insulating my attic - two questions


Old 12-06-03, 08:28 PM
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insulating my attic - two questions

I have a 1922 house in NJ that has no insulation. I have a couple of questions about the finished attic.

First, about the ceiling joists on the ceiling of the top most floor; I dont want to blow in loose fill insulation, I want to put in batt or kraft faced between the joists. Question; does it make sense to put in such insulatin so that it sticks over the top of the joists (is deeper than the joists)?

Secondly, being sensitive to vapor and heat loss, what would be the best way to insulate this attic. I have created a webpage to illustrate. The top part of the attic will be vented in summer and winter, the horizontal walls will be insulated, Question; do I simply insulate the diagonal sections or do I vent them?

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Old 12-07-03, 04:46 AM
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First I am going to name the different sections of your attic and I would like you to lable them on your webpage so there is no misunderstanding. Starting from the bottom of the second floor going up.

The entire second floor is the ceiling for the first floor. The sections of the second floor that is not heated behind the knee wall (explained next) are the "ATTIC FLOORS".

The vertical walls that goes from the floor to the roof rafters are known as "KNEE WALLS".

The diagnol sections from the knee walls to the collar ties (explained next) are your "SLANT CEILINGS".

The horizontal wood members that connect opposing rafters are known as collar ties and when sheet rock is applied to them form the second floor "CEILING".

So the sections of the structure of the second floor that should be insulated are the ATTIC FLOORS, KNEE WALLS, SLANT CEILINGS and CEILING. The reason for this is the THERMAL BOUNDARY which by definition means the surface of a structure that separates the areas of a structure that you want to heat/cool from areas of the structure and outside that you do not want to heat/cool.

There are several problems with insulating the different sections with this type of construction and they all have to do with attic ventilation, which by the way is a misnomer. What attic ventilation does is ventilate the underside of your entire roof. The attic vents are designed to work in conjunction with each other especially the eave and the gable vents to create what is known as FREE VENTING. An example of free venting is dipping a straw into liquid and putting your finger on the end of the straw and lifting the straw out of the liquid. The liquid remains in the straw until you lift your finger from the end of the straw and all the liquid leaves the straw. The lifting of your finger illustrates free venting. Where the part of the straw dipped into the liquid represents the lower vents (eave or soffit) and the end of the straw where you put your finger represents the high vents (gable in your case). If you apply the insulation to any of the above named sections that impede free venting, you increase the probability of a moisture problem dramatically. This applies to every single piece of insulation you install.

For example, let's say you have to install 40 pieces of insulation on your slant ceiling. You allow a space between the insulation and the underside of the roof to allow for free venting. You inadvertently install one piece of insulation without this space. The result is that small section of the roof is not properly ventilated. It will reduce the life of the shingles applied over that section of the roof dramatically. In other words, the shingles over the section of the roof that have proper ventilation will last longer than the shingles over this small section that does not have proper ventilation. Also because of the inadequate ventilation for this small section the probability that moisture will form in this area increases, the probability of forming mold and mildew in this area also increases.

So the rule for installing attic insulation is fairly simple, that is, you can install all the insulation you want provided that it does not impede free venting. Remember, free venting applies to the entire underside of the roof.
Old 12-07-03, 06:11 PM
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dvarga, read this thread. The woman has the same scenario as you:

In your diagram question #2, you definately want to insulate this area but you need to maintain an air gap between the insulation and the underside of the roof. Thats where the foam baffles come into play.
Also, about insulating between your ceiling joists. You should lay kraft faced insulation (paper facing towards living space) that will be even with the joists, then you can roll a second layer of unfaced insulation perpendicular to the joists causing the top layer to criss cross sealing any gaps.
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