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Old 01-04-2015, 11:09 AM   #1
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Air sealing and insulating a tri-level attic


I've been researching in preparation for air sealing and adding insulation to my attic. Currently it has blown in fiberglass to the tops of the joists, and I plan to blow in cellulose on top of this. I think I have a pretty good handle on what needs to be done overall, but I do have a few questions.

The house is a tri-level (built 1971), and the upper and lower attic spaces are connected. The ridge of the lower attic is about at the same level as the soffits of the upper attic. Should I block off/air seal this connection between the two? There is separate access to each space, so I don't need the connection for access. There are ridge vent/soffit vents for each attic space, but I'm not sure they would function properly since the two spaces are connected. If I do seal it, I assume treating it like an open soffit and sealing with XPS at the joists in the upper attic would make the most sense?

I plan to seal the knee walls in the lower attic with XPS. I have some sloped framing as well (above stairs)- would I do the same thing there? Obviously blowing insulation on top of this wouldn't stay put. The existing insulation in the stud/joist cavities is kraft-faced batts with the paper on the drywall side. Any concern with moisture if I seal the attic side?

Thanks in advance!
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Old 01-04-2015, 02:18 PM   #2
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If you adequate intake and exhaust on both attics and it is balanced, you don't really need to block them off.

If you want to seal it off, feel free to but I would just use a piece of wood. I don't like exposed foams where they can be avoided.

Where is the home? We will need to know that for vapor retarder recommendations.

If the XPS is in line with the kraft facing on the insulation, the odds are that it is okay. Make sure the XPS is sealed up tight and pull the insulation to seal the back side of the kneewall prior to re-installing it. Be sure to align it properly and then install the XPS.

Cellulose will settle out nicely and doesn't slide that much when you put it down. We put it on pretty decent slopes without much issue.
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Old 01-04-2015, 02:40 PM   #3
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I'm in Indianapolis- didn't realize I hadn't included that in my profile.

I'll have to count soffit vents and do the math to see if things are balanced or not. I don't have a continuous soffit vent, so if anything maybe I'll need to add a few vents.

Sorry if this is a dumb question, but what do you mean by XPS being in line with the kraft facing?
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Old 01-04-2015, 10:07 PM   #4
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That they are in line and next to each other.
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Old 01-05-2015, 01:23 PM   #5
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Welcome to the forums!

Split level house with two attics joined may give problems... IMO, in winter, the cold soffit air from the upper attic would spill over the edge and mix with the lower attics air flow- a really cold lower attic floor; use some house wrap to separate the two attics rather than wood... due to the higher ridge line of the upper attic, I would think it would try to exhaust air from both attics in the summertime, limiting the lower ridge's effectiveness. You may get a mixing of attic air because of the higher stack height on the one and diagonally a lower soffit height on the other. Best to keep them in line, though I haven't read any studies on multiple side-by-side but stepped attics and air flow, just my opinion from my reading. Which would require separate attics.... you could try it and see if air flow problems arise (wet rafters, spots on sheathing, rusty protruding fasteners at roof deck) but then way more work to house wrap than before adding insulation.

The knee wall foam board goes on the attic side (prevents wind washing) with your existing paper faced cavity fill next to drywall, same as an exterior wall. This will keep the cavity warmer/dryer and stop the wood studs from thermal bridging, cover it with required ignition/thermal barrier as per local codes. http://www.buildingscience.com/docum...apor-retarders

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Old 01-06-2015, 07:20 PM   #6
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If you do add soffit venting, try to locate it close to the gutter rather than the house, for positive wind pressure= less rain/snow in attic. The best is double louvered rather than the round vents; pp. 616---- http://books.google.com/books?id=Z8a...0vents&f=false

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