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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
New Member
Posts:64
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| 01 Aug 2010 11:28 PM |
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Very close to starting our new home construction project. Went down the path of ICF and it was simply too expensive in my area. Going conventional and looking to build for energy efficency. Originally planned to build 2x6 walls fully spray foamed with huber zip system sheathing. Got to reading about thermal bridging issues in studed walls so came up with the following idea.... Build 2x4 framed exterior walls, use plywood or huber zip sheathing and then glue 2" hard ridgid foam to exterior wall surface. I thought I would use 2x6's to frame the windows and doors. Overall this gives me the same 6" wall, but addresses bridging problems. I figured I would spray foam fill the remaining 2x4 wall cavity. My biggest concern is possible wall breathing/moisture/condinsation issues between the foam/plywood/spray foam cavity. I'm thinking the 2" is thick enough to stop any problems. Thoughts please?? |
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Alton
Veteran Member
Posts:2157
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| 02 Aug 2010 07:41 AM |
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I have designed and built with 2x6 construction that used 2" of foam board on the exterior and fiberglass or foam between the studs. Worked exceedingly well in Columbia SC and KY. I have also worked with gluing foam board to walls. (We used Mastic no. 11 adhesive.) A somewhat slow procedure. I think it is best to use nails or screws instead of glue or adhesive. Just use a few fastners and let the closed cell foam glue the foam board to the studs. By the way, petroleum based glues will dissolve the foam board unless it is covered with another type of skin.
If you build with 2x6's I suggest you consider using the Arkansas method that conserves lumber. You know, single top plates, roof trusses with energy heel aligned directly over the 2x6 stud, hollow box headers foam filled, etc.
Another thing we did was to sprinkle Roach Proof (boric acid) on the bottom plate before the insulation was installed. I do not believe that roaches can develop immunity to boric acid like they do to other chemicals.
I think a better way to eliminate thermal transfer of energy through studs would be to use a staggered stud arrangement of 2x4's instead of 2x6's. With bottom and top 2x6 plates the wall would not increase in depth (5.25" plus the materials on both sides). Spray foaming the cavity wall would insulate the studs.
Time-wise I believe that you will find that SIPS will be faster than stick framing.
Good luck on your build. If this is your first time to build, then in addition to having a builder you may want to find a local person to guide you through the process. Building can be very frustating when things do not go smoothly. A lack of experience on the homeowners part can wreak havoc. If you were nearby I would help you for free since I am now retired. |
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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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lzerarc
Basic Member
Posts:423
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| 02 Aug 2010 12:43 PM |
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I too am planning something like this as well. I have not complete ruled out ICF, however since I am building it myself, I am sure I can do 2x6 stick framing much cheaper.
Being in Iowa, I am thinking 2x6 exterior, and was playing around with 24" oc spacing w/ trusses aligned, single plates. I have to check local codes to see if that is acceptable on load bearing walls. I was also planning on using 2x8s to frame the opening around the windows to allow solid nailing for the windows. 2" of rigid foam, or possibly 1.5" over 1/2" sheathing. Batt on the inside. Another option is the Dow SISin 1", which may not give me quite as good R as the 2", however would greatly increase the build time on the exterior.
My question is, you wouldn't happen to have a sqft cost for materials only in this construction type would you? Also what benefit would you be gaining for going 2x4 and spray foam. Why not 2x6 and batt? Might still be cheaper? I would also like to explore spray foam, but imagine it will be quite expensive vs batt.
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jonr
Senior Member
Posts:5341
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| 02 Aug 2010 02:10 PM |
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I'd be interested in the figures on staggered 2x4s vs 2x6s. Or perhaps some type of high density foam spacer that adds to the width of a 2x4 to make it 2x6.
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kschweitzer69
New Member
Posts:64
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| 02 Aug 2010 03:01 PM |
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I priced my house with ICF and 2x6 frame with full spray foam insuation and the ICF was 25K more. I'm sold on ICF, but bank appraisal did not put the value to the ICF we needed to justify the build. I felt stick built spray foam would give us similar result if detailed correctly at a whole lot less cost. To answer your question about the 2x4 wall with spray foam. Is that technically 2x4 wall is all you need with spray foam and I simply don't feel comfortable with going 2x6 and using fiberglass or some other fluff material. I feel the foam is the only thing that protects me from zero condinsation issues. I don't like the air flow through the wall other insulations allow for. Also the 2x4 with 2" exterior foam puts us at 6" thick wall which is normal for doors and windows. Getting thicker than that adds additional challenges. For the record in our area spray foam for 2x4 wall is $1.40 sq/ft. I can get the 2" foil foam board for $12 a sheet and then there is the cost of the ZIP OSB sheathing. The staggered 2x4 wall approach with 2x6 top and bottom is intreging to me and it addresses some issue, but does not seal entire envelope like the rigid foam would in my opinion. I'm also concerned about strength of the staggered wall for external shell application, although I think the spray foam would add some shear strength to the system. My concerns about the 2" rigid foam are about brick ties or any other exterior mounted items for the home. I don't think moisture will form with that thick of exterior rigid foam along with the fully foamed interior wall. |
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lzerarc
Basic Member
Posts:423
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| 02 Aug 2010 04:08 PM |
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is that $1.40 spray foam price for your floor area sqft of your home or the wall area sqft of your home? |
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kschweitzer69
New Member
Posts:64
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| 02 Aug 2010 04:15 PM |
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1.40 per square foot of wall area. |
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Dana1
Senior Member
Posts:6991
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| 02 Aug 2010 06:05 PM |
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Posted By kschweitzer69 on 01 Aug 2010 11:28 PM
Very close to starting our new home construction project. Went down the path of ICF and it was simply too expensive in my area. Going conventional and looking to build for energy efficency. Originally planned to build 2x6 walls fully spray foamed with huber zip system sheathing. Got to reading about thermal bridging issues in studed walls so came up with the following idea.... Build 2x4 framed exterior walls, use plywood or huber zip sheathing and then glue 2" hard ridgid foam to exterior wall surface. I thought I would use 2x6's to frame the windows and doors. Overall this gives me the same 6" wall, but addresses bridging problems. I figured I would spray foam fill the remaining 2x4 wall cavity. My biggest concern is possible wall breathing/moisture/condinsation issues between the foam/plywood/spray foam cavity. I'm thinking the 2" is thick enough to stop any problems. Thoughts please??
Condensation issues only arise when warm humid air comes into contact with surfaces that are colder than the dew point of that air, or through water-vapor diffusion through the material to cause a dew-point conditiont. There are other issues that arise when both the inner & outer surfaces are to water-vapor impermeable to allow the assembly to dry. The performance of any given stackup can be very much climate dependent. In hot humid climates the source of the humidity is the outdoor air, in cold climates it's the indoor air. The vapor permeability of rigid foam differs by foam type and the permeability of any foil/poly/vinyl/other facers it may have. The typical pink or blue foam sheathing is extruded polystyrene (XPS), and semi-impermeable at 2", but still an order of magnitude more permeable than foil or poly sheeting. Open cell foam is moderately permeable at 3.5" thickness, but denser closed-cell foam is almost a vapor-barrier at 3.5". Assuming the 2" sheathing is XPS and the cavity fill is open cell foam, the XPS is ~ R10, and the o.c. foam is ~ R12-R14, so more than 40% of the total R value is on the exterior of the studs & structural sheathing. If this is a heating dominated (cold) climate, the condensation hazard comes in the winter, when humid interior air diffuses water vapor through the wall & o.c. foam. The dew point of ~68F air at 30% relative humidity (dry, but not super dry- a healthy RH), occurs around 37F- only the parts of the stud or sheathing that stay below 37F for substantial periods are at risk. The outdoor temp at which the sheathing is 37F is at 16F- if it stays that cold or colder for weeks on end, (as it does in Fargo or International Falls, but not Peoria) you have a condensation problem. If your January average (not average high, not average low, but overall monthly average) temps are above 16F, the risk of damaging condensation is low, if you've made it all air-tight. If that stackup is a problem in your it can be rectified by using air-tight methods and vapor-retardent latex on the interior, to make the interior less vapor permeable than the exterior XPS, which puts the sheathing closer humidity environment of the cold winter air. If your January average temps are a LOT colder than 16F you may need to use a true vapor barrier between the gypsum & studs to keep the sheathing & outer parts of the studs from loading up with moisture. Alternatively you can get a lot of mileage out of using cellulose instead of foam as cavity fill, which can buffer quite a bit of winter moisture without loss of performance or damage, releasing it seasonally as conditions shift. (Don't use interior side vapor barriers with cellulose- it needs to be able to release that moisture more quickly than the exterior XPS by itself would allow. Standard latex paints are usually vapor-retardent enough. Attention to air-sealing the wallboard is critical.) If this is in a hot humid climate with summertime dew points that stay in the 70s, as long as you don't use vapor barriers or vapor retarders (or foil/vinyl wallpaper, etc) on the interior, that stackup is good to go. The XPS is vapor retardent enough that the AC keeps up with the very minor latent load that represents quite easily (your ventilation air will have orders of magnitude more humidity to be conditioned than anything diffusing through the exterior foam to the studs.) But the performance changes if you're talking beadboard EPS, foil-faced iso rigid board and closed cell foam cavity fill- the above was just an example. Details matter. |
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kschweitzer69
New Member
Posts:64
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| 02 Aug 2010 07:48 PM |
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Dana1 - Thanks for the very detailed evaluation of moisture risk on this wall structure. Allow me to share a little more about our climate and wall construction plans so you can drill down a little deeper into evaluation. We are located near Cincinnati, OH which according to building science is basically on the border of zones 4 and 5. We tend to get hot humid summers and fairly cold spells in the winter, so we have to engineer for both types of situation. I was planning to use the foil faced 2" foam Rvalue of 13 on exterior, then stud up a 2x4 wall with the open cell spray foam. I'd like to keep total wall thickness around 6" to keep window and door trimmings within standard dimensions. The detail I'm not sure about and am concerned about is the exterior wall sheathing basically the area between the spray foam wall and exterior rigid foam. I'm not a fan of standard OSB, but have considered the Huber zip system in this application with the felt covered exposed front. If not the zip system I would probably use 1/2 plywood. Either way would rather avoid house wrap if possible. Is one of these better than the other? In my zone is the 2" rigid going to keep from having "sweating" or "condinsation". Does the foam need to be seem taped and completetly sealed or is their merit in letting air through to help house breath? The exterior of the shell will be mostly brick. There will be some cultured stone and minimal vinyl siding in gables. Knowing this additional information any additional contribution of thoughts would be much appreciated. I'm thinking with this configuration if detailed right we should be able to get about a total wall Rvalue of R26. |
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lzerarc
Basic Member
Posts:423
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| 03 Aug 2010 09:03 AM |
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For brick you will definately want a cavity wall situation complete with weeps and mortar nets. This means you should have a pretty water tight sub surface behind your brick. The foam would need to be taped off. However for typical commercial construction, I detail out a cavity wall with 2" of rigid foam usually on 8" block or precast walls. I call for the block to have a spray applied vapor barrier. This allows some water to get through the foam, but stop there. In your case, the Zip would be perfect behind the foam. Since you have the zip, I would not really think you would need to tape up and seal the foam. However if you do, that will just make your house that much more tight. I assume your HVAC will also have an ERV or HRV with this type of construction? |
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Dana1
Senior Member
Posts:6991
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| 03 Aug 2010 11:12 AM |
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Posted By kschweitzer69 on 02 Aug 2010 07:48 PM
Dana1 - Thanks for the very detailed evaluation of moisture risk on this wall structure. Allow me to share a little more about our climate and wall construction plans so you can drill down a little deeper into evaluation. We are located near Cincinnati, OH which according to building science is basically on the border of zones 4 and 5. We tend to get hot humid summers and fairly cold spells in the winter, so we have to engineer for both types of situation. I was planning to use the foil faced 2" foam Rvalue of 13 on exterior, then stud up a 2x4 wall with the open cell spray foam. I'd like to keep total wall thickness around 6" to keep window and door trimmings within standard dimensions. The detail I'm not sure about and am concerned about is the exterior wall sheathing basically the area between the spray foam wall and exterior rigid foam. I'm not a fan of standard OSB, but have considered the Huber zip system in this application with the felt covered exposed front. If not the zip system I would probably use 1/2 plywood. Either way would rather avoid house wrap if possible. Is one of these better than the other? In my zone is the 2" rigid going to keep from having "sweating" or "condinsation". Does the foam need to be seem taped and completetly sealed or is their merit in letting air through to help house breath? The exterior of the shell will be mostly brick. There will be some cultured stone and minimal vinyl siding in gables. Knowing this additional information any additional contribution of thoughts would be much appreciated. I'm thinking with this configuration if detailed right we should be able to get about a total wall Rvalue of R26.
There seems to be great confusion about what it means to let the
assembly "breathe". AIR leaks should always be treated as part of the
problem, never the solution. WATER VAPOR PERMEABILITY is the
"breathability" that we're looking for, as the mechanism by which the
assembly can purge itself of damaging moisture. A square inch of air
leak can be worth a whole wall's worth of vapor diffusion through latex
paint & gypsum, or a seam taped layer of half-inch CDX. Air- sealing
is job-1! Then it's a matter of adjusting the vapor permeability of the
different layers of the stackup to avoid accumulating or trapping
moisture. "Vapor" always refers to water vapor, not air. (The
molecular size of water vapor is about half the size of the most-common
gases in air- N 2& O 2, and will pass relatively
freely though many materials that block air.) Air-tight, but vapor
permeable on select surfaces is what we need to control the moisture
hazards in building assemblies. With foil-faced R13 (yes, seams stagger, taped & edges foam-sealed) on the exterior of a 2x4" open cell foam studwall you have 50% or more of the total R outside of the studs, MORE than enough R to protect the sheathing & studs from condensation for Cincinnati's ~28F January average temp. With 2 layers of foil-faced iso on the exterior, moisture transfer via diffusion from the structural sheathing outward is for all practical purposes completely blocked- the stackup MUST dry toward the interior, but the exterior R value is high enough that you don't end up with seasonal accumulation of moisture in the studs & sheathing as long as you don't trap it there with highly vapor retardent materials on interior layers. With half the R on the exterior condensing conditions for 68F 30%RH air don't occur on the exterior sheathing until it's been +6F or colder outdoors for a significant number of hours- you can probably make this stackup work in places as cold as Duluth MN. In this case there's no practical difference between the different sheathing systems from a moisture point of view. The Zip system is easier to make into a full air-barrier, but the taped iso on the exterior is already an air barrier, as is the foamed cavity fill. But from a cost/performance point of view you might consider using wet-spray cellulose rather than o.c. foam, which will give it a somewhat higher R, but significantly higher thermal mass. Athough cellulose is slightly more air permeable, that won't be a performance hit here. If you lay a bead of caulk or adhesive underneath the stud plates as the studwalls go up, then caulk/tape at the sheathing seams the air-barrier aspects of the studwall will be quite high, and the additional air-barrier benefit of the foam quite low relative to the performance benefits you get out of the thermal mass of the cellulose in your climate. Wet-spray cellulose also lowers the potential pest-hazards- ants & termites tunnel right through o.c. foam, but the borate fire retardents used in wet-spray cellulose are lethal to those insects (but safe for mammals- used in everything from soaps & detergents to cosmetics, etc.) You'll need to include a venting cavity between the brick veneer and the iso, foam-sealing the iso around the penetrating masonry ties, and ventilate the cavity with bottom course weeps & top course vents. Insuffciently vented, there will be condensation cycles in summer that might eventually corrode the exterior facer of the iso. Sun on rain or dew wetted masonry surface drives a LOT of moisture into the cavity, which can condense on the iso as temps drop overnight if not allowed to purge via convection to the exterior. Masonry is a moisture reservoir, soaking it up liquid moisture via capillary action. Solar heating it vaporizes the liquid, and the RH in the cavity goes temporarily sky-high. The masonry can't re-absorb the moisture it released until it cools down below the dew point of the cavity air, but when that happens the foil is also a the dew point of the cavity air. Masonry walls are one of the few places where a controlled air leak is the preferred drying mechanism. Using silane or acrylic masonry sealers on the exterior can reduce the volume of rain/dew moisture taken up via capillary draw, without dramatically reducing the ability of the moisture to dry via vapor permeation. Use of metal/polyethylene capillary breaks between the foundation & masonry can also block the wicking of ground moisture upward into the masonry. (copper flashing is good, since it also blocks termites.) With a ~R26 clear-wall value, can we assume you're also planning to insulate the foundation to at least R16? (If not, it will present a performance-robbing heat-leak relative to the rest of the thermal envelope.) |
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Dana1
Senior Member
Posts:6991
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| 03 Aug 2010 11:31 AM |
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To max out the thermal & hygric performance of your wall design, you might want to read this in some detail:
http://www.buildingscience.com/documents/reports/rr-0903-building-america-special-research-project-high-r-walls
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kschweitzer69
New Member
Posts:64
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| 03 Aug 2010 04:10 PM |
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Thanks for the link to building science, I have been through that site extensivly the last several days studying up. I did see that article that you reference, although no proposed systems exactly like the one I want to do. If we should need extra space would 1.5" foam work in our area as well? I believe the R-value is R9.75 for it. I'm also wondering what the recommended spacing is for the brick to the foam insulation to allow for proper moisture control. The cellulose is a resonable idea that I would entertain, however, I still feel that spray foam is best. Cellulose according to many has a tendency to settle and I don't want product effectiveness to diminish over time. Based on your other comments I think the 1/2 plywood sheething may make the most sense in this setup. Since we will be tape sealing the foam the zip system almost seems like overkill with additional tape layer. Plywood is probably more econmically priced and more effective for us. |
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jerkylips
Basic Member
Posts:359
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| 03 Aug 2010 05:56 PM |
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Posted By kschweitzer69 on 01 Aug 2010 11:28 PM
Very close to starting our new home construction project. Went down the path of ICF and it was simply too expensive in my area. Going conventional and looking to build for energy efficency. Originally planned to build 2x6 walls fully spray foamed with huber zip system sheathing. Got to reading about thermal bridging issues in studed walls so came up with the following idea.... Build 2x4 framed exterior walls, use plywood or huber zip sheathing and then glue 2" hard ridgid foam to exterior wall surface. I thought I would use 2x6's to frame the windows and doors. Overall this gives me the same 6" wall, but addresses bridging problems. I figured I would spray foam fill the remaining 2x4 wall cavity. My biggest concern is possible wall breathing/moisture/condinsation issues between the foam/plywood/spray foam cavity. I'm thinking the 2" is thick enough to stop any problems. Thoughts please?? You've received lots of good info, I'll add one thing that I didn't really see addressed. It sounds like you're trying to get to a finished wall equivalent to 2x6 construction. I don't know that I'd get too hung up on that. extension jams for windows don't really cost all that much, and going a little thicker will improve your overall r-value. If you look back at some of my older posts you'll see what we did, but in short we did 2x6 walls, 1" of spray foam & then r19 batts (r-value in the cavity around r22, factoring in slight compression of the batts) plus 1" blue board on the exterior. on the non-bracing walls, we did just the 1" foam board, no OSB. In the structural areas, it was sheathed with 1/2" osb + 1/2" foam board. The overall thickness of the walls is only 1/2" thicker than a standard 2x6 exterior wall. Because the blue board is cheaper than osb, the overall cost difference was minimal (a couple hundred bucks) but it gave us complete air sealing, a thermal break, and roughly r28 walls. |
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lzerarc
Basic Member
Posts:423
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| 04 Aug 2010 09:03 AM |
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Posted By kschweitzer69 on 03 Aug 2010 04:10 PM
Thanks for the link to building science, I have been through that site extensivly the last several days studying up. I did see that article that you reference, although no proposed systems exactly like the one I want to do. If we should need extra space would 1.5" foam work in our area as well? I believe the R-value is R9.75 for it. I'm also wondering what the recommended spacing is for the brick to the foam insulation to allow for proper moisture control. The cellulose is a resonable idea that I would entertain, however, I still feel that spray foam is best. Cellulose according to many has a tendency to settle and I don't want product effectiveness to diminish over time. Based on your other comments I think the 1/2 plywood sheething may make the most sense in this setup. Since we will be tape sealing the foam the zip system almost seems like overkill with additional tape layer. Plywood is probably more econmically priced and more effective for us. leaving 5/8"-3/4" would be roughly enough air space for a house between the brick and foam. The trick is getting the mason to not clog the cavity with mortar. Make sure they use a mortar net and through wall flashing to get the moisture out along with the weeps and brick vents. |
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Dana1
Senior Member
Posts:6991
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| 04 Aug 2010 10:04 AM |
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Posted By kschweitzer69 on 03 Aug 2010 04:10 PM
Thanks for the link to building science, I have been through that site extensivly the last several days studying up. I did see that article that you reference, although no proposed systems exactly like the one I want to do. If we should need extra space would 1.5" foam work in our area as well? I believe the R-value is R9.75 for it. I'm also wondering what the recommended spacing is for the brick to the foam insulation to allow for proper moisture control. The cellulose is a resonable idea that I would entertain, however, I still feel that spray foam is best. Cellulose according to many has a tendency to settle and I don't want product effectiveness to diminish over time. Based on your other comments I think the 1/2 plywood sheething may make the most sense in this setup. Since we will be tape sealing the foam the zip system almost seems like overkill with additional tape layer. Plywood is probably more econmically priced and more effective for us.
Wet-sprayed cellulose is dimensionallly stable, locked in place by it's water activated adhesives. Dry cellulose dense-packed to 3lbs ft 3 or more also will not settle over time, even without adhesives, and substantial seasonal humidity cycling. (I can point you to the published science on the latter, if you read Danish.) Settling on the order of 10% in the first decade occurs in open-blow attic installations (~1.6lbs ft 3 density) and that loss of loft is included in the manufacturers specified R-value. In wall cavities there's typically ~5% over the first 2 decades dry blown using "2-hole method" @ ~2.0-2.5lbs ft 3 density. Going "blown in bag" wet-spray dense-pack is ~50% more material (and more thermal mass), and the R value of a 2x4 cavity will increase by another ~R1, but it's more labor intensive than open-sprayed wet-spray at 2lbs density since it requires installing the mesh, and inserting a dense-packing hose one cavity at a time. But even 2lb open-spray cellulose will outperform half-pound foam in your stackup & location, and will NOT settle over time. A masonry cavity of 1inch would be the minimum 1.5-2" is common. If you went with 1.5" of iso you're talking ~R10 exterior/R13 cavity, very similar to my first analysis with 2" XPS + opencell. Moisture won't get stuck in the plywood unless the average temps are around 16F outside, and won't be much problem unless that persists for a few weeks every year. Since wintertime outdoor average is ~28F, I'd say you have PLENTY of margin. If you can, wait on installing the cavity fill until the windows & doors are installed, then pressurize/depressurize the place with a blower door (or some big window fans). Buy a professional smoke-pencil and a 1-part foam sealing gun (or a small 2-part foam kit) & caulk gun, then run around finding & fixing all of the air leaks you can find (there'll be a lot of them- trust me.) Then insulate, but take care to air-seal the wallboard as best you can too. Re-test before painting, to find any leaking cavites that may have cropped up (or snuck by the first time.) Air-sealing really IS job-1. A well-insulated wind-tunnel is a waste of good insulation. |
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awhite
New Member
Posts:17
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| 04 Aug 2010 12:58 PM |
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kschweitzer69,
I'm curious how you plan to attach your brick ties through the rigid foam. I'm looking at building a similar wall here in Alabama.
Thanks. |
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| greencountydowns.com |
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Alton
Veteran Member
Posts:2157
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| 04 Aug 2010 01:50 PM |
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Awhite,
In effect, here is what I told kschweitzer.
Brick ties can be attached with longer nails if you know where the studs are. Screws hold even better but cost more and take longer to install. Brick ties have small holes so be sure you find a nail that is long enough with a diameter small enough that it can go through at least the 2" of foam plus the 1/2" OSB sheathing and then about 1.25" into the stud (approximately 3.75"). If long nails can not be found with small enough diameter, then the holes in the brick ties can be enlarged various ways.
In the Souteast area I have caught more than one recommended brick layer ignoring the brick ties we had already installed according to code. In other words, Trust but Verify.
kschweitzer, I hope you do not mind me walking on your lines. |
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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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| 09 Aug 2010 02:18 PM |
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Alton,
No worries, you have been a great resource for me. Glad to help anyone else out there seeking this information.
Dana1,
So if I understand what you are saying correctly the wet/dense pack cellulose in the walls is going to perform better than the open cell spray foam? And probably at a lesser cost. Also settling within the wall assembly will not occur? Cellulose is going to give us thermal mass that foam cannot provide, correct? Please note also that the rigid foam we are using is foil faced 2" foam not the XPS you mention above. I'm going to assume this does not affect the science of the wall system. We plan to tape foam seams and use either Zip board or 1/2 plywood as sheathing. If we used zip board we would not bother taping since we are taping the foam.
Another question regarding this foam has cropped up. I have thought about getting 4" of the foil faced rigid foam to place in the attic between the bottom truss chords and then have closed cell foam sprayed on the top of the bottom truss chords to bridge and seal the rigid foam seems and add Rvalue to the wood on the bottom of the truss chord. The only reason I'm considering this method is because I have ran onto an opportunity to get foam at a deeply discounted price. While this would only be a rating of R26, I would think I would get much better performance considering the air sealing that has taken place. |
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Dana1
Senior Member
Posts:6991
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| 10 Aug 2010 02:55 PM |
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Yes, wet sprayed dense-packed cellulose in an air-tight assembly will outperform most open cell foam. It has comparable or higher R that is stable across temperature (oc foam loses it slightly at big delta-Ts), and has many times the thermal mass of the foam giving it an edge during the shoulder seasons. Wet sprayed goods will not settle- it's glued in place. Even dry dense-pack won't settle until/unless it goes through massive wetting/drying cycles. Cellulose also protects the structural wood by wicking any leak or condensation moisture away. oc foam would dry more quickly after leakage events though. Low density dry blown is where most of the cellulose settling issues occur. Low density wet-sprayed clings forever- literally glued in place.
Tape the Zip, even if you tape the foam- multiple air-barriers are more likely to achieve & maintain the perfection you're seeking. If you use 1/2"ply or OSB, primer the seams 2" either side of the joint with a decent latex or latex/acrylic primer to form a good bonding surface on which to apply Zip or housewrap tape. It's nearly impossible to make it too air-tight, and every layer in the assembly counts.
Air sealing is job-1- seriously! If it's not air tight all of the business about vapor permeability etc becomes "who knows, who cares?, overwhelmed by the orders of magnitude higher air-transported moisture. A square inch of air leak is worth a whole wall's worth of vapor diffusion.
4" of iso between the truss chords is a good start, but not sufficient total R by itself independent of air-tightness. Even used/factory seconds iso board is going to be a lot more expensive per unit R than dry-blown cellulose. The compressive strength of iso is high enough to handle a cellulose overblow though, if you're going to do it anyway. Blowing the cellulose deep enough to cover the truss chords by at least 3" will give it significant performance boost since 3" of cellulose puts ~ R11 over the thermal short-circuit of the wood. (Low density wet-sprayed is used in attics too, if you're concerned about soffit ventilation currents moving the stuff around on you.) Going R50 with rigid board is several times more expensive than with cellulose. It's cost-effective with cellulose, but with iso, not so much. Going code minimum (or less) just because you're using more expensive insulation is a bad tradeoff if you can get 1.5-2x minimum with cheaper goods for the same money.
It's not as if you're saving anything labor wise on the air sealing doing a cut'n'cobble + foam-sealant in the attic with iso. You do gain something in R/lb. though, if you're looking to go high-R and doing an all-cellulose show would run into dead-load rating issues on the truss chords. 4" of iso + 8" of cellulose would make R50+, and form a thermal break over the truss chords with substantially less loading than 15" of cellulose.
I know I sound like a cellulose sales-droid sometimes, but I swear I'm not associated with anybody in that biz. It's just that in terms of performance per dollar it usually works out favorably. Foams are great for their air-sealing qualities, and closed cell foam can be used to adjust vapor-permeability, with a favorable R/inch, but in $/R, where you have the space to fill it up, cellulose tends to be a value-leader. If you're going high-R on the attic, be sure to frame it with sufficient space to let the insulation extend at full depth all the way to the exterior of the framed wall ( use "energy-heel" trusses, or frame it with studs that are a foot or so longer on the upper floor, etc.) |
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