attic (partially finished) insulation


Old 01-17-04, 05:51 PM
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attic (partially finished) insulation

We have an old (1900-1920) house. The attic space is partially finished. There are areas along the west and north sides and at the southeast corner between the finished portion and the roof that are unfinished. There is access to these areas through short doors in the kneewall. There is an original wood floor that a previous owner drilled, filled with blown in insulation and sealed the drill holes with corks. There is fibreglass insulation on the outer side of the kneewalls and on top of the finished ceiling between the rafters. There is no insulation between the rafters in the unfinished portions. In the area where the floor ends and the roof extends out past the house walls (where the soffit is on the outside) there appears to be some type of mica based insulation covered with fibreglass batting. A heating duct (which is the heat source for the finished portion of the attic) runs around the southeast corner (in the unfinished portion) above the floor where the roof and floor meet. It is wrapped in fibreglass insulation and covered with cardboard boxes. This past summer the roof was reshingled and there are now a total of 7 roof vents installed at various points on the roof.

The attic is comfortably warm in the winter but tends to be quite hot in the summer even though we have central air conditioning.

Currently there are ice dams developing at the southeast corner. (Likely due to the heat duct melting snow?).

Any ideas or suggestions regarding the heat duct and how to better insulate it without having to go to great expense would be appreciated.

Also, why is there insulation in the soffit area? Isn't the soffit vented to allow air to circulate into the attic and out the roof vents? Should this insulation be removed?

Thanks in advance,
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Old 01-18-04, 08:25 AM
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One possible solution

You may want to wrap your heating duct in some reflective material check out they make a product that seems well suited to your needs here. I have never done it but I am guessing the you would want to pull off the current insulation, wrap the duct with the proper air space required by the product and then if you so desire, put all the fiberglass wrap back on that you are using now.

I don't know where you can get this in Canada, but it is carried at our local Lowes store. may be able to direct you better.
Old 01-18-04, 01:02 PM
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Sealing and insulating ductwork is something I would strongly recommend, however, I would do so for different reasons which would not address your present querries. I would also strongly recommend you clear your soffit area to allow for free venting, which in my opinion is a contributing factor to the ice damming problem, but is not the source of the problem.

Everyone understands the old saying "A chain is only as strong as its weakest link". This is universally accepted and applied. If we apply this to your situation concerning the thermal boundary (Insulation applied to areas that separate a conditioned space from an unconditioned space) the weak links are the access doors you have in your knee walls.

There is a simple test to determine if this is true. Take a lighted cigarette or a blown out match that is still smoking and run it around the perimeter of the attic access door. If the smoke is moved by air currents either coming from or going to the attic access, it is without question the primary source of the ice damming problem, in my opinion.

The solution here is relatively simple and inexpensive, but the benefits are enormous. Install some foam weather-stripping around the perimeter of the jamb of the access door. Install window latches on the sides to close the access door tight and glue some rigid board insulation to the back of the access door. This will reduce the probability of ice damming occurring dramatically, which in turns reduces the probability of roof damage and water penetration associated with ice damming. On top of that, it will reduce your energy cost by a minimum of 5%.

As far as you air conditioning in this same area of the house. There are different forms of heat transfer, namely conductive, convective and radiant. Insulation prohibits heat transfer by addressing conduction. Or if you prefer, it prohibits heat from the house escaping during the winter and heat from the outside from entering the house during the summer. Since conduction deals with how heat travels through a material, the material used to prohibit conduction has a characteristic to retain the heat attempting to traverse it. This applies during the heating and cooling seasons. Your comfort level in this area during the winter is a direct result of the insulation because the dominant heat transfer mechanism during the winter is conduction, which insulation specifically addesses. It does not mean that convective and radiant heat transfer does not exists during the winter in your home. It does mean that they are not dominant.

In the summer the dominant heat transfer machanism is radiant. conductive heat transfer during the summer does play a very important role, however, it is not dominant, radiant is. The hotter an object gets, the more heat the object will radiate and the closer you are to this object, the more heat you will feel from this object. For example, a pot is boiling water on a stove, the closer you put your hand towards the side of the pot, the more heat you sense. Mass also plays a major factor as with the pot, your hand has to get within a few inches of the pot before you sense the heat, whereas, if the oven was on, you would sense it within a few feet.

To understand why your cooling does not work well in this area is to merely apply the aforementioned. Since insulation retains heat, the amount insulation involved, the proximity of the insulation to this area and the dominant heat transfer mechanism being radiant during the summer, are by far the reasons why your cooling does not work well in this area. Understanding the source of the problem allows you to properly address the problem.

For example, you can reduce the amount of heat retained by the insulation by induced convective heat transfer. What this means that air at a lower temperature that passes over an object at a higher temperature will extract heat from the warmer object. That is exactly what an attic fan does. It will reduce the cooling load needed for this area which will increase your comfort in this area and lower your energy bills. What actually works better than this but in most cases not possible is having trees shade your roof during the summer. But with that in mind, light colored roofing shingles have the same effect but not as great as tree shading.

There are other solutions that can increase your comfort cooling in this area. The most common people opt for is the adjusting of the dampers in your ductwork to compensate for the additional heat gains created by the radiant heat transfer. This in most cases will increase your cooling costs and unfortunately this is what most HVAC people recommend.
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