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birgco
Joined: 01 May 2007
Posts: 302
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 Posted: Thu Mar 27, 2008 3:53 pm Post subject: |
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| Quote: | | As for your outside foam insulation issue, would you consider having a custom made flashing that would cover it? Thats what I did at my daughter's house (the foam could not be placed inside - the first floor of her house is a horse stable - yes your read right.) |
Hi Francis,
I am not in favor of placing foam on the outside or inside of a double wall.
Of course there are many opinions on the best technique to achieve a condensation free double wall, but many experts believe it is critical to let the wall breathe from the inside and the outside. Loose or wet blown cellulose in a double wall is supposed to be dense enough so that no vapor barrier is required. The other issue with exterior applied foam is the difficulty with installing siding materials (unless you are planning on using synthetic stucco which presents a whole different set of problems.)
Check out the BSC web site and good luck with your house construction. |
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Dick Russell
Joined: 08 Mar 2006
Posts: 9
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| Posted: Fri Mar 28, 2008 4:44 am Post subject: |
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Regarding foam outside, I had been referring to the use of XPS to insulate the concrete foundation wall, and the issue of protecting that from UV and abrasion forces. In some areas, termite and carpenter ant infestation is a problem, although I wouldn't have that particular problem. For that, though, there is at least one vendor of borate-treated EPS foam panels. For covering either type of foam, there are a number of materials that can be applied, in sheet or brush-on form.
As to dense-packed cellulose in a wall, yes, it is a lot tighter than the FG batt, so that air infiltration is greatly reduced. But let's not confuse "air barrier" and "vapor barrier," or even "vapor barrier" and "vapor retarder." Good sealing between framing joints and proper detailing of housewrap will address the air barrier issue, and dense-packed cellulose will provide much better insulation performance than the typical FG batt.
For water vapor transmission problems, recognize that cellulose is quite permeable, and a good vapor retarder on the correct side of the wall is appropriate for most climates. Poly film is not a good idea in all but the most bitter cold climates. A very well sealed drywall (against air leakage), covered with latex paint, can provide the necessary retarding of water permeance from inside in a heating climate, yet allow "drying to the inside" when required at times.
An interesting paper was published by Dr. John Straube a few years back, on quantifying just how much water actually moved into a wall painted or otherwise provided with a 1 perm vapor retarder. In absolute terms, it is so small that even over a couple months of bitter winter weather, the sheathing and framing can absorb the moisture and still stay within the range normally encountered in "dry" wood. Moreover, as OSB and plywood absorb moisture, their perm ratings go up, so that a good deal of the moisture they can absorb will be passed on through to the outside.
It's usually air infiltration into the wall cavity (convective transport) that causes wall moisture problems. This type of transport is far greater than happens through just diffusion through a painted wall.
Where building codes require a vapor "barrier" (usually it is a "retarder" that actually is required), one could use the Certainteed "MemBrain" product, which is a retarder (1 perm) in dry air but which becomes more permeable in more humid air, allowing that "drying to the inside" in humid summer conditions. |
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mx2
millennium club
Joined: 18 Jan 2006
Posts: 1964
Location: Miami, Florida
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| Posted: Fri Mar 28, 2008 2:02 pm Post subject: |
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| Quote: | | how and when is the insulation in the cavity between sheathing and vapor barrier installed? |
Leave small segments of the plywood sheathing/firestop off at certain intervals to allow for filling the cavity with blown-in, then attach the missing plywd/firestop segments (like putting the lid on a cookie jar) 
You do not want to compromise your vapor barrier by slitting it. And vapor barriers should not allow for air movement, so that is not the issue. On the contrary, the blown-in will also conteract somewhat against air movement. However, vapor permeable or vapor barrier is the question. You don't want warm air entering your cold wall...you're screwed if that happens. Make sure you insulate your junction boxes, basboards and windows well.
Also, if your installing batt in between the studs, will they be kraft faced? Why not instead install (or maybe even add...being superinsulated and all)) foilfaced/kraft rigid behind your gypsum...just make sure you face the reflector towards the correct side of the wall...I always get that confused (in or out?) LOL...anyway, food for thought.
mx2.5
_________________
*Art of Architecture: The conscious use of skill and creative imagination in the production of an aesthetic building.
*Science of Architecture: The calculated use of technical skill and knowledge in the construction of a functional building. |
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mx2
millennium club
Joined: 18 Jan 2006
Posts: 1964
Location: Miami, Florida
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| Posted: Fri Mar 28, 2008 2:09 pm Post subject: |
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You know...it would have been good if I actually read your last post. You already undersdtand the principles very well. I think your original question is a valid one and to maintain the integrity of the membranes you should think outside the box. I would protect the vapor barrier at all costs and if anything, mess with the exterior wall...perhaps blow in from over the top of the outer wall, from the outside...then patch in the exterior wall sheathing and finish. I dunno...would love to hear what you finally do. Most people ask their questions, get their ideas and then disappear forever...leaving us wondering forever.
mx2.5
_________________
*Art of Architecture: The conscious use of skill and creative imagination in the production of an aesthetic building.
*Science of Architecture: The calculated use of technical skill and knowledge in the construction of a functional building. |
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