Attic Insulation and vapor barrier northern IL

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Old 02-15-15, 07:38 PM
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Attic Insulation and vapor barrier northern IL

I am planning to re-insulate and drywall the attic floor (2nd floor ceiling) this year, do to sagging ceilings and a mouse infestation that has been resolved. I have a lot of 2 inch extruded polystyrene left over from doing our basement. Is it possible to use it in between the attic trusses? I was planning to cut it for a tight fit, and spray foam perimeter and cracks for air sealing. I would then put unfaced batts on top for fire protection. On top of that I would add 8-12 inches of blown insulation. Would this be an effective fire barrier for the xps? Would I be able to put the xps on top of the poly and not cause a double vapor barrier due to them abutting each other?
 
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Old 02-15-15, 08:22 PM
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If you have or are going to remove the existing drywall and insulation you will be able to review the back of the roof deck for any signs of condensation damage. If you don't see any evidence of that then I would question the need to add a poly vapor retarder in addition to the foam which will already be acting in that capacity. IMHO, having two items acting as vapor retarders on the warm side of the assembly is not necessarily problematic since the dew point temperature should never occur at or between the retarders but it is more an issue of why have two.

What is/was the original insulation in terms of the material and R-value? What are you going to add above the foam, why add fiberglass batts and blown material? I would just add one type of material, preferably cellulose above the foam. It will give good thermal performance as well as fire protection. The drywall below the foam will protect it from the interior in the event of fire.

Do you have enough Xps to do the whole ceiling just from basement leftovers?

Do you have any idea what the relative humidity in your house is before you get too crazy about vapor retarders. If you have high humidity and you tighten up the ceiling, you may cause issues on windows or wherever the next coldest surface is.
 
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Old 02-16-15, 01:15 AM
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Just to make sure we are on the same page it is the attic floor we are talking about. The roof deck is just fine. My wife uses a space heater in the winter for our 8 month old ( 76 degrees). I believe the extra heat is defusing through the drywall where the orginal vapor barrier has a hole thus causing condensation on the drywall. I have read a lot of info on the vapor barrier subject however; In the double barrier concept the examples have been poly or xps on both sides of the wall cavities not double stacked on one side. I would like to do the whole attic with two inches of xps for the air sealing. The poly may not be necessary but would there be thermal bridging (not sure if that’s the right term) between the studs and the attic (studs meaning the bottom of the truss assembly that you attach the drywall to)? I want to avoid future condensation.

The original insulation is blow cellulose 12-16 inchs thick mix with a lot of mouse poop. I thought if I added batts on top of the xps it would finish out the truss cavity giving me more spark resistance for the xps buried underneath. I have about 40 sheets of 2 inch xps. This should be able to do the majority of my attic space. My reasoning behind the xps is I want to drop the ceiling in one room at a time. If I have the xps installed and sealed to the attic trusses and not the drywall beneath; I would not have a vacuum effect blowing the cellulose from other rooms around. This would also give me more time to install the drywall in the dead of winter with some insulation already installed.
In the winter I keep the humidity set between 25-35. I use a dehumidifier in the summer only in the basement though. When I gut the ceiling I plan on gutting the walls of the room too and installing xps in the cavities. Thoughts? I did the exposed part of my basement this way and its never been warmer.
Thanks for your reply.
 

Last edited by ryane; 02-16-15 at 03:39 AM.
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Old 02-16-15, 04:41 AM
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What is the original vapor retarder? The fact there would be a hole in it would not mean you would get condensation forming on the drywall. You have to have a sufficient amount of moisture reach a surface that is cold enough to condense that vapor. If there is insulation over the drywall then it would be unlikely the ceiling would ever reach the dew point and condense. The vapor, if in sufficient quantity, could flow to a place in the attic where the sheathing temperature is cold enough to condense, but that is why ventilation systems should be considered carefully.

Moisture will be diluted in concentration by the fact that incoming air will help to exhaust moisture through a well designed venting system. The secondary function of the system is to promote evaporation of condensation that does occur, since there are going to be times when conditions will not allow for complete dispersion of the moisture before it condenses.

The only concern about your loading the back of the foam with fiberglass and then blowing cellulose over it is the cellulose is a higher density and will somewhat compress the fiberglass so you will not get the r-value you might be thinking.

If your house is operating at 25-35 % RH area other points to consider..., are your walls insulated and is the rim area of the framing as it rests on the foundation air sealed. That reading would be fairly low in my area and an indication that there is a fair amount of air leakage into the structure. Make sure you are getting a good reading and remember that as you tighten things you will typically raise the RH.

Your question about thermal bridging at the truss bottom chord is not really a great concern from a vapor issue. Vapor will migrate through the wood but not enough to be problematic. Thermal bridging is more a term applied to the concept of the insulation itself, you will be covering the truss chord from the attic with your newly installed insulation.
 
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Old 02-16-15, 05:53 AM
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Original vapor barrier is probably 10 mil. We donít have water lines near the water staining. The hole in the poly is on a interior wall where it meets the ceiling. It looks like they made a plate to nail the drywall to on the ceiling because it was not under a truss. The whole in the barrier is fist size. Your right about the fiberglass being compressed I forgot that I had read that in a different forum. Iíll try your first suggestion of the xps and blown cellulose on top. Thanks again.
 
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