Safe for wood to touch chimney?

Old 10-28-09, 06:20 AM
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Safe for wood to touch chimney?

Dumb question probably...but here goes.

I have a steam boiler fuled by oil. There is a concrete block chimney going up from the basement to the roof (one story ranch). There is a gap around the chimney where it passes through the floor and also when it passes through the ceiling. Since the attic is open to the outside (through vents) it also makes my basement effectivly open to the outside.

The basement gets very humid and I want to run a dehumidifyer down there, but obviously I can't dehumidify the world ...

Is it okay to close the gap with wood, or what should I use? I don't think the hot air going up the chimney is hot enough for the flash point of wood but I figure its best to ask and make sure.
Old 10-28-09, 07:04 AM
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Although many older homes were built tight against the chimney, wood should be held back, I believe it is, 2". To close your gap, use some metal flashing and seal it with fire rated foam caulk. It is important to seal both top and bottom. This gap is referred to as the chimney chase and has been costing you some $$ over the years. So, good idea, seal it up.

While you are at it and have some extra foam, locate and seal all vent pipes and electrical holes. Check any recessed lights, they should be air sealed and IC (insulation contact) rated. You can also find a substantial listing of "thermal bypasses" that the energy auditors look for when recommending efficiency upgrades.

You also mention excess humidity in the basement. In addition to the dehumidifier, try to locate the source of the moisture and correct the problem there. Any spaces with a dirt floor are noted as having excess moisture and should be covered with a vapor barrier.

Hope this helps
Old 12-10-09, 11:12 PM
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Hi Bud,

I have this same situation. Can you explain in more detail how exactly to seal the opening with the flashing and foam caulk?

And what type of flashing should I buy?
Old 12-11-09, 04:25 AM
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Originally Posted by Bud9051 View Post
Although many older homes were built tight against the chimney, wood should be held back, I believe it is, 2".Bud
The 2" rule is from IRC R1001.15 Chimney clearances. Any portion of a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a [minimum air space clearance to combustibles of 2 inches (51 mm). Chimneys located entirely outside the exterior walls of the building, including chimneys that pass through the soffit or cornice, shall have a minimum air space clearance of 1 inch (25 mm). The air space shall not be filled, except to provide fire blocking in accordance with Section R1001.16.

Which leads us to:

R1001.16 Chimney fireblocking. All spaces between chimneys and floors and ceilings through which chimneys pass shall be fireblocked with noncombustible material securely fastened in place. The fireblocking of spaces between chimneys and wood joists, beams or headers shall be to a depth of 1 inch (25.4 mm) and shall only be placed on strips of metal or metal lath laid across the spaces between combustible material and the chimney.
Old 12-14-09, 01:02 PM
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Thanks ... it has been awhile and I still have not gotten around to this. Now that winter is close I probably should stop putting it off.

This metal (flashing), I assume it goes on the wood joists? Or both the joists and the brick? How can you secure it, since there obviously isn't enough room to get a hammer/drill in there?

First time home buyer, so clueless on these kinds of things :S

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