# Knee walls and vapor barrier



## Gary in WA (Mar 11, 2009)

Little short (SPF R-6 per in.http://energyguild.hubpages.com/hub/Spray-Foam-Insulation-Is-It-Really-That-Good) on the sloped ceiling code requirement of R-38.... will you be furring down the rafters to add more insulation? http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=Kansas

Be sure to keep that daughter safe with window egress per code; http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_3_sec010.htm?bu2=undefined

The asphalt coated paper is on the wrong side for your location, should be next to the drywall; http://www.buildingscience.com/docu...heet-310-vapor-control-layer-recommendations/

Why Masonite board? for decoration?

Any gaps between vapor retarder paper facing are fine as it is area weighed, not supposed to be air barrier; *Page 5*; http://www.buildingscience.com/docu...por-retarders/?topic=/doctypes/researchreport

Push it in so the facing is in full contact (including corners) with the drywall/Mb, then let it expand out slightly. Best to add some rigid foam board on the stud faces (tape seams foil tape, run fb under knee wall to block air flow there, and up to block air-flow to rafters as the upper/lower attics should have their own ventilation) to get minimum required R-20 for your area. Check local AHJ for foam ignition/thermal barrier required. This will stop attic wind-washing of the fibrous cavity insulation. http://www.buildingscience.com/documents/information-sheets/installation-of-cavity-insulation/

Insulate the access door, air seal the wiring/plumbing holes in top plates; http://www.habitat.org/env/pdf/ceiling_and_attic.pdf

http://www.google.com/url?sa=t&sour...sg=AFQjCNHwd56o0AxLi8-V03E5cMUmwWATQw&cad=rja

OK, had to reread your post-so much info.... lol. Better than not enough. 
1" XPS has a perm rating of 1.1 or 1.5 per inch, depends on brand. So R-15 (1-1/2")XPS is; 1.1perms divided by 1.5" = 0.73perms.------ or two inch thick= divided by 2 = 0.55perm. for 2" 

Your .73 perms (XPS) is a vapor retarder, same as the asphalt faced paper; 0.3-0.5 perms but it is variable; http://www.builditsolar.com/References/PermeabuilityEEM-00259.pdf

Variable; http://www.buildingscience.com/documents/reports/rr-0004-air-barriers-vs-vapor-barriers

AND the previous link showed hardboard- standard= 11perms and hardboard (Masonite)- tempered= 5 perms. Long story short- IMO, use the open cell (EPS) with no worries. BUT, lol, using the XPS/kraft/Masonite it could still dry to attic if not a facer on fb; "double vapor barrier"- http://commercial.owenscorning.com/assets/0/144/172/174/e45fe07d-5cc9-4e4b-866a-5e35d75090ec.pdf

Read pages 5-7 here: http://www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design

Use this for your min. fb thickness for your Zone to prevent cavity condensation; http://www.buildingscience.com/docu...ulating-sheathing-vapor-retarder-requirements

OR; tell me the closest large city for your local climate and I'll figure dew-points for you with different thickness foam boards?

Gary


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## Gary in WA (Mar 11, 2009)

Ah, the Masonite is under the drywall.... the Kraft facing is permeable enough to permit drying to the inside and still use fb; Ginger vs. Mary Ann; http://www.buildingscience.com/documents/insights/bsi-024-vocabulary

AND, using both together; http://www.buildingscience.com/documents/reports/rr-0004-air-barriers-vs-vapor-barriers

Gary


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## duffman56 (Mar 9, 2012)

Sheetrock is old and original, masonite board I'm guessing was used for added strength?

As for location - we are actually an equal distance from Nebraska and Oklahoma border, and about 60 miles East of Colorado border. I believe this puts us right on the border between zones 4 and 5. Humidity is usually very low here, MUCH lower than central and Eastern KS. 


Gary - very informative, thank you. Just want to make sure I understood everything correctly:

Does the old insulation (w/ asphalt coated paper) need removed so I can install insulation with the vapor barrier against masonite board? Since I'll be installing it backwards (with paper away from me instead of facing) is there a way to secure it to hold it in place, at least long enough for me to get the foam panels up? 

OR - since the fb acts as a vapor retarder (as does the primer) should I cut out the kraft paper vapor barrier altogether, to avoid trapping the moisture inside? I might point out that I'll be adding at least 3" of fb, not sure what the permeability would be down to - .37? According to the site you listed [http://www.buildingscience.com/docu...lating-sheathing-vapor-retarder-requirements] it sounds like I'd be OK without the kraft paper vapor barrier. 

The info I found on EPS suggests it's closed cell, but that it does have a higher rate of permeability than XPS. If I used as much as 4" (to attain R-16) would this still be an effective _vapor permeable_ air barrier? Does this affect the answer above, regarding using faced or unfaced fiberglass batts? What is the best way to secure these fb panels? (I'll likely have to use two 2" layers, as that's all that's available at Menard's in smaller quantities) I can see it being difficult to find the stud with a screw after going through 4" of fb. 

After reading parts of [http://www.buildingscience.com/docu...or-retarders/?topic=/doctypes/researchreport] I thought it worth mentioning that this bonus room has a bath tub that will be used. 

I know I'm asking a lot of questions, but I really want to do this right. :thumbsup:


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## Gary in WA (Mar 11, 2009)

1. Do you have a link to the fb for me? The paper facing requires on the heated room side to stop moisture from there.

Best is removing the paper facing, though not required as it is less when you don't need it and more when you need it then. 2. Why so much insulation, are you keeping heat in? Only R-5 for your location, and that is added benefit of being protected from most of the moisture/rain/wind because of in attic; http://www.buildingscience.com/docu...ulating-sheathing-vapor-retarder-requirements Echo, echo.... 

There is a balancing point, IMO, ;http://www.google.com/url?sa=t&rct=...VjgAkW8XzKgQkGg&bvm=bv.44442042,d.cGE&cad=rja

I think you should spend time air-sealing attic, outlet holes in existing drywall/Masonite, and caulking/foam under the drywall behind the baseboard; http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_11_sec002_par021.htm

http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_11_sec002_par022.htm

3. Did you air seal the attic or basement/crawl yet? http://www.conservationtechnology.com/building_design.html#MoistureTransmissionPathways

http://www.wag-aic.org/1999/WAG_99_baker.pdf

Gary


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## duffman56 (Mar 9, 2012)

http://www.menards.com/main/mobile/...d-polystyrene-insulation/p-1934680-c-5779.htm

Those are the foam panels I'm planning on using. Two layers, still not sure how to secure them.

I know it's a lot of insulation, but the room can get extremely hot in the summer, the attic holds in very high temperature heats, which is why I'm trying to insulate between room and attic side to such a great extent. 

Do I need to remove old insulation (with reversed paper facing), or is it ok as is to add the fb over?


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## Gary in WA (Mar 11, 2009)

Pull the paper face to be safe. The RayLite boards have a 3-5perm rating. Depending on density, which they don't say. You should be fine, IMHO.http://www.buildingmaterials.umn.edu/products/raylite.html

Gary


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## duffman56 (Mar 9, 2012)

So no vapor barrier (at least not between Masonite and fiberglass insulation). 

What about a method of securing the foam board? 4" is a lot to go through and find the center of a stud.


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## Gary in WA (Mar 11, 2009)

Facing installed at the drywall is fine, not at the foam. Use 3"nails and fender washers, then bigger nails with washers. Or the cap nails for house wrap (if long enough) online for foam fasteners.... get creative. Pre-measure studs from a fixed location- wall, corner framing, mark on the roof sheathing....

Gary


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## duffman56 (Mar 9, 2012)

3" nails wouldn't be long enough. Maybe I can use the fender washers on something longer?


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## Gary in WA (Mar 11, 2009)

Gutter spikes and washers.

Gary


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## duffman56 (Mar 9, 2012)

Just re-reading your post Gary - haven't started on the project yet with things so busy for the holidays. 

You said "facing at the drywall is fine, not at the foam". Does this mean I need to remove all of the insulation that is currently in the knee wall space? 

If that is the case, could I get away with unfaced fiberglass insulation? I'm not sure how I'd attach the paper-side, since sheetrock is already up.


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## Gary in WA (Mar 11, 2009)

IMO, flip it around so the paper facing is against the drywall. This will help stop adding moisture to the wall cavity from the wet room. The paper facing does not require stapling to be effective; http://docs.google.com/viewer?a=v&q...0pf2_7&sig=AHIEtbQylPzdnxaB0Oqk9yRrrdP1VBztDg 

Gary


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