Batt insulation w/ cedar shingles

[TimNY]

Arch. specified fiberglass batt insulation to be installed under existing cedar shingles applied over lath (vaulted ceiling).

Never seen this done before; I think it's a bad idea due to the moisture that will come through the shingles.

However, i looked in ch 11 and also at the manufacturer's instructions and don't see anything to prohibit it.

Some manufacturer's have 20 pages of install instructions, other have 4 pages. None of which really addresses moisture. One manufacturer did mention a mold possibility with damp locations (not the manufacturer of the insulation specified in this instance)

Is there any standard I should be looking at?

Anybody seen this before?

 

[steveray]

As long as they have the air space...it should be fine....if your roof leaks...it doesn't matter what it is made out of...your insulation is going to get wet..IMO

 

[MarkRandall]

Not sure what your concern is. It is installed in the framing cavity with air space and properly vented, correct? If so, it is fine.

 

[TimNY]

Thanks for the replies.

Perhaps my concern is unwarranted. As I said, I don't see anything to prohibit it.

General consensus seems to be it is no problem. My concern is the moisture from the shingles permeating the insulation and reducing R-factor or worst case causing a mold issue.

Underside of shingles are definitely damp after a rain; with 100% humidity all of the ventilation in the world won't reduce moisture until the sun comes out.

Again, thanks for the help. Something I hadn't seen before so I wanted to get some feedback.

 

[mtlogcabin]

My concern is the moisture from the shingles permeating the insulation and reducing R-factor or worst case causing a mold issue.Underside of shingles are definitely damp after a rain; with 100% humidity all of the ventilation in the world won't reduce moisture until the sun comes out.

There should be an underlayment installed which would prevent the moisture from getting to the insulation

 

[TimNY]

mtl that's what I thought. That's what I would do, but I see nothing in the code that requires it.

 

[mtlogcabin]

2009 IRC

R905.7 Wood shingles.

The installation of wood shingles shall comply with the provisions of this section.

R905.7.1 Deck requirements.

Wood shingles shall be installed on solid or spaced sheathing. Where spaced sheathing is used, sheathing boards shall not be less than 1-inch by 4-inch (25.4 mm by 102 mm) nominal dimensions and shall be spaced on centers equal to the weather exposure to coincide with the placement of fasteners.

R905.7.1.1 Solid sheathing required.

In areas where the average daily temperature in January is 25°F (-4°C) or less, solid sheathing is required on that portion of the roof requiring the application of an ice barrier.

R905.7.2 Deck slope.

Wood shingles shall be installed on slopes of three units vertical in 12 units horizontal (25-percent slope) or greater.

R905.7.3 Underlayment.

Underlayment shall comply with ASTM D 226, Type I or ASTM D 4869, Type I or II.



R905.7.3.1 Ice barrier.

In areas where there has been a history of ice forming along the eaves causing a backup of water as designated in Table R301.2(1), an ice barrier that consists of at least two layers of underlayment cemented together or a self-adhering polymer modified bitumen sheet shall be used in lieu of normal underlayment and extend from the lowest edges of all roof surfaces to a point at least 24 inches (610 mm) inside the exterior wall line of the building.

Exception: Detached accessory structures that contain no conditioned floor area .

 

[TimNY]

Thanks mtl.

I am going to have to research that tomorrow in the office.

I interp it to mean felt paper is required over solid sheathing, not spaced sheathing. Although I may be wrong, I have just never seen it on spaced sheathing.

It would certainly solve the problem.

 

[Jobsaver]

Underside of shingles are definitely damp after a rain; with 100% humidity all of the ventilation in the world won't reduce moisture until the sun comes out.

The underside of solid sheathing gets slick with condensation here in the south. I do not see the difference . . . just make sure you have the ventilation so the moisture will evacuate when the sun comes out!