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Exterior wall insulation construction
Last Post 01 Sep 2010 05:22 PM by Dana1. 51 Replies. |
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kschweitzer69
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| 10 Aug 2010 03:51 PM |
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Dana1 - Thanks for your additional input. Seems like cellulose gives you the most bang for the buck, now I have to find a local contractor. We have lots of vaulted ceilings in our house plan that give small cavity space between roof and drywal ceiling. I've heard people claim in these situations that their room becomes drafty. Is cellulose a remedy to this? Or is the spray foam better? What if I had 1" of closed cell sprayed across entire ceiling and then had cellulose blown in on top of that? I find it interesting that most associate energy efficiency with the spray foam instead of cellulose. Does air flow through cellulose like it does fiberglass? Is open cell foam and cellulose comparable as far as air flowing through it? I figured if I used the spray foam I could be more lax about all the taping you mention. Perhaps 1" of closed cell on the walls with cellulose filling rest of cavity is best? I'm after the simplest most cost effective way to get this done. I think using the rigid foam in the attic is going to be incredibly labor intensive with very little gain. Thoughts? |
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Dana1
Senior Member
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| 10 Aug 2010 06:43 PM |
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Posted By kschweitzer69 on 10 Aug 2010 03:51 PM
Dana1 - Thanks for your additional input. Seems like cellulose gives you the most bang for the buck, now I have to find a local contractor. We have lots of vaulted ceilings in our house plan that give small cavity space between roof and drywal ceiling. I've heard people claim in these situations that their room becomes drafty. Is cellulose a remedy to this? Or is the spray foam better? What if I had 1" of closed cell sprayed across entire ceiling and then had cellulose blown in on top of that? I find it interesting that most associate energy efficiency with the spray foam instead of cellulose. Does air flow through cellulose like it does fiberglass? Is open cell foam and cellulose comparable as far as air flowing through it? I figured if I used the spray foam I could be more lax about all the taping you mention. Perhaps 1" of closed cell on the walls with cellulose filling rest of cavity is best? I'm after the simplest most cost effective way to get this done. I think using the rigid foam in the attic is going to be incredibly labor intensive with very little gain. Thoughts?
Relative to standard density fiberglass cellulose even in a low density open blow such as an attic reduces air movement by about 90%, with an average R-value of ~R3.5/inch. At standard wall-cavity density the reduction on air flow is somewhat higher, and the R-value more like R3.6-3.8/inch. Dense-packed to 3.2lb+ density in a cathedral ceiling or wall cavity the reduction on air flow relative to fiberglass is more like 98% with an average R value of 3.8/inch. Open cell foam reduces the air flow by 100%, and runs ~R3.5-R3.8/inch at room temperature, but very cold outdoor temps reduce it's total R value (by ~5% @ 0F) whereas cellulose retains it's full-R (even in open-blow, low density installations.) In your wall stackup the outer edge of the foam would never be as cold though, so it'll pretty much behave as-rated. They're not all the same density or R value (eg, Demilec is rated at 0.7lbs density, and R3.8 per inch, whereas Icynene is rated at 0.5lbs density and ~R3.6/inch, but they're always tweaking the formulas & retesting for a marketing edge. The actual installed densities & R value depends a lot on the quality & experience of the installer. This is true of cellulose installers as well, but it's a lot easier to fix a sub-standard cellulose job after the fact.) By itself the cellulose is a good enough air-retarder from a strictly thermal point of view (there isn't a large fractional heat loss from that minimal air-movement), but not good enough to qualify as an air BARRIER from a moisture-transport point of view. But combined with reasonable attempts at air-sealing the wallboard & sheathing the assembly the differences in air-tightness between celluose & foam are academic. National Fiber even warrantys their goods against causing moisture damage in a
dense-pack cathedral ceiling application as long as there are no
interior vapor barriers. At 3.2lb density the air transported moisture through the cavity over a winter won't
exceed the moisture buffering capacity of the insulation. The whole house air infiltration and individual wall assembly leakage is then governed far more by the quality of sealing around plumbing & electrical penetrations, sealing around windows & doors, etc, than whether the cavity fill is cellulose vs. o.c. foam. It's possible to achieve 1 ACH/50 (that's air changes per hour @ 50 pascals pressure) with dense-packing and some detailing. Cellulose is often injected into dead-air spaces like vaulted ceilings in retrofit situations to reduce drafts. But if the house is truly air-tight with no gaps in the insulation there really shouldn't BE any drafts to speak of. Vault cavites like that can often provide thermal-bypasses when the structure isn't air tight though, and stuffing it full with even low density cellulose knocks back 90% or more. (In wall cavities, every vertical 16" or so of standard density cellulose is as good as a fireblock made of 2x framing timber.) Air-sealing the ceiling with foam doesn't have to be a full-coat to achieve air-tightness, but a 1" layer of closed cell would provide both an air seal and an interior-side class-II vapor retarder. Flash & fill using 1" of closed cell foam is becoming fairly common, using minimum amounts of foam for air sealant & vapor control, but utilizing cheaper goods for filling out the total R. Even using foam as cavity fill taping & sealing the iso & sheathing are worthwhile. Air under pressure from wind or stack effect wants to move, and it moves moisture with it. Moisture from behind brick cladding or siding can get sucked back to the studs in a surprising number of ways. Controlling air movement through walls is the first 90% of avoiding moisture problems, the next 9% being capillary draw & flashing detail, with vapor control more like 0.9% of the problem (for as much attention as it gets.) But it's still useful to analyze where in the assembly the average winter dew point occurs, since that's potentially both an air leakage and vapor diffusion moisture issue. A good stackup should be resilient to minor air leakage, and capable of drying from minor water intrusions without damage. |
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kschweitzer69
New Member
Posts:64
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| 15 Aug 2010 09:36 PM |
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Dana1 - Thanks for your thoughtful evaluation. Basically you are saying that Cellulose (Nuwool) in wall cavities is going to give me near the same performance as the open cell foam and by my local pricing at half the cost. Wet cellulose .70cents sqft. Open cell foam $1.40 sqft. And 1" of closed cell in the attic with cellulose ontop will not yeild any moisture condinsation problems in my climate? How would the cellulose alone perform? Another question that came up for me is I have a large attached garage on the house. I planned to do the wall stackup as previously discussed around all walls of the home that border living space. I planned to take a less serious approach for the attached garage. I was just going to have exterior walls of the garage framed with 2x4 studs and sheathed with 1/2 plywood. My question is how to properly protect the plywood sheathing from moisture that will come through the brick exterior. Would 1/2 rigid foam board work? Or is that more problem than it's worth in my climate? Not a house wrap fan, but maybe that is the route to go? Maybe the old school way of roof felt? Thanks. |
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Dana1
Senior Member
Posts:6991
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| 16 Aug 2010 01:44 PM |
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Posted By kschweitzer69 on 15 Aug 2010 09:36 PM
Dana1 - Thanks for your thoughtful evaluation. Basically you are saying that Cellulose (Nuwool) in wall cavities is going to give me near the same performance as the open cell foam and by my local pricing at half the cost. Wet cellulose .70cents sqft. Open cell foam $1.40 sqft. And 1" of closed cell in the attic with cellulose ontop will not yeild any moisture condinsation problems in my climate? How would the cellulose alone perform? Another question that came up for me is I have a large attached garage on the house. I planned to do the wall stackup as previously discussed around all walls of the home that border living space. I planned to take a less serious approach for the attached garage. I was just going to have exterior walls of the garage framed with 2x4 studs and sheathed with 1/2 plywood. My question is how to properly protect the plywood sheathing from moisture that will come through the brick exterior. Would 1/2 rigid foam board work? Or is that more problem than it's worth in my climate? Not a house wrap fan, but maybe that is the route to go? Maybe the old school way of roof felt? Thanks.
With a ventilated attic and good air-sealing practices at the attic floor/conditioned ceiling interface cellulose alone would work just fine using nothing but standard latex paint on the ceiling. In very cold climates special vapor-retardent latex primers might be necessary. On your garage studwall with the brick facing, protecting the plywood & from the brick could be as simple as painting the exterior with vapor-retardent latex paint and taping the seams, or using standard 15lb felt (a double-layer, if the garage-interior side is ever going to be insulated & finished.) Housewraps are not designed for that application- they're far too vapor permeable (typically 30+ perms). Felt has variable vapor retardency- it becomes more permeable (even highly permeable) as it takes on nore moisture, but if the cavity betweent the brick & ply is well vented the average moisture content of the felt will be low- low enough that it will have a permeability of 5-10 perms- enough to limit the peak drives at the plywood layer. Half-inch XPS has a permeablity of ~2 perms, but half-inch foil-faced iso with taped seams would be under 0.05perms. But if the foil facers get dinged during assembly or corroded over time the iso itself may eventually load up with moisture and will eventually become highly permeable. There are 1/4" & 3/8" thick very large-sheet fan-fold XPS products out there that test in the 1-5 perm range that would be appropriate for vapor control. Those also make good air-barriers when foam-sealed at the edges. |
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kschweitzer69
New Member
Posts:64
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| 16 Aug 2010 03:30 PM |
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I've seen the fan fold product before and that may fit the bill well to protect the plywood in that area
I was planning to use the foil faced rigid foam in my other wall stackup as previously discussed. You have brought
concern to my attention regarding this as to potential moisture issue as described above. Should I be using the XPS
or something else instead of the foil faced 2" on my main living space exterior walls?
In the attic you are basically saying that cellulose with attention to detail of sealing would be adequate. Is this the reason
people tend to use the foams and have great success is because they are mostly fool proof? Other ways of insulating with cellulose
for example really force you to detail every penetration to ensure air infiltration is stopped. The biggest problem I have with the foam
is potential health hazard and double the cost, for little gain as you describe. |
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Dana1
Senior Member
Posts:6991
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| 16 Aug 2010 04:52 PM |
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Spray foam is hardly foolproof, (and screwups with foam can be hard to fix) but with a full spray-foam job it's usually pretty easy to get adequate air-tightness with foam, whereas air-sealing with fiber insulation requires more attention to detail. In terms of R/lb cellulose is typically between 3-4x that of open cell foam, which can be a factor when going for high-R in a trussed roof with low static load specs on the truss chords.
Health hazards from using spray polyurethane foams are highly
exaggerated- the industry has come a very long way since the
urea-formaldehyde products of the 1970s. The largest benefit is the
air-sealing qualites, which can be very important if it's the primary
air-barrier. But in your stackup (and in most new construction, as long
as it isn't too geometrically complex) it's pretty straightforward to
achieve those ends by other methods. Not much air gets through an
aluminum facer with taped seams & foamed edges. If it can be made
air-tight, going high-R with low (but not zero) air-permeability goods
like cellulose yields a higher performance per dollar.
Go ahead and use the 2" iso everywhere, if it can be had for cheap. If you ventilate the cavity well and foam/tape seal the foil facers well it works fine behind brick- just be sure to tape & seal any dings it gets. You may have to limit the total thickness of the iso to 3.5" in order to keep a 1" cavity and still meet code for total seperation between the brick & structural wall though. See: http://www.pima.org/SpotLightFiles/polyisoarticlereprint3.pdf A 1.5" layer over a 2" layer would get you there. The the more religion you have about using capillary breaks between the foundation and wall/insulation and using masonry sealers on the exterior brick, the less likely it is that the foil facers will ever have issues. (It's not as if you live in a tropical climate where condensation conditions on the cavity interior will be frequent or long-lived.) As long as the average humidity in the cavity is reasonable the foil will last forever.
Where you aren't looking to add R value it's cheaper to do the vapor control bit by other methods (fan fold XPS or vapor-retardent latex.)
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kschweitzer69
New Member
Posts:64
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| 17 Aug 2010 12:50 AM |
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I think I follow all that you are saying. I'm beginning to wonder if I should get 1.5" foil faced foam and install the 3/8 fanfold underneath of it for my living space walls instead of going with full 2". The fanfold would cover alot of the seems of the plywood and seems of the fanfold could be seam staggered with the 1.5" foam board. Would this be a better setup? If done would I still need to tape seem the plywood? I'm just not crazy about painting plywood seams for taping. Seems like that could be a messy process. Paint dripping on foundation walls, etc. Doing this reduced my R-value slightly, but also give a little more wiggle room to leave larger gap between brick and foam. I would have to think that stackup would still be fine for my climate zone. Thanks again for all your thoughts and opinions. |
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Dana1
Senior Member
Posts:6991
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| 17 Aug 2010 10:37 AM |
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Posted By kschweitzer69 on 17 Aug 2010 12:50 AM
I think I follow all that you are saying. I'm beginning to wonder if I should get 1.5" foil faced foam and install the 3/8 fanfold underneath of it for my living space walls instead of going with full 2". The fanfold would cover alot of the seems of the plywood and seems of the fanfold could be seam staggered with the 1.5" foam board. Would this be a better setup? If done would I still need to tape seem the plywood? I'm just not crazy about painting plywood seams for taping. Seems like that could be a messy process. Paint dripping on foundation walls, etc. Doing this reduced my R-value slightly, but also give a little more wiggle room to leave larger gap between brick and foam. I would have to think that stackup would still be fine for my climate zone. Thanks again for all your thoughts and opinions.
Naw, stick with the full 2" of iso, tape the seams with FSK tape, foam-seal the edges. Caulk/glue the seams of the sheathing as you put it up whereever the seams coincide with the framing. Stagger the seams of the iso & the sheathing by at least 12", and finish the air sealing from the interior. Any sheating seams that aren't on framing can be primed & taped from the interior before installing the cavity insulation. Alternatively you could use housewrap between the iso and sheathing as a continuous air-barrier, but methinks priming & taping a few seams would be less material expense and less labor. Any way you do it, verifying the air-tightness of the sheathing with a pressure door test is advisable prior to the cavity insulation. It's an extra step, but fixing the air leaks is 10x easier before the insulation goes in than after. |
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Jere
Basic Member
Posts:106
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| 17 Aug 2010 02:02 PM |
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Posted By Dana1 on 17 Aug 2010 10:37 AM
Any way you do it, verifying the air-tightness of the sheathing with a pressure door test is advisable prior to the cavity insulation. It's an extra step, but fixing the air leaks is 10x easier before the insulation goes in than after.
Pressure testing before insulation... won't most of the air leak through the roof, soffits, etc.? |
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I built my home with the help of Pierson-Gibbs Homes, "The Hands on House". They build the shell, you finish it.
www.p-ghomes.com |
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Dana1
Senior Member
Posts:6991
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| 17 Aug 2010 02:19 PM |
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Or how about through the open windows & doors?
In order to pre-test the sheathing, you obviously must wait for the (hopefully air-tight, but you'll find out) attic floor, doors, and windows to already be installed, and plug any flues/vents that have yet to be hooked up to at least the vent-dampers, if not the whole appliance. Insulating the wall cavities is usually a much later operation, after all of the plumbing & electrical are at least roughed-in.
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Jere
Basic Member
Posts:106
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| 17 Aug 2010 03:23 PM |
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Alright, my question was kinda silly eh...
My attic doesn't have a floor, just drywall on the ceilings in the rooms below... the cellulose insulation is blown in on top of the drywall. I guess the drywall contractor could hang the drywall on the ceilings that are right below the attic spaces, before the insulation in the exterior walls goes in. The drywall contractor could come back to hang the rest after pressure test is done and the wall insulation is installed. The drywall contractor probably won't like it as they like to hang everything at once, but that's their problem and I'm sure they would get over it. lol |
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I built my home with the help of Pierson-Gibbs Homes, "The Hands on House". They build the shell, you finish it.
www.p-ghomes.com |
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kschweitzer69
New Member
Posts:64
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| 19 Aug 2010 09:43 AM |
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Dana1 - Thanks, so 2" foil faced foam around living spaces and 3/8 fanfold around garage, right? Now if I do the wet blown cellulose in wall cavity. Am I going to need an interior vapor barrier besides just the drywall paint. I've read some forums where people with the wet blown cellulose complain of saturated drywall by not putting in a vapor barrier. Any truth to this? Since you are familar with my wall stackup and know that I plan to use plywood sheathing on exterior would 3/8 suffice since I'm bricking the home and putting thick foam over it? Or should I just stick with 1/2 plywood? If 1/2, could it be 3ply or should I just do the 4ply? Thanks |
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Dana1
Senior Member
Posts:6991
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| 19 Aug 2010 11:38 AM |
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Posted By kschweitzer69 on 19 Aug 2010 09:43 AM
Dana1 - Thanks, so 2" foil faced foam around living spaces and 3/8 fanfold around garage, right? Now if I do the wet blown cellulose in wall cavity. Am I going to need an interior vapor barrier besides just the drywall paint. I've read some forums where people with the wet blown cellulose complain of saturated drywall by not putting in a vapor barrier. Any truth to this? Since you are familar with my wall stackup and know that I plan to use plywood sheathing on exterior would 3/8 suffice since I'm bricking the home and putting thick foam over it? Or should I just stick with 1/2 plywood? If 1/2, could it be 3ply or should I just do the 4ply? Thanks
With taped foil-faced goods on the exterior you can't saturate the cellulose from summertime exterior vapor drives. With R10 on the exterior of and R R13-ish interior wall it won't accumulate moisture in winter, because it'll stay above the dew point of the interior air in your climate. Putting a vapor barrier on the interior wall would create a moisture trap- DON'T DO IT! And be careful about paint selections for the interior as well to avoid unintended moisture traps. Saturated drywall is more likely to occur if the paints & exterior sheathing are are too vapor retardent, which don't allow the assembly to "breathe" water vapor in either direction. Moisture can get into the assembly by a number of methods (leaks, capillary draw), but it gets out primarily via vapor diffusion. With foil on the exterior it has zero capacity to dry in that direction, so you HAVE to allow it to dry toward the interior. Stay away from oil/alkyd paint, foil or vinyl wallpapers, etc. Stick with standard grades of latex or acrylic and you'll be fine. Look up the perm rating of any interior wall finish- anything over 2 perms would be fine, under 1perm you're looking for trouble. The other way people get into trouble with wet-sprayed cellulose leading to drywall issues is by not giving it sufficient drying time before putting in the drywall and paint. This is more common with 2x6 studwalls wet-sprayed in winter without heating the interior than with 2x4 construction applied in the summer, but it really needs some time even with 2x4 studwalls. Give it at least a couple of days at reasonable temperature & humidity before putting up the drywall, and at least another week at reasonably conditioned indoor temps before painting (even with latex, which is only about 1/10th as water-vapor breathable as unpainted wallboard.) A quick 1-pager on the topic lives here. Done right it celluose adds considerable moisture resiliency to the structure, but done wrong you can trap the moisture used for activating the adhesive into the assembly, which could take over a year to fully stabilize. (Or even decades, if you lock it in with a poly vapor barrier.) If the schedule doesn't have sufficient drying time, dry blowing "blown in blanket" behind stapled-on-mesh works. Dense-packing to 3lbs+ is advisable if going that route. It's more labor intensive than open-stud wet spray, but has a slight performance edge. A cheezy promo video of wet-spraying cellulose. A cheezy promo video of cellulose blown-in-blanket. The half-inch plywood sheathing is structural, to prevent racking & shear forces from distorting or collapsing the studwall. Since most of the racking loads are wind driven, and the brick reduces those forces by a goodly amount you MIGHT be able to get away with thinner stock, but the whole plan would need an engineering review to say for sure. (It's probably cheaper to stick with half-inch than to pay for the analysis though.) Whatever goods you put there should be APA rated for sheathing applications at your stud spacing. If you're trying to save a few bucks, going to fully rated OSB sheathing rather than unrated or under-rated thinner plywood would be a safer bet. |
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kschweitzer69
New Member
Posts:64
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| 19 Aug 2010 03:37 PM |
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Makes sense, wouldn't using OSB sheathing open me up for a better potential of wall moisture issues since OSB does not allow a wall to breath and dry out? I think the only OSB i would consider is the ZIP system, which cost as much as plywood. Would you agree? I'm using 5/8 plywood for roof sheathing. |
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Dana1
Senior Member
Posts:6991
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| 19 Aug 2010 04:57 PM |
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Posted By kschweitzer69 on 19 Aug 2010 03:37 PM
Makes sense, wouldn't using OSB sheathing open me up for a better potential of wall moisture issues since OSB does not allow a wall to breath and dry out? I think the only OSB i would consider is the ZIP system, which cost as much as plywood. Would you agree? I'm using 5/8 plywood for roof sheathing.
OSB and plywood have very comparable vapor permeability when the humidity is low to moderate (as it always will be, in your stackup.) Plywood only has the advantage at very high humidity levels. See: http://www.buildingscience.com/docu...ure_03.jpgFor a longer-winded more scientific version: http://www.vtt.fi/inf/pdf/workingpapers/2005/W22.pdfThe relevant conclusions being: "The water vapour permeability of a material typically increases with higher relative humidity. That influence is also seen in Figure 4. However, the relation on relative humidity is different between OSB and plywood. At average relative humidity less than 30 % OSB has higher water vapour permeability than plywood whereas at higher RH levels plywood has higher values." and "OSB -products have more resistance against short period water contact with the board surface than the plywood products. This is probably due to the wax that the OSB boards have at least on the surfaces. Plywood products start to absorb water immediately after water contact, but the suction flow decreases till the final moisture level is reached." OSB surely DOES allow drying, but it will take somewhat longer than plywood if it actually gets WET, but it takes a longer exposure for OSB to become wet. Since the cellulose would wick any condensation and minor leakwater away, the chances of significant wetting of either OSB or plywood are quite low until/unless you get a significant leak event. With foil on the exterior, all drying of the sheathing has to be via the cavity insulation. On the XPS fan fold section you'll get some drying toward the exterior. But don't sweat the differences here- by putting the OSB inside the R10 of iso you're keeping it much warmer & drier than it would be in a roofing app under shingles or directly behind siding or brick in a wall. Putting a good fraction of the insulation outside of the structural wood has MANY benefits. |
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Alton
Veteran Member
Posts:2164
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| 19 Aug 2010 05:35 PM |
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I agree with Dana1's logic. With your stackup of the wall I think you are safe in using OSB or plywood. This should result in some savings.
I have not used plywood for walls or floors for several years. I have not heard of any problems after the homeowner moved in. |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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kschweitzer69
New Member
Posts:64
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| 19 Aug 2010 11:15 PM |
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So all parties agree that stock grade OSB with caulked or taped seams and then a layer of 2" foil faced foam is going to be a good wall system with good performance with zero moisture issues. OSB in my mind is a fine product until it gets wet, then I feel it is very hard to ever get fixed. That is where I believe plywood is better is that it is more forgiving in an instance of moisture either forming by condinsation or other methods. It seems very difficult for the OSB to ever recover from that. Perhaps the 2" ISO makes my wall fool proof to the point that this is not an issue. As you can tell I'm a hard sell on OSB. |
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kschweitzer69
New Member
Posts:64
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| 20 Aug 2010 01:58 AM |
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Would a tape like this over stock OSB work to seal seams? http://www.amazon.com/Intertape-4379-Masking-2-Inches-Yards/dp/B000BQWGUA. If not is their something out there that would adhere directly to OSB without having to paint joints? |
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Dana1
Senior Member
Posts:6991
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| 20 Aug 2010 02:14 PM |
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No tape adhesive will stick to unpainted plywood or OSB long enough to matter. The paint makes a very solid bond with the wood, and gives a better bonding surface for tape adhesives. Normal non-destructive humidity cycling of bare wood would be enough to break the bond with tape adhesives (but not with decent quality primer paints.) If Huber could avoid painting Zip sheathing merely by using a better tape adhesive, they surely would!
Almost all purpose-specific air sealing system tapes like those used with Huber Zip, Tyvek, etc. would be adequate. What your looking for is generically called "sheathing tape" or "housewrap tape". The adhesives for general purpose or masking tapes won't likely last more than a decade best-case. Goods designed for masking off or sealing EIFS (like the one you linked to) probably isn't going to cut it as sheathing tape. (The same company does make sheathing tape, but with different polymers & adhesives than that product.)
If you go a full 2" on the iso condensation potential at the sheathing drops to about zero in your climate. The only way the sheathing moisture gets high enough to matter is if there's a real leak, or if you create a moisture trap by putting something like poly on the interior. And cellulose cavity insulation will moderate normal seasonal humidity cycling. It's at lower risk of damage from condensation in your wall & insulation stackup than it is in the racks in the shed down at the lumber yard (which would not be the case were wrapped in Tyvek under vinyl siding without a rainscreen gap, where it also seems to do fine, most of the time.) You have a wythe of brick, an inch of air, 2 sheets of aluminum foil and 2" of iso between the OSB and the outdoor environment- it's not likely to be rained-on. Then, with,semi permeable paint, gypsum, and 3.5" of hygric-buffering fiber between the OSB and the interior environment, and located depth in the R that experiences interior dew point conditions less than 0.1% of the heating season hours in a decade. I can't think of how it could get very much better than that. It has to stay below the dew point for weeks to saturate the cellulose enough that the OSB would take on substantial condensation moisture. It's moisture content will rise over the winter, but it'll stay comfortably within the range it can handle.
Viewing it graphically, take a look at figures 9 & 10 in this document: http://www.buildingscience.com/documents/reports/rr-0903-building-america-special-research-project-high-r-walls Your stackup will perform approximately like case 2b, (since the exterior insulation is about the same R value as the cavity fill in both cases.) The relative humidity at the sheathing is what's being depicted in fig. 9, 100% (the top) is where it hit's the dew point. During week-long cold snaps it begins to approach the dew point, but doesn't exceed it. Then looking at figure 10, you'll see that your stackup (case 2b) spends far fewer hours than the others at risk from air-leakage condensation. Bear in mind, these are simulations for MINNEAPOLIS, which is about 15F colder in winter than Cincinnati's climate! Mentally shift the lines of the graph in fig 10 up 15F, and leave the indoor dew point reference in the same place. In an average year you'll have zero hours with the sheathing below the interior air's dew point, but you might have a few hours of exposure during the cold-snap of the century, but even that wouldn't come close to overwhelming the buffering capacity of the cellulose, even if you don't air-seal the interior very well. |
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JohnyH
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