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Question DetailsAsked on 1/8/2012

why can't we put additional insulation in the rafters of the attic - along the ceiling as well as on the floor.

I have been told that I can't do this that the air flow will be hindered. If I leave the vent in the middle of the roof open and the soffet what is the reason that insulation can not be put on the roof of the attic without drywall?

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3 Answers


You are correct that the soffits and ridge vents must remain open to allow air to flow in your attic space.

I'm not an insulation expert, but I'll try to explain it as best I can:

The main reason is mold.

The insulation itself also allows air to flow. Insulation doesn't stop air flow (per se), as much as it stops the transfer of moisture. Hot air has moisture, cold air has less. (We know air takes the path of least resistance, but let's pretend the surface of insulation is perfect, with even air pressure across).

Batt insulation is rated for the amout of resistance (R) the insulation gives to this transfer. As the hot air passes thru, the moisture is stopped by layers of insulation, this moisture simply evaporates back out from the way it came or hotter side. The air, when it finally reaches the other side of the insulation, has less moisture, so it has less heat. (It also has less force to push through, since it is closer to the temperature on the other side, or more balanced). This transfer goes back and forth all year long as the outside air tries to balance with the inside air.

If you add another layer of insulation to your existing insulation, you stand a chance that the moisture on the first layer cannot evaporate back out (it hits the other layer of insulation and is trapped). So in times of extreme heat/cold differences (dead of winter, height of summer) moisture will make it further through the insulation. But during the rest of the year, the difference isn't enough to push the moisture back out.

This moisture will set somewhere between the two layers of insulation, and mold will form.

This is also why you should never compress insulation; the rating is based upon the layers and amount of air space between those layers. Pressing an R31 insulation down into a space reduces its rating significantly; it is better to get a lower R value that isn't pressed down. (The higher the R value, the thicker the batt. typically.)

Replacing your existing insulation with a higher rated insulation is a better way to improve your insulation value. Keeping your attic area well ventilated (to prevent extreme temperature differences) is another way. Also, batt insulation comes with foil or paper, or without. This paper goes toward the heated side as the last stop between moisture and the area. If you put paper on top of exisitng insulation, you are definately going to get moisture trapped at the paper, and the paper will mold. In your attic, the paper should be touching the ceiling of the lower floor. It is often installed upside down in attics. . .

Again, I'm not an insulation expert, but that is how I understand the situation. Hope that helps and good luck!

Answered 8 years ago by Kenny Johnson


I am a Keystone Help Trainedweatherization contractor. The information provided by Kenny is very goodand accurate. To address your personal situation I would have to know howmuch and what kind of insulation is present. Are there baffles present? Baffles allow air to flow from the soffit to the ridge vent against the sheathingof the roof. This airflow is considered important because if theinsulation is in contact with the under-side of the roof deck the shingle warrantywill probably be voided.

If your attic is well air sealed, you should be able to add to the insulationbecause there will be very little moisture entering the attic space. The other consideration is the amount of ventilationpresent. The formula for calculating area of ventalation needed uses attic area and is as follows:

Have you had any problems with moisture build up in theattic?

Answered 8 years ago by Cost Rite


Coming to this question late, but might be able to shed a bit of light on the subject for future readers with similar question:

1) insulating the underside of the roof risks trapping moisture in the insulation. This can be from condensation of attic air moisture (from the house or from outside air) which then condenses to droplets or frost when it gets into the insulation far enough to hit the freezing front (under a cold roof), or from moisture trying to evaporate to the attic from wet sheathing or from vapor leakage through the roof underlayment, both of which would normally evaporate or sumlimate (convert directly to water vapor from ice) off the underside of the sheathing and be carried away by the air moving from eave to ridge vent. The insulation prevents this water vapor removal, commonly resulting in wet insulation and a situation equivalent to leaving a drape or curtain in close contact over a very cold window - it will get wet, then freeze at the glass surface, causing rot.

2) in hot conditions, the attic ventilation on the underside of the sheathing significantly cools the roof. Insulating that surface can drive the shingle and sheathing and underlayment termperature up as much as 40-50 degrees on hot sunny still days, which is way above design tempeature for the roofing system and wood. Basically like putting the roofing system in a steamer cooker because the heat and any moisture is trapped between the hot roof surface and the insulation. I have seen houses with 6-8 inches of insulation on the underside of the sheathing actually running liquid tar from the bottom layer of shingles and tarpaper because the roof got so hot in the summer even in moderate climates- and the underlayment becomes extremely brittle and degraded in this type of heat, rendering it useless. The wood fibers also degrade, significantly reducing the load carrying capacity of the roof.

3) in that position in a ventilated attic, while it would somewhat reduce heat radiating to the house in the summer (at the cost of roof heat per item 2 above), it would be worthless in winter, because you have cold exterior air moving between it and the attic floor so it would be seeing cold tempeatures on both surfaces - so it would effectively not be insulating at all. To do any good in winter against heat loss from the house through the upstairs ceiling to the colder attic, the insulation has to be on the attic floor.

4) Ditto to absorbing heat from the hot outside air passing into the attic from the eaves in summer, continually passing hot air (which then heats up more from roof heat) over the cooler attic floor, transmitting heat into the air conditioned house. Insulation on the attic floor retards tht heat flow, under the sheathing it would have no effect on that heat source.

Answered 6 years ago by LCD

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