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Insulating a timber-frame building with cellulose — moisture issues?

Taylor Burt | Posted in Energy Efficiency and Durability on

Hi all, thanks for all the great information here…I really appreciate it.

Background:
We’re in the process of building a timber frame workshop & office. Only one of the “bays” will be insulated and heated…a 12’x24′ space, with a 9′ ceiling. There will be insulation directly above the ceiling, and then storage space above that. Below the frame is a root cellar, separated by 8″ joists. Basically we are making an insulated, conditioned cube within a section of the frame.

Right now I am thinking of doing the following:

Walls:
— From inside: drywall; 2×4 strapping; Intello membrane; 2×6 studs w/ dense cellulose; pine board sheathing; 2″ Roxul board; batten & board siding (ie. battens as first layer, screwed to sheathing through Roxul)
— 2×6 studs would be flush with the outside of the timbers
— The outside boards & Roxul would extend down to the bottom of the 8×8 sill.

Ceiling:
— From inside: 4×6 decorative joists; T&G ceiling boards; air barrier, 2×10 joists w/ dp cellulose; 1x pine boards
— Air barrier membrane would continue over ceiling, with 10″ joists on top also dense-packed with cellulose

Floor:
— From inside: Finished pine flooring; 3×8 joists w/ dp cellulose; vapor retarder membrane; strapping; root cellar

My biggest concern is whether I am going to run into moisture issues at the 8×8 hemlock sill. Sill sits on both copper termite barrier & thick layer of EPDM rubber, so no issue of capillary moisture. To the inside and on top would be dense cellulose, & to the outside would be pine boards & Roxul board. Any reason I should be worried about this?

Also, I am hesitant to add an interior air barrier to the floor because of the vapor retarder on the bottom of the joists (there to prevent high humidity from root cellar from migrating upward). I assume that the vapor retarder will function as an adequate air barrier, as it is tied into the bottom of the sill….but maybe not?

Thanks again!

ps. Climate Zone 5b
Likely heating with electric baseboard, or similar.
Use will be an office with no cooking, no plumbing, occupied by 1-2 humans 4-6 hrs/day.

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Replies

  1. Taylor Burt | | #1

    Oops, last paragraph should refer to no interior air barrier in the FLOOR, the walls and ceiling will have the Intello, or similar barrier.

  2. GBA Editor
    Martin Holladay | | #2

    Taylor,
    A few comments:

    1. As described, your wall has no water-resistive barrier (WRB). Building scientists strongly recommend, and building codes require, that every wall have a WRB. In your case, you would probably want to install the WRB (for example, plastic housewrap) between your board sheathing and the Roxul insulation. For more information, see All About Water-Resistive Barriers.

    2. You are planning to build a timber frame with infill walls. This type of construction is notoriously hard to air seal -- air leaks show up between the posts (which shrink) and the infill framing. By far the best solution to this problem is to install a continuous exterior layer of rigid foam that covers the entire timber frame as well as the infill walls. This type of continuous foam layer needs to be carefully sealed at the foam seams.

    3. The biggest problem with your ceiling assembly is the relatively low R-value (R-34).

    4. I strongly recommend that you install a plywood subfloor under your pine board flooring, to act as an air barrier and to prevent dust from slipping down into the insulation layer.

    5. You didn't describe what type of material you were thinking of installing under your floor joists. You describe it as a "vapor retarder membrane." What's that? Polyethylene? A smart retarder? Asphalt felt? Something else?

    Detailing the ceiling of a root cellar is tricky. The cellar is cool and damp; the floor above is warm and somewhat humid. The best type of insulation for this type of ceiling is foil-faced polyiso, installed as a continuous layer on the underside of the floor joists.

  3. Taylor Burt | | #3

    Martin, thanks as always for the thorough response.

    1) WRB: I thought I might be able to get away with large overhangs & no WRB, but now I will likely include one...either asphalt felt, or something I could also air-seal (Pro Clima Mento?..see (2b)

    2a) The appearance of the timbers inside the building is a low priority...efficiency is a high priority. If I were to infill as described, but run the air barrier (ie. Intello) continuously over the inside of the posts, that seems like it would solve the problem, yes?

    2b) If I wanted to avoid using foam on the outside, a continuous exterior air barrier (one of Pro Clima's products, for instance) over the sheathing would work, if carefully detailed, correct? I understand this can be difficult to do correctly, but perhaps that would help air seal if the infill-style insulation poses problems.

    3) I can add deeper joists to increase the insulation in the ceiling, thanks.

    4) I was considering a plywood subfloor...but was worried by basement humidity, the joists drying, etc...would the vapor permeability of the plywood eliminate any problem with the joists not being able to dry out to the outside due to cool, moist root cellar?

    5) I was considering something like Pro Clima's DA (https://foursevenfive.com/product/da/). You suggest a vapor barrier...foil faced polyiso. I remember an article (Resilient Food Supply Systems) where you talk about your root cellar under your house...what do you have installed in the ceiling and between the joists? Has that been working for you?

    Thanks so much again...very much appreciated!

  4. GBA Editor
    Martin Holladay | | #4

    Taylor,
    Q. "WRB: I thought I might be able to get away with large overhangs & no WRB, but now I will likely include one...either asphalt felt, or something I could also air-seal (Pro Clima Mento?)"

    A. If you can afford a WRB that doubles as an air barrier, you won't regret the investment.

    Q. "The appearance of the timbers inside the building is a low priority; efficiency is a high priority. If I were to infill as described, but run the air barrier (ie. Intello) continuously over the inside of the posts, that seems like it would solve the problem, yes?"

    A. If the studs are the same depth as the posts, so that the studs are co-planar with the posts on the interior as well as the exterior, it makes it easier to install a continuous air barrier. As long as you do a careful installation job, Intello can work as an interior air barrier.

    Q. "If I wanted to avoid using foam on the outside, a continuous exterior air barrier (one of Pro Clima's products, for instance) over the sheathing would work, if carefully detailed, correct?"

    A. Yes, that's correct.

    Q. "I was considering a plywood subfloor, but was worried by basement humidity, the joists drying, etc."

    A. I think your worries are groundless. The advantages of the subfloor vastly outweigh your perceived disadvantages. If you are framing with green lumber, or you are framing under rainy conditions, it's always a good idea to allow your framing lumber to dry out before insulating. Just get your roof on and wait a while for everything to dry.

    Q. "I was considering something like Pro Clima's DA (https://foursevenfive.com/product/da/). You suggest a vapor barrier, foil faced polyiso."

    A. The advantage of foil-faced polyiso is that it performs better in a humid environment than cellulose. A vapor barrier in this location is OK, as long as the vapor barrier has R-value. I can't think of any reason why you want vapor to flow through your floor assembly. It's best if the moisture in your root cellar stays right where it is.

    Q. "I remember an article (Resilient Food Supply Systems) where you talk about your root cellar under your house. What do you have installed in the ceiling and between the joists? Has that been working for you?"

    A. My house was built by a hippie who had no regular job and no money. My house is not a model. One of these days (ha!) I'll tear apart the ceiling of my root cellar to see what's going on. I'll say this: whatever is happening down there is irrelevant to this discussion.

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