Insulating exterior of masonry chimney in attic *Update*

[pgmr]

Since installing our stove in the centrally located, masonry chimney, we've had some problems with ice damming (mostly on the North side of the house). We have 10-12" of blown in cellulose and liner is insulated with perlite.

I took a quick reading of the chimney block temperature up in the attic this morning: It was ~ 50°F while outside air temp was 28°F. I speculate that it is warm both from the stove heating the brick/block below and some heat from the liner. Am contemplating insulating the outside of the chimney to limit the amount of heat escaping into the attic space.

Any recommendations on insulating the exterior block of the chimney up in the attic space? Glue on foam panels? Wrap in unfaced fiberglass?

Thanks.

 

[webby3650]

I wouldn't reccomend putting anything cumbostible on the chimney, If the exterior of your chimney is only 50 degrees it seems like the system is operating just fine.

 

[pen]

If anything, I'd wrap it in unfaced fiberglass. I wouldn't use glue or foam for the potential fire risks.

pen

 

[firefighterjake]

I wouldn't reccomend putting anything cumbostible on the chimney, If the exterior of your chimney is only 50 degrees it seems like the system is operating just fine.

Agreed.

 

[LLigetfa]

If anything, I'd wrap it in unfaced fiberglass. I wouldn't use glue or foam for the potential fire risks.

pen

+1
Also, don't stuff any insulation into gaps between the brick and the wood framing.

 

[Backwoods Savage]

22 degree temperature difference! Why worry?

 

[pgmr]

Thanks for the replies, all.

I'm not really that concerned about the flammability of materials on the outside of the chimney block. If the temperature ever gets that high outside of the chimney block, there are plenty of other things up there that would be just as happy to ignite. But, there's no point in adding anything else that can contribute to a conflagration, so I'll probably just use the unfaced fiberglass.

Even though the temperature differential was only ~22°F, that is a large mass that is continually radiating heat into a space that should not have extra heat being introduced. Granted, the sun does a fine job of heating the attic when it is not snow covered. However, when there is snow cover (like now), if enough heat escapes into the attic, it can easily raise the roof temperature above the freezing point (especially on days where the ambient is just below freezing), causing ice dams at the eaves.

If insulating the chimney does not cure this problem, then I'll need to look into adding additional insulation to the attic space itself.

BTW, there is continuous ridge vent and plenty of soffit venting, so it is most likely not a ventilation issue (though snow cover can block the ridge vents, I suppose).

 

[pgmr]

Thought I would post some observations after insulating the chimney. We've had a couple snows since I wrapped the chimney with R-25 unfaced fiberglass. Over the course of a few days after the snow, there was some melting around the chimney itself (maybe out 16-18"), but the rest of the roof remained snow covered. There was some ice on the top of the solid gutter covers, but not nearly as thick as it had been and the number and size of icicles was greatly reduced.

While this is purely anecdotal evidence, it does appear that reducing the amount of heat from the chimney into the attic space has greatly reduced the size of the melt area and consequently, the amount of ice at the eaves.

I overlapped the fiberglass 8-10", then pinned the end in place with a piece of stiff steel wire. Then I wrapped 3" wide plastic non-adhesive tape around the insulation to help hold it in place.

If you are having similar problems at your own home, I recommend giving it a try.

 

[Renovation]

Thanks for reporting back, Pgmr. That's very thoughtful of you.