I have been wrestling with this question for quite some time. I have
a two story house with a concrete block chimney with clay tile liner.
I have a oil fired forced air furnace with an add on wood stove piped
into the same chimney. I have been heating my home with this set up
for close to 30 years so I am not a wood burning greenhorn. I have
never had a chimney fire or at least one that I was aware of. I keep
the chimney clean and burn only seasoned hardwood. My question is
that my chimney has about a 3 inch air space which goes up all the way
into the attic and adds alot of warm air into the attic which I think
causes ice to form at the eaves. Would I be making a big mistake to
insulate around the chimney where the chimney extends into the attic?
What about insulating around the whole chimney inside the attic? I
would guess the outer block temp gets pretty warm but not over 200
degrees F. Although, a chimney fire, if one did occur, could approach
or exceed that temp..... Or should I just leave well enough alone and
put up with the ice buildup at the eaves?
ps I have about a foot of fiberglass batt insulation in the attic
Unfaced fiberglass insulation will not burn. If you hold it in a direct
blue flame (like a gas stove burner) it will melt but will not catch fire.
As long a your chimney has a proper draft I see no reason why you can not
insulate any gaps. Modern construction actually requires a non-combustible
firestop around all chimneys to retard the spread of a fire.
Isn't it the other way around?
It really isn't the warm air that causes the ice to form, but the humid warm
air that will rise up through any opening and carry the moisture with it.
This moisture will for the ice.
The air rising through your ceilings, light fittings etc and loft
trapdoor carries lots of water vapour.
Each of us produces around 2.5 litres of water vapour each 24 hours.
During the night we sweat 330 ml of water keeping our temperatures
Children and animals produce more, as they tend to be more active.
Drying things on radiators, leaving bathroom and kitchen doors open
adds to the total.
All this humidity rises as wet air is lighter than dry air.
Once in the loft space it condenses onto or into the nearest cold
Hot always moves to cold.
This action creates the ice in your loft.
And of course it also makes an uninsulated roof warmer than the roof
which melts the snow on the roof makes an ice dam and floods into the
The solution is to stop the warm air rising into the loft space by
almost vapour proof barrier between your comfort zone and the outside.
This is usually a plastic waterproof sheet that covers the whole of
area throughout the home.
Insulation is a separate thing designed to keep the heat inside the
most insulation is not water vapour proof -so, does not help.
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