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Dense pack cellulose in sloped ceilings — need vent chutes?

ars777 | Posted in Energy Efficiency and Durability on

I have an 1800’s story and a half log home. There are knee walls on the 2nd story which have been insulated in a variety of different ways — but most knee walls are treated as unconditioned space.

An insulation contractor proposed bringing the knee wall floors and the upper attic floors up to R50 by adding more blown in cellulose.

He proposed stuffing the rafter bays in the knee walls and dense packing the sloped bedroom ceilings (connecting the knee wall to the upper attic) with dense pack cellulose.

I expressed some concern because it was my understanding that air entered the soffit vents at the base of the knee wall, flowed through the knee wall, up into the upper attic, and out the ridge vent.

When I mentioned this, he changed his proposal to include installing vent chutes before dense packing the sloped ceilings.

My questions are:

1. Is it better to have the vent chutes installed or was his original proposal better?

2. Would each rafter bay need a vent chute? or just enough to get air flow from the knee wall up into the upper attic? i.e. is the primary need to connect the knee wall air flow to the upper attic or is it important that none of the rafters have dense pack cellulose without ventilation?

3. In a somewhat leaky home, is there concern regarding blowing in additional cellulose — over old material that has signs of rodent tunnels, etc. We have tried to address the rodent activity since we purchased the home and I am not aware of an active issue, although I am sure there may always be activity. i.e. how or when do you determine if you need to vacuum out and start new?

4. In an ideal world, is it better to make each knee wall part of the conditioned space? Or would one only do this if intending to use the knee wall space for bookshelves, closet space, etc. If so, what is the optimum way to insulate the outside wall and the underside of the roof in the knee wall area as well as the sloped ceiling? And do you then need to introduce intake venting for the upper attic?

We are likely to re-roof next year (I have a few questions on that as well) and I am wondering if we should take these interim steps now or if we should address when the roof comes off (a few knee walls have already been converted to living space and there is not much access to improve insulation in those areas).

Thanks much for any thoughts or insights.


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  1. GBA Editor
    Martin Holladay | | #1

    Many questions.


    1. The best approach is to make sure that the triangular attics behind the kneewalls are inside the home's conditioned space. More info here: “Two Ways to Insulate Attic Kneewalls.”

    2. If you decide to install cellulose insulation between your rafters, you need a sturdy ventilation baffle in each rafter bay. More info here: How to Build an Insulated Cathedral Ceiling.

  2. charlie_sullivan | | #2

    " i.e. is the primary need to connect the knee wall air flow to the upper attic or is it important that none of the rafters have dense pack cellulose without ventilation?"

    It's to make sure that all of the roof deck is vented, so it can't accumulate moisture.

    As far as starting over vs. adding cellulose, I don't think that other debris in the cellulose degrades its performance significantly, but if there are ways that rodents are getting, or have gotten up there from below, there must be holes big enough for them, which are also big enough for air leaks. It's easier to seal the air leaks in the attic floor before you add cellulose than after.

  3. ars777 | | #3

    Thank you for the responses. As mentioned in Martin's article that he linked to, I think it will be easier / more effective to bring the kneewalls into the conditioned envelope. Not an easy task, but with the multitude of ways kneewalls have been handled thus far, I think it will be very difficult to properly air seal them in the present state. Thank you again.

  4. ars777 | | #4

    One other issue: The outside wall of the kneewall is the original early 1800's logs w/ more modern chinking. Is there an issue spray foaming on log?

    The home has wood siding on the outside -- over the logs with exposed log on the inside. I am not certain what, if anything, is between the siding and the logs. Is there a concern of moisture getting trapped between the log and the spray foam on the inside? Or are you confident the log wall and ceiling in the knee wall can be spray foamed?

    Thank you.

  5. GBA Editor
    Martin Holladay | | #5

    In most cases, installing spray foam on the interior side of the log walls will not cause moisture problems. The spray foam will add an air barrier, which is good, and the log wall should be able to dry to the exterior (unless there are strange layers of inappropriate material between the log wall and the siding).

  6. Dana1 | | #6

    If you are going to re-roof with insulation above the roof deck, and plan to install the requisite amount of above-deck R-value relative to the total based on IRC Chapters 8 & 11 prescriptives, it's safe enough to dense-pack cellulose below the roof deck now. It's even safer if you also install an interior side smart vapor retarder such a MemBrain or Intello Plus. The warmer the climate, the less risk, and the less exterior R you need for the long haul.

    As a fraction of the total it takes, by climate zone, it takes the following minimums to be above the roof deck:

    Zones 1-3: R5 out of R38 total, which is about 13%

    Zone 4C (Pacific northwest: R10 out of R49: 20%

    Zone 4A/B: R15 out of R49: ~30%

    Zone 5: R20 out of R49: ~40%

    Zone 6: R25 out or R49: ~50%

    Zone 7: R30 out of R49: ~60%

    Zone 8: R35 out of R49: ~70%



    These are minimums, an designed to keep the average temp at the roof deck in winter above the average dew point of the interior air, which minimizes the amount of moisture that the roof deck can accumulate. If these ratios are maintained, the interior side can be left fairly vapor open (standard latex paint is good) which allows the roof deck to dry quickly toward the interior.

    It's the RATIO, not the absolute R-value that determines the average temp at the roof deck, so if the R value under the roof deck is higher than what it would have to be to add up the prescriptive total, the above-deck R must grow proportionally to keep the roof deck warm enough. Conversely,if the total R is going to be sub-code, you can safely cheat the above-deck R as long as the minimum R-ratio is met.

    If you're not going to put insulation above the roof deck when you re-roof, you can use close-cell foam applied directly to the underside of the roof deck at the same ratio of foam/total R with no risk of wet insulation at the foam/fiber interface, and the roof deck is protected by the vapor retardency of the closed cell foam. Going with 2" (R12) closed cell foam is sufficient protection for the roof deck, but going much beyond that can create a moisture trap.

    Even though it's a grey area in the code to go there, if you use a smart vapor retarder on the interior with only 2" of closed cell under the roof deck with the rest as dense-packed cellulose it's still pretty safe, even if the R-ratio is significantly below spec. Building Science Corp ran a series of WUFI simulations using only latex paint as the interior vapor retarder indicating that it's even with dense packed fiberglass (which buffers far less moisture than cellulose can handle), even if only using latex paint as the interior vapor retarder, and a smart vapor retarder would give it even more margin. The discussion of the BSC simulations can be found in BA-1001, which can be downloaded by clicking the link in the lower right corner on this page:

    See the tableson page 12, the column titled: 2" ccSPF + spray fiberglass". Even in International Falls MN/ climate zone 7 the 2" foam is protective with an asphalt shingle, marginal only if a light colored "cool roof" metal roofing is used. R12 out of R49 is only about 25%, which is WELL below the prescriptive 60% required under the IRC prescriptives

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