Celotex? Old roof underlayment

Under the very old asphalt shingles on my house is 2 layers of 1/2 inch sheet board, marked "Weyerhaeuser" "insulation sheathing board" "1B" "R1.92" and "COMBUSTIBLE MAY SMOLDER OR BURN IF IGNITED" (and indeed it does burn easily). It appears to be wood fiber composition board. It may be 30+ years old. A small patch of it is rotted and below that is good tongue and groove board, which may or may not be the very same T&G boards that make up the exposed rafter ceiling of the room below. I have no idea yet if there is one or two courses of T&G, but I do know there is electrical wiring in the ceiling so it is either sandwiched between courses of T&G or it is lying between the T&G and this sheathing board stuff.
The roofer says this sheathing board is not up to current building code and should be removed. What is this sheathing board called? The roofer calls it celotex (?) but the celotex I find on the web seems to be an entirely different product...?
    Una
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Celotex was a brand name that made siding and roofing stuff. I believe they went bankrupt long ago and someone bought their name.

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Una wrote:

Sure. Weyerhaeuser is still in business, though. Anyway, my questions are along the lines of:
What is this product called? The brand name or the general class name. What is its R value? Is it more flamable than what we might replace it with? Is the product on my roof in good condition or bad condition? Does it tend to rot? (The roof leaks, and may have done so for years.)
It is brown, except the upper surface which is black and oily. The printed text on the upper surface is brown on the black and in places still easy to read. The board feels rather soft and crumbles easily.
The roofer proposes to strip this stuff off completely, put on a 4 inch insulation, put a 1/2 inch plywood or OSB decking on top of that, then felt and shingle.
    Una
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Una wrote: ...

Sounds like good plan to me -- the celotex is probably still fine except for the places it is obviously not, but it is of marginal R value (I don't know the numbers ottomh, but it wasn't really an insulating product but an inexpensive sheathing product).
Consequently, what the roofer is proposing will probably save more in heating/cooling costs in a relatively short time than the expense.
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Unheated attics are to be insulated at the floor, unless I missed something
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ransley wrote:

Cathedral roof sounds to me...t&g showing in the interior.
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On Apr 28, 7:03pm, snipped-for-privacy@att.net (Una) wrote:

I didnt see the roofer proposing 4" of insulation, which is worthless in a vented attic
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ransley wrote:

...
Guess you didn't see the "good tongue and groove board, which may or may not be the very same T&G boards that make up the exposed rafter ceiling of the room below", either. There is no attic afaict.
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Correct, no attic. It is a low angle roof over cathedral ceiling with exposed structural members and tongue and groove planking. That room is cold in winter and hot in summer; a well insulated roof would pay for itself in just a few years. I guess my questions are mostly academic, but I also wonder if stripping off the fiber board is really necessary. I suppose it presents a technical problem if 4 inch insulation is to be put on above deck. And I would like to be sure that new insulation will make a significant difference.
    Una
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Una wrote:

If there was no insulation other than the celotex before, it will make a significant difference. A quick look didn't find data in my handbook, but I'm comfortable an inch of old celotex isn't anything remotely approaching an inch of a modern solid-foam in effectiveness.
One disadvantage I see given what you've already stated in leaving it there is you can't see the condition of the sheathing underneath before you cover it back up -- you've already mentioned it's leaked before and there is visible damage in places. I'd want to know what else is there hidden before putting the new back on.
I'm still of the opinion sounds like your roofer is doing you well--I'd go with him on this one from all I can tell from here (which, of course, isn't a lot :) ) but I think his is the right call.
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dpb wrote:

Second the recommendation to follow the roofers plan, what he suggests is very reasonable. 4" of rigid foam insulation will be a huge improvement.
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Agreed. Starting from clean, intact sheathing is the right way to go.
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From original post:

I'm guessing that the old stuff has an R value of 1.92.
The 4" foam would be much better, and keeping this old stuff really wouldn't help much.
Sounds like you've got a good roofer.
HTH, Vicki
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Una wrote:

You think? :-)
Well, the job is finished. The old inch of fiber board is gone. On the whole it was in very good condition but in some areas it was mush. The T&G under the fiber board was in very good condition throughout. Whew!
Thanks for the feedback, all!
    Una
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Celotex is a brand name for a wood fiber sheathing board. It comes in 4 foot x 8, 10, 12 foot lengths, sometimes for roofing it is available in smaller sizes. It is made in various versions, in the early 50s it had a very smooth surface and was used for wall covering before drywall was popular. I still see the tar impregnated version use as exterior wall sheathing under siding or brick veneer. It is generically known as "insulboard" and a common brand name of "ten-test" in many areas and is used in many cheaper building techniques but has been replaced by many newer products. Its original name of "insulboard" implied it added insulation value, but it was miniscule.

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