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Old 08-05-2010, 11:07 AM   #1
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Attic furnace and insulation


Hello,

We are renovating a new place, the top two floors and attic (no basement) of a very old brick row house with mansard roof. We know there were previous problem with ice damming, almost certainly due to the furnace in the attic.

What has been recommended is spraying icynene to insulate the roof and leave the ceiling between the second floor and attic uninsulated to allow air circulation into the attic. I'm worried about the attic being warm/moist in the winter...I am hearing different things from different people and wanted to see if there was consensus on a good solution.

Thanks!

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Old 08-05-2010, 12:11 PM   #2
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Attic furnace and insulation


Where do you live? That will get you the best answer from someone familiar with your climate and house construction.

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Old 08-05-2010, 12:41 PM   #3
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Attic furnace and insulation


Massachusetts. Brick house, 1880s, mansard w/slate tile.
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Old 08-06-2010, 09:09 AM   #4
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Attic furnace and insulation


You are correct. The heat from the furnace, warms the roof, melts the snow, the roof overhang remains frozen, creating the dam wall, the warm water rises up the roof under the higher snow/ice and into the home. Not good.

Spraying foam onto the under side of the roof is a good idea from the point of insulating the roof and preventing the heat from melting the snow.

It is not so good when you have a leak in the roof, that will go un-noticed for years, until the rotten wood collapses on your head.
Even when you notice you have a leak, stripping the underneath of the roof to find the leak is a horrible job and will take a long time, the point where the water comes out, may be a long way from where it gets in.

A better idea is to move the furnace from the attic, or
Insulate the roof, with sheets of polystyrene/blueboard etc; pushed as a tight fit between the rafters, carefully leaving a gap between the insulation and the roof to let and leaks show themselves.

Leaving the second floor uninsulated will cause a problem.

The moisture created within the home, by washing, bathing, cooking, breathing, rises into the roof, finds those cold rafters, creates mold and in time wood rot.

The solution to this is, pull down the ceiling, fill all the spaces with closed cell tight fitting insulation board, then fit a water vapour proof membrane under the joists, taking care to leave no holes or gaps for the water vapour/moisture to rise through, then fit you new ceiling.
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Old 08-06-2010, 01:41 PM   #5
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Attic furnace and insulation


thanks perry... having both the roof and attic floor insulated presents a new problem: lack of an exterior combustion intake to the furnace. if the attic is sealed up tight on top and bottom we'll have a problem.

also, if the attic floor is uninsulated and the roof is tightly foam-insulated, the attic shouldn't be much colder than the rest of the house...so is condensation really going to be a problem?

ceilings on the second floor are currently down anyway, so now is the time to do something...
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Old 08-06-2010, 08:09 PM   #6
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Attic furnace and insulation


You will get ice damming even without the furnace in the attic when the sun comes out and warms the roof.

If you're concerned about the furnace heating the roof, then do not turn the furnace on when there's snow on the roof.

Instead, use space heaters. Of course, ensure that your attic is well insulated. Up North, you probably need R-59.
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Old 08-07-2010, 07:18 AM   #7
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Attic furnace and insulation


Good point, have to say that, a lot of modern furnaces are balanced flue.
You could consider putting the furnace in a box, with a dedicated air supply from the outside using a six inch pipe.

This will ensure there is no air change in the attic and that the furnace burns cold air, rather than burning air you heated at great cost.

The remaining problem is, each of the rafters is in direct contact with both the inside warm air and the outside cold air, a lot of your heat is lost to the outside by conduction through the fabric of the home. This needs to be addressed. This applies to the roof and every part of the home that is attached to the roof, walls and floor.

Then you need to consider the cost of heating the attic space, which is probably not used to live in? If you do not live in it, is there any point in keeping it warmer than the rest of your home = heat rises.
Keeping the heat downstairs saves money and leads to warm feet.

Just as warm air rises so does water vapour, think equator, the sun heats the sea, water vapour rises, moves inland causing storms etc, as it moves towards the North Pole.

Water vapour/moisture automatically moves to cold, it will move into your walls, roof, floor trying to find a cold spot to condense or a crack to escape to the cold outside.

In our effort to save power and heating and to enjoy a comfortable life, without mold or wood rot expense, we are moving towards a type of Scandinavian home, where each room is in effect a sealed plastic box, finished with wood or drywall etc. Ensuring that all holes, cracks are blocked and there is no infiltration of cold wet air into the walls, floors or ceilings.

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