Kneewall insulation planning stage – Thoughts appreciated
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.
– 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.)
– 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.
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