Adding water shield or house wrap to chimney interior?
We are having our brick and block chimney partially rebuilt, and I was wondering about the possibility and wisdom of adding something like Tyvek house wrap or an Ice and Water shield inside the cavity.
The chimney abuts the exterior of the house as shown in the first photo.
The second photo shows the top of the chimney interior (it is partially deconstructed). There is a block wall against the exterior sheathing, which is some type of fiberboard. I’m not sure how much of a gap there is between the two. There are a couple of inches of air gap between the block wall and each of the three liners.
The fiberboard has been damaged by water over some unknown number of years. There is one place where it has worked all the way through the board into the wall just above the chimney, as shown in the third picture. Note the missing mortar between the blocks.
The current game plan is to rebuild up from the point that they have already torn down to; add a new cap, new flashing, siloxane coating, etc.
Should I have the chimney torn-down to the bottom of the sheathing so I can replace it? The hole is maybe 4″ x 4″, so I was thinking of just patching it.
Would it be unwise to spray some fire-resistant “Great Stuff” foam insulation into the hole or between the blocks where the mortar is missing?
Would it be wise and worthwhile to try to slide some Tyvek or water shield between the block wall and the sheathing? I was thinking of buying a 9′ wide roll and sliding it down until it reached the top of the fireplace opening in the wall. I wouldn’t be able to tack it down except near the top of the chimney without having it torn down farther.
If I added Tyvek, how might I tie into the flashing or roofing to keep the water from running between the Tyvek and the sheathing?
Any thoughts would be appreciated.
Replies
It's certainly wise to consider doing what you can to block air infiltration through the chimney area -- areas such as this are often a major source of air infiltration.
It's also VERY wise to consider doing what you can to head off damage in the inevitable case that water gets into the interior spaces of the chimney (and from there to the house). Such damage can be insidious, occurring over a period of decades before it's noticed.
And of course you want to exercise a degree of caution with regard to fire hazards.
In my opinion (though folks here will gladly tell you I'm no expert) you definitely want the masonry that faces the house to be tightly sealed against fire. There should be no gaps in the masonry that could "project" a finger of flame into the house structure. Fire-rated foam would probably be OK for this, though I have a small concern about how it would hold up against weather, should a water leak develop.
Tyvek is at least OK for air sealing (and better than nothing for water), if you can arrange to tape it appropriately along the edges. While Tyvek will melt readily if it gets too hot, it will not support flame, and should not represent an increased fire hazard. Of course, if you were Junkhound you'd probably have a piece of sheet metal in one of your sheds that would do the job better, but most of us are not that lucky (and have wives that are not that unlucky).
Can't Do It.
While it would seem sensible to add a weather barrier between the masonry of the chimney of the chimney and the unprotected sheathing, it isn't...and actually would be a building code violation to do so in most cases.
Why?
Because most building codes require a minimum clearance of at least 1 inch from masonry surfaces of chimneys to combustible materials and at least 12" of clearance from any flue to combustible sheathing.
Your chimney built as it is remains a fire hazard because the flue and masonry walls are too close to or are in direct contact with the combustible sheathing. In short, you already have neither required clearance. Adding flammable housewrap against the block will only make matters worse.
While the best approach would be to demolish the chimney completely and build it anew using appropriate clearances, it likely isn't practical and you should consider instead a non-combustible material other than housewrap, like sheet steel or aluminum, which can provide both weatherproofing and a non-combustible surface to reduce the chances of fire.
Thanks for your insights.
Thanks DanH and manhattan42 for the quick replies.
Given that the sheathing itself is flammable, I don't see any problem with covering the wall with flammable wall wrap (e.g. Tyvek), other than the thickness of the wrap having to be deducted from the air gap. Of course, this assumes that the wrap is tight to the wall (which I couldn't guarantee without more deconstruction). But I agree that sheet metal is probably the better solution.
I had an uncle that worked at the Simplicity Pattern company. He could get these large rolls of aluminum (as I recall) that had been used for printing the patterns. Too bad both he and the rolls are gone now.
There is some clearance between the block wall and the sheathing because I can push the sheathing toward the block wall from the inside. This gap may have been code-compliant at the time the chimney was built and could still be now, depending on the local code.
I agree that the integrity of the block wall against fire penetration is a major concern. I know that the current wall is weak where the hole is in the third picture. I doubt that there is a through hole there, though, because the water would have drained away instead of eroding the sheathing. Short of dropping a web cam or a long borescope down the shaft, I don't see how to check it
elsewhere without deconstruction. Hmm, maybe a trouble light down the center of the block...
I don't have a copy of our local code (the final arbiter), but I haven't seen anything before that says that 12" is typically required between the flue and combustables, except in the case of the chimney acting as a support for the structure (which is not the case here). For example, see the InspectAPedia article on "Masonry Chimney Fire Clearances from Combustibles" at http://www.inspectapedia.com/chimneys/Masonry_Chimney_Clearances.htm.
Most of the things I have seen show a 2" gap between the outer wall of the chimney and combustible framing and only an expansion gap between the flues and the outer walls of the chimney.
Given all this, it seems to me that the "best" thing to do would be to tear down the inner block wall (which would necessitate having the outer brick wall at least partially dismantled), replace the sheathing, cover that with sheet metal flashing, rebuild the block wall an inch or two (per code) away from the recovered house wall, then (if necessary) rebuild the flues farther out from the block wall.
I also agree that this leans toward being an impractical last resort and that it would be more practical to slip a non-combustible flashing into the air gap.
I guess I need to go find a copy of the local code...
Thanks, again.