I had some problems last winter with ice forming on the eaves of a section of my homes roof. Conditions were optimal. This section of roof was distinct from the rest of the homes roofs in that the chimney from our masonry furnace travelled thru the attic space. It is a 4/12 pitch roof. It is a chimney composed of 12" clay liner encompassed by 17"x 27"(?) chimney block. The other sections of roof in the house performed fine. All clearances from combustibles meet code. We have a composite roof. Of course, my evaluation is that the design of a trussed, cold, properly insulated roof inherently protects the eaves from ice damming by keeping the temperature differential between the eave and the outside minimal so that the melting and freezing cycle doesn't create the formation of ice. Now let me just mention that we didn't sustain actual ice damning or any necessary damage, things are to code, but just too much icycle formation and some nuisance ice up a few inches above the gutters (but pretty thick). Now, my conclusion is that the heat from the chimney rising thru the attic, heat this space and defeats the roof from 'staying cold', thus creating the heating/thawing cycle at the eaves etc. I cannott find this issue addressed in any of my construction books (of which I have quite a few) and any web searches reveal nothing. Can anyone tell me the best way to insulate the chimney section going thru the attic to keep the heat from reaching the eaves? I was thinking about building a box of rigid insulation maybe 4" thick or so, with adequate air space. Is some kind of venting necessary? Will I violate any codes. Is there somewhere I can be directed to see the proper tecnique? I'd be willing to buy a book if I knew it would address the issue precisely. I live in north Idaho.

Hi,

Your house roof chimeny blocked due to cold or what exactly, it is better i you clear something more.

Thanks Angel!

whatever you decide, be sure to maintain a 2" airspace from all sides of the chimney. Perhaps you could construct a 'chase' around the chimney with plywood and rigid foam. Just keep at least two inches from the chimney. build it all the way to the roof deck. my .02$

Mark

Building a full chase around the chimney basement to roof often results in higher air infiltration rates. Blocking the chase with metal (sealed with the appropriate high-temp caulk) throttles that back, and works as fire-blocking as well.

Otherwise, a non-flammable material (galvanized steel or similar) forming the chimney-side wall of the chase with a minimal air gap is usually allowed. In most building codes, clay lined masonry chimneys allow minimum of 1" air gap between masonry chimneys & combustible materials as long as the interior of the clay liner is a full 12" or more from the outside of the masonry, otherwise it's a 2" min to combustibles. Exposed flooring/sheathing/trim/siding etc. are allowed to abut the masonry as long as it's within the 12" to interior liner rule.

You might be allowed to build a chase around it with blown rock wool in direct contact with the chimney. In that case, go hog wild- make it 3-4" thick and get R10-12 out of it. If that doesn't fix it, you should be able to apply ISO or XPS on the outside of the rock-wool insulated chase.

Thanks, Dana. That was good help.