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Community and Q&A

Kneewall insulation planning stage – Thoughts appreciated

Joe Duchek | Posted in Energy Efficiency and Durability on

Hi All,

I have read all the available articles and Q&A on kneewall insulation, including Dana’s comments on playing the fool on multiple occasions and therefore would appreciate your views on my situation / questions / possible approach.

Situation:

– Climate Zone 5 (Chicago area)
– Walk-in closet over attached 2-car garage (20ft x 8ft space)
– Garage attic has soffit and ridge venting and this attic is isolated from the rest of the home’s attic space, except for one HVAC duct which comes from the “main” attic. This duct runs above approximately half of the kneewall ceiling
– Reasonable access to both sides of the kneewall through 2ft x 3ft scuttleholes. There is reasonable working space in both sides of the attic.
– No reasonable access to ridge (above room) without removing drywall.
– The diagonal space between where the kneewall starts to slope to the ridge (on the attic side) and underside of roof deck is small (i.e. only a few inches). This space is currently filled with fiberglass batts (running vertically to the ridge area, between 2×4 kneewall studs).
– The vertically installed fiberglass (exposed to the attic-side and not thick enough) is seemingly blocking / retarding most of the air path to the ridge vent. Hint: The fiberglass is dirty in this section.
– Joists below kneewall (2x10s, 12in o.c.) are not blocked / not air-sealed, and are only partially filled with batts (e.g. R-19 ish). These cavities are relatively easily accessible.
– No apparent moisture issues.
– Roof rafters are 2x8s. Significantly sloped, shingled roof.

My DIY inclination is remove the ineffective batts currently below the kneewall and re-fill the underside cavities with fully-fitting unfaced fiberglass or blown cellulose (see question below), and then close off those cavities by air sealing with rigid foam and spray / caulk. I would then consider adding another layer of insulation (again unfaced fiberglass or blown cellulose) on the vertical side, again air-sealed with rigid foam spray / caulk. (If rolling a perpendicular layer of unfaced insulation, I would create 24in o.c. wood “channels” to fit the insulation in. If blowing cellulose, I would create the rigid barrier with Thermax and some wood support and then fill the space with cellulose and air seal accordingly.)

Questions:
– I suspect Martin & Dana would recommend insulating the underside of the roof deck instead? Even with an unconditioned space (garage) below? If yes, then is it possible to estimate how much I lose by “insulating with a blanket” approach rather than insulating the underside of the roof?
– For the joist space under the kneewall, I think that I could potentially get as high as R-35 with cellulose vs R-30 with unfaced fiberglass into the cavities (ignoring thermal bridging from the wood underneath). Can a first-time blower (me) using machine from a box store still get a better R-value / air blockage result into this horizontal space using cellulose as compared with “pulling through” properly fitting fiberglass?
– How much of a concern is the blocked ridge vent? I could remove the fiberglass at that section and try to get a layer of rigid foam between the roof studs (with a 1 in vent space at the underside of the roof deck). I could also then try to fill the remaining space with insulation (either pancaked rigid or fluffy stuff), but overall this element will be a challenge.
– Is the entire effort likely not going to not produce reasonable improvement if I can’t better insulate above the kneewall?

Overall I would be happy with reasonable improvement, but also happy to be told that I would be playing the fool for under-taking suboptimal efforts.

Again, I appreciate any thoughts.

Regards,

Joe

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Joe,
    First of all, let's clarify the terminology -- because some of your descriptions are confusing.

    You need to insulate four different areas. (Some of these areas come in matched pairs.)

    1. You need to insulate the floor assembly of your walk-in closet. If I understand correctly, these are 2x10 floor joists with a garage below.

    2. You need to insulate your kneewalls -- one on the left, and one on the right. These are vertical walls, about 4 feet high, I assume. (Alternatively, you need to insulate the roof slope above the triangular attic behind these kneewalls.)

    3. You need to insulate the sloped ceiling of your walk-in closet -- one on the left, and one on the right.

    4. You need to insulate the horizontal ceiling in the center of your walk-in closet.

    The most effective way to insulate (2), (3), and (4) is with rigid foam above the roof sheathing. But that's expensive.

    If you can't afford to install rigid foam above your roof sheathing, you're stuck with the "playing the fool" approach described elsewhere by Dana Dorsett. It's possible to play the fool fairly well, or play the fool poorly, but in either case it's not a lot of fun.

    I don't recommend trying to slide fiberglass batts into hidden framing cavities. I imagine that you can use a borrowed or rented cellulose blower to improve the situation in the floor assembly. It won't be perfect, but it will be better than what you have.

    In the other areas -- (2), (3), and (4) -- your access sounds terrible, and your likelihood of success are only so-so.

    If you want to play the fool, remember that air sealing is extremely important. Pay attention to air leakage and potential air leakage paths.

    One last question: What's your plan to address the HVAC duct in the unconditioned attic above the closet ceiling? If you have to remove the drywall ceiling to address this problem, at least you'll have access to the attic above the ceiling.

    -- Martin Holladay

  2. Joe Duchek | | #2

    Hi Martin,

    Thanks for the detailed response. I appreciate it. And apologies for the confusing terminology. We can stick with numbers; they are more my strong suit :) I have also attached a few pictures to illustrate.

    Let me clarify the situation:

    - 1 and 2 (floor joists and attic side kneewalls) are easily accessible. Getting polyiso (Thermax) into this space won't be difficult. I will just cut the 4x8 sheets in half to get them into the attic.
    - 3 (sloped ceiling) is a tight space. Improvement options somewhat limited.
    - 4 (horizontal ceiling) is currently inaccessible.

    Responses to your points comments / questions:

    1. You need to insulate the floor assembly of your walk-in closet. If I understand correctly, these are 2x10 floor joists with a garage below.

    - Correct. 2x10 joists below closet with garage further below. I will fill these cavities with blown cellulose. Should I plug one side of the cavity (with polyiso) and blow in from the other? Or plug both, leave a hole in one and patch the hole once done? Third option would be to fill from topside, but existing carpet would need to be removed / replaced.

    - Also, thanks for the recommendation / confirmation about not sliding fiberglass through the cavities.

    2. You need to insulate your kneewalls -- one on the left, and one on the right. These are vertical walls, about 4 feet high, I assume. (Alternatively, you need to insulate the roof slope above the triangular attic behind these knee walls.)

    - Correct. This is also doable. I was thinking of rolling unfaced fiberglass perpendicularly across the existing vertical kneewall studs (properly supported with 1x10in wood channels) to get more R-value on the walls and deal with the existing thermal bridging of the vertical studs. I would then cover the perpendicular insulation with Thermax and properly air seal. Or I could just cover the existing fiberglass with rigid foam and proper air sealing and forget about the extra unfaced insulation and associated labor.

    - Separately I am also considering the "insulate the roof slope" option, but feel somewhat less confident about this. I guess I could get to about R-30 using an initial layer polyiso inside the joists to create the air vent channel, then fluffy stuff, then more polyiso over the entire interior roof slope joists. I don't think that I am going to spray foam in this space.

    3. You need to insulate the sloped ceiling of your walk-in closet -- one on the left, and one on the right.

    - Yes. I need to remove the existing fiberglass in this space to see what's possible. I could probably get some rigid foam in here with a bit of cutting and snugging and also leave some room for air venting at the same time. Thoughts?

    4. You need to insulate the horizontal ceiling in the center of your walk-in closet.

    - Yes, but inaccessible currently. Not sure what's up there at the moment. I will think about this as part of larger retrofit.

    5. What's your plan to address the HVAC duct in the unconditioned attic above the closet ceiling? If you have to remove the drywall ceiling to address this problem, at least you'll have access to the attic above the ceiling.

    - Open to suggestions. I currently have a "thermal envelope disaster" in the main part the attic which is another part of this problem but deserves a separate post. Specifically, we have a second floor HVAC unit, and the supply duct, as well as a combustion air supply pipe, extend from the second floor into the main VENTED attic. Duct work is also in the attic. Needless to say, it is not cold in this attic even during the Chicago winter. This area is probably the best candidate for attic side roof slope insulation, but I need to assess the soffits and dormer areas a bit more. Again, open to thoughts. I have attached a picture of this too.

    Though all of the above is fussy work, it's more of a hobby for me than an immediate "emergency". I don't mind the slow but steady challenge. We keep the house cool in the winter and warm in the summer. Therefore utility bills aren't bad. It's the principle of wasteful energy loss that bugs me more.

    Finally, thanks for mentioning the ideal situation (rigid foam above the sheathing). Not yet ready for a new roof, but will keep it in mind :)

    Regards,

    Joe

  3. GBA Editor
    Martin Holladay | | #3

    Joe,
    Q. "I will fill these 2x10 joist cavities with blown cellulose. Should I plug one side of the cavity (with polyiso) and blow in from the other? Or plug both, leave a hole in one and patch the hole once done?"

    A. Either way will work, as long as you know what you are doing. Dense-packing cellulose is a skill; you may want to hire a contractor for this work. Here is a link to an article with more information: How to Install Cellulose Insulation.

    Q. "I need to remove the existing fiberglass in the sloped ceiling to see what's possible. I could probably get some rigid foam in here with a bit of cutting and snugging and also leave some room for air venting at the same time. Thoughts?"

    A. Again, as long as you know what you are doing, and as long as you can achieve the minimum code R-value for roofs, you can do this work. If I were doing this work, I would remove the drywall for good access. Here is a link to an article with more information on your options: How to Build an Insulated Cathedral Ceiling.

    Q. "I currently have a thermal envelope disaster in the main part the attic which is another part of this problem but deserves a separate post. ... Again, open to thoughts."

    A. It sounds like you know what to do. Fixing these problems isn't rocket science, but it's a lot of work. If your budget is adequate, and you can find a competent contractor, all you have to do is sign a check.

    -- Martin Holladay

  4. Joe Duchek | | #4

    Hi Martin,

    Thanks again for the advice provided above regarding my kneewall insulation project. Coming back with a few more questions:

    1) If I install polyiso from the floor to the rafters on the attic side of each kneewall (as the air barrier for the existing R-19 fiberglass), and have cellulose blown into the newly air-sealed floor joists underneath the walk in closet, (i.e. the "good" option in the Fine Homebuilding article / "playing the fool" option), does the remaining floor joist space in the unconditioned part of the attic need to be filled with insulation? I think no because the garage below is unconditioned and the soffits are allowing unconditioned air into the space, and therefore both above and below this space is outside of the thermal envelope of house. But if it would help, then it's the easiest to add.

    2) Regarding the attic side vertical wall insulation (the "good" option), it seems that (based on responses to other Q&As) that R-19 in a 5in stud cavity with 1in of polyiso (in climate zone 5) would be okay-ish / bare minimum? Would you recommend the polyiso be a bit thicker? The box stores around here don't stock anything thicker than 1in polyiso. I am keeping my eye out for a good reclaimed foam option, but haven't found it yet.

    3) I have soffit and ridge venting, but the sloped ceiling section on the attic side currently does not have ventilation chutes / baffles. Instead, the sloped cavity is just filled / blocked with fiberglass insulation. How important is it to get ventilation flowing through this space? Does "if your going to vent a roof, then do it right" hold true here? It seems that I must either insert site-built baffles (also made of polyiso) or take the attic side air barrier all the way to the roof sheathing. Leaving it as-is seems to be the worst option due to wind-washing of the fiberglass in the sloped area. Is the taking the air barrier vertically to the roof sheathing (even if air sealed well) risky in this case? (I have to deal with this issue regardless of which insulation option I go with.)

    I realize that taking out the drywall in the sloped and horizontal ceiling sections of the closet is the optimal way to address several of these issues, but I'm not (yet) ready to tackle that, mainly because I don't yet have a great plan for the existing overhead HVAC ductwork.

    4) In terms of heating the walk-in closet, we have tolerance for the temperature to always be few degrees higher or lower than the rest of the conditioned space (e.g. 60 in winter, 80+ in summer) and with that in mind, I can't come up with a more cost / energy efficient way to condition the space. The best option may be to just remove the HVAC ducting entirely (when the time is right) and leave the closet door open. I don't think that this space warrants its own minisplit heat pump ($$$) and any newly installed electric option (floor heating, space heating, etc.) would also be wasteful most of the time I think. Am I missing any options?

    Regards,

    Joe

  5. GBA Editor
    Martin Holladay | | #5

    Joe,
    Q. "Does the remaining floor joist space in the unconditioned part of the attic need to be filled with insulation?"

    A. No.

    Q. "R-19 in a 5-in stud cavity with 1 in of polyiso (in climate zone 5) would be okay-ish / bare minimum? Would you recommend the polyiso be a bit thicker?"

    A. You can get away with the 1 inch of polyiso, which is indeed (as you correctly note) a little thin, because the triangular attic behind the kneewall won't be quite as cold as the exterior air. That said, thicker rigid foam would, of course, be better. If you choose to install two layers of 1-inch rigid foam, you can do that. More info here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Q. "How important is it to get ventilation flowing through this space?"

    A. Essential.

    Q. "Does 'if your going to vent a roof, then do it right' hold true here?"

    A. Yes.

    Q. "It seems that I must either insert site-built baffles (also made of polyiso) or take the attic side air barrier all the way to the roof sheathing. Leaving it as-is seems to be the worst option due to wind-washing of the fiberglass in the sloped area."

    A. You're right.

    Q. "Is taking the air barrier vertically to the roof sheathing (even if air sealed well) risky in this case?"

    A. I'm not sure which air barrier you are talking about. If you are talking about the 1-inch thick vertical polyiso on the exterior side of your kneewall, the answer is: Yes, it's risky.

    Q. "In terms of heating the walk-in closet, ... am I missing any options?"

    A. It's hard to answer this question. Of course, it's up to you whether you want to condition this space -- you don't have to. You could heat it with an electric-resistance baseboard heater if you wanted to, and you could cool it with a window-mounted air conditioner (assuming there is an exterior wall). Or you could install a ductless minisplit in there. Or, as I said before, you could leave it unconditioned.

    -- Martin Holladay

  6. Joe Duchek | | #6

    Thanks Martin. You interpreted my questions correctly. The feedback is greatly appreciated.

    Regards,

    Joe

  7. Joe Duchek | | #7

    Hi All,

    I am making progress retrofit insulating my walk-in closet (knee wall) over garage and now have a few sanity-check questions as I move to the sloped ceiling and upper attic sections. I am in climate zone 5A.

    Thus far I have dense-packed the floor assembly (2x10 joists) with cellulose and have also put polyiso on the attic side of the vertical knee walls.

    The sloped ceiling section consists of 2x8 rafters, 24in o.c. with drywall tacked directly to those rafters, leaving me just over 7 inches of space to work with. (Martin previously suggested removing drywall, which would be the optimal solution for both furring out and adding rigid foam, but we have decided not to remove drywall at this point). Therefore I think that my best bet is to use R23 mineral wool with a 1/2 inch of rigid foam as a site-built baffle, leaving a 1.25in ventilation gap (which connects the lower attic to the upper attic). I have tested this approach on one rafter bay and it works, but is time-consuming. See attached picture.

    For the upper attic, I am considering loose-fill cellulose. I am thinking about cutting a hole in the ceiling drywall at one end of the room, working the flex tube to the opposite end of the attic and then slowly pulling the tube back as the area fills with cellulose. The ventilation baffles used for the sloped ceiling will extend through most of the rafter bay in the upper attic section to keep the cellulose from touching the roof sheathing.

    Questions:
    - Considering the limitations that I have put on myself (no significant drywall removal), are there any better approaches / modifications that I am missing?
    - Do the plans outlined above or the attached pictures elicit any concerns? (The vertical polyiso in the attached picture will be foamed / taped in due course, and the remaining exposed fiberglass will also be covered with polyiso once the sloped section is done.)

    Regards,

    Joe

  8. GBA Editor
    Martin Holladay | | #8

    Joe,
    It looks like you are doing a good job. Congratulations -- it's fussy work.

    As long as you can extend the site-built ventilation baffles so that they terminate above the top of the cellulose that you plan to blow into your attic, your plan should work. When you are blowing cellulose, keep the hose away from the ventilation chutes -- you don't want to accidentally fill the ventilation channels with cellulose.

  9. Joe Duchek | | #9

    Thanks Martin. I appreciate the confirmation and the encouragement.

    Regards,

    Joe

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