      I certainly come from the perspective of a colder climate. So my advice may not be as relevant to you in zone 3.
      I am saying insulating along the roof slope is typically ideal. Thr air sealing is primarily to prevent energy loss in your case but will also reduce moisture loading. But you're right the venting should help remove that moisture.
      It's pretty hard to advise on the specifics of your case--which has a lot going on-- from a keyboard afar. I would just say do everything you van to make your air barrier completely continuous. So where you have complicated roof/floor intersections that interrupt drywall, there may be benfit to something like a spray foam to connect the disparate planes above and below.

      1. derekr | | #19

        Ok just to go over everything one more time to make sure that this wouldn’t cause any problems

        I’m in zone 3

        I have a vented soffit at the bottom of the roof on the first floor of the A frame, venting all the way to the top

        The small space behind my knee wall on the first floor will be unheated, I also will not be able to air seal that small space on the inside with drywall because of my ductwork and electrical, but it will be sealed on the outside of that space with drywall in the heated area, I want to put r38 rockwool in that part of the roof where it’s unheated and not air sealed

        The heated part of the house will be dry walled

  9. adrienne_in_nj | | #20

    You have an A-frame with a vented roof that is vented from where the roof begins (at the top of the crawl space) all the way to the ridge vent. Basically, the roof is acting as 2 of your walls. In a non-A-frame house, the air barrier can be located at the sheathing layer. The sheathing can be taped at the seams to make an airtight enclosure. In this case, your "walls" are vented, because your vented roof is also acting as 2 of the walls, so the air barrier cannot be on the outside. Therefore, you must have the air barrier on the inside. It looks like you will be using the drywall as an air barrier. When you have a kneewall, it is recommend that you keep the "attic" in the conditioned space. Here's a good article that explains the why and how: If you choose to follow the advice in the article, you will need to put drywall (or some other continuous air barrier material) onto the sloped roof behind the kneewall. Ideally, it should have been put there before the area got filled up with ductwork, but stuff happens. It's happened to me too. Maybe you can cut some smaller pieces of drywall or rigid foam that you can more easily slide into place. After it's installed, you'll need to seal the seams with tape or something else. How were you planning to install airtight drywall? With acoustical sealant? Here's more info on airtight drywall:
    It kinda has to be planned ahead of time so you can plan the penetrations, such as electrical junction boxes. And I absolutely hate that the HVAC contractor put duct(s) in between your roof rafters. It will be very difficult to air seal those areas, and the ductwork displaces that R38 insulation that you had planned for the roof. In addition, if you live in a humid climate, you may end up with condensation in there because you could potentially have humid air in contact with cold ducts. Is there any way to bring them out of the roof and maybe run them in a chase just inside your air barrier (the drywall?) I know it's a pain, but with the ducts in the roof, they are basically outside, and the system will be more efficient if it's located completely inside the conditioned space. As far as your 2x12's that you were concerned about, it seems like these are fire blocking. It's almost like your house is balloon framed, with the second floor platform supported by the roof rafters, and those 2x12's are fire blocking at the floor level. For that reason, I'm wondering if it was proper for you to notch them out to make room for your ventilation baffles? Or if it would have been better to treat the 1st floor roof slope as a wall and have it unvented? Is it really a roof? Or is it a sloped wall? I guess it's both. Also, that area directly under the 2x12 blocking is actually like another rim joist, so you need to figure out a way to continue the air barrier from the first floor ceiling to the underside of the second floor floor. You mentioned spray foam, but how would you make it continuous with the top side of the drywall if the drywall is already installed? How would you get in there with a ceiling already in place? Maybe a thin strip of drywall that you can install at the perimeter of the ceiling and can then spray foam before the rest of the ceiling is installed?

    1. derekr | | #21

      The duct work is only within the roof rafters for like 1 foot then turns into the second floor joists to reach the second floor, the ducts are also insulated shouldn’t be condensation, so it will be r38 through the whole roof except for that 1 foot of space where the duct turns

      I’m not doing the drywall I’m hiring someone else to so I’m just going to have them seal it the way they normally do

      I’m in zone 3 and I’ve already talked with multiple builders down here and they said it’s ok for the drywall to be on my knee wall and none on the roof at the bottom in the attic area as long as all my heating and air is within the dry walled area

      The 2x12 the top roof is resting on was just used for a connection, it was connected to the bottom rafters first then the top rafters were lined up with bottom rafters and connected to the 2x12

      There’s no drywall in that attic space in the article you sent, it just has foam board on it that was used to achieve the correct R value because the rafters weren’t deep enough, I used 2x12 rafters and rockwool to achieve r38 without the use of foam board

      1. adrienne_in_nj | | #22

        In my previous post, I said that you can use drywall OR rigid foam on the roof behind the kneewall. The rigid foam in the drawing in the linked article is used as an air barrier (in addition to adding more R value.) The article states: "If you’re creating a vented roof assembly with air-permeable insulation (fiberglass, cellulose, or mineral wool), the insulation should be protected by a durable air barrier on both sides of the insulation. The top-side air barrier is usually the panel or baffle forming the bottom of the ventilation chute, while the bottom-side air barrier can be drywall, OSB, ThermoPly, or rigid foam. To ensure a tight air barrier, the seams of these panels should be sealed. It’s also important to caulk the joint between these panels and the kneewall top plate."

        I doubt that any drywall contractor that you hire would have a normal practice of sealing the drywall or has even heard of airtight drywall. If you want airtight drywall, you will need to research how you want it sealed and ask the contractor to do it to your specs and see if they agree. An easy way would be to use acoustical sealant at all the top plates, bottom plates, first and last stud on each wall and around windows, doors and other penetrations. Alternatively, you could use dywall gasket and have it all attached in place before the drywall contrator arrives so they don't have to do any "extra" work. Probably too late for the junction boxes, but you can caulk them after the drywall is installed. If you don't want to spec airtight drywall, then what is your plan for an air barrier? If you don't have a plan for an air barrier, you will not have an air barrier.

        Is there any code enforcement where this house is being built? If so, will there be a blower door test?

        You said, "I’m in zone 3 and I’ve already talked with multiple builders down here and they said it’s ok for the drywall to be on my knee wall and none on the roof at the bottom in the attic area as long as all my heating and air is within the dry walled area." Isn't your HVAC system in this attic area behind the kneewall? Which does not have drywall on the roof slope? If so, then all your "heating and air" is not "within the dry walled area."

        You said, "The duct work is only within the roof rafters for like 1 foot then turns into the second floor joists to reach the second floor, the ducts are also insulated shouldn’t be condensation..." If you could move the kneewall into the interior by just one foot (it may be loadbearing,) you can get the ducts out of the roof. As a bonus, with that wall termporarily removed, you may be able to get an air barrier onto that sloped roof by sliding it in from above.

        I looked again and I see that it appears that the 2x12 at the second floor rim joist area is a continuous horizonal board. That doesn't mean that it's also not acting as fire blocking or structural.

        1. derekr | | #24

          My duct work is in the attic area but none of the air exits into there, all the air vents exit into the space on the other side of the knee wall and upstairs, the only air in my attic area will be outside air from my soffits, none of my house air will get into my attic because it’s sealed on the other side of my knee wall

          No different than all the other houses around me with duct work in their attics

          If I was able to seal that part of the roof at the bottom (I can’t because of ducts) the outside air would still be going into roof from the soffits the same as it is now and my house air would still be on the other side of the knee wall as it is now, nothing changes by me air sealing that small section

          The knee wall can’t be moved in, the house has been finished framing for awhile, it’s knotched into the roof

          1. adrienne_in_nj | | #25

            I understand everything that you're saying. This is a green building site and presumably you want to build a green home. Just because all the other homes in your area have their HVAC system in the unconditioned attic does not mean it is a good idea. The topic has been covered extensively on this website and others and it's known to waste a lot of energy.

            And I wouldn't actually consider that space an attic. It's more like a closet with HVAC equipment, which I would call a mechanicals closet. Since you are going to have this closet as unconditioned space, what is your plan for the crawlspace below, which also runs under conditioned space? Where exactly is the location of the air barrier? Have you planned a continuous air barrier? Can you take the drawing that you previously posted and use a red pen to draw a continuous line around the house to represent the air barrier?

          2. derekr | | #26

            Yes I want the house to be energy efficient, but in this particular case I was more concerned about the safety of the house because there’s no way for me to get dry wall in there now, it’s impossible, unless I have the hvac people come back and undo all the work they did and I’m not doing that

            From what builders here have told me and a few people online I shouldn’t have a moisture problem this way, that was main concern since I can’t drywall that area now

            I’m putting r23 rockwool in my crawl space, I’m air sealing that from above with a floor underlayment and tape, have a 6 mil vapor barrier on the crawl space floor

  10. Deleted | | #23


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