wood shingles over foam board

I’ve seen some related posts, but here’s my issue. The 1925 house has cedar shingle siding, and the south face needs replacement. I’m planning a full strip (on that face), blow in dense-pack cellulose to the empty plaster/lath walls, then add foam board to break the thermal bridge plus give a vapor seal.
Building Science shows what to do with strip siding (vinyl, alum, or clapboard), but what do I do with shingles?
The issue is an air break between the siding and the foam board. Furring wouldn’t really work with shingles. Is there some kind of screen material or dimpled membrane that could be attached in sheets to the foam layer? Would felt paper be enough?
How important is the air-gap/drainage-plane? Could I just attach to the foam board without the gap?
While I’m at it, is 1″ XPS the right foam? Will the shingles hold over everything using long ring-shanks, or can I get by without ring-shanks (hard to pull out, if the siding needs to be redone)? Note that the sheathing was tongue-in-grove, but with years of shrinkage, it’s more like plank-with-gaps, and probably somewhat brittle with age.
Am I missing anything?
thanks,
—mike…
Madison Renovations
Cambridge, Mass.
Replies
Furring DOES work, lay up 1x3 at the exposure, same as a roof with skip.
Spheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations
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>Furring DOES work, lay up 1x3 at the exposure, same as a roof with skip.horizontal layout? There will be an air gap, but won't water get trapped on the furring? Or do you just leave gaps along the length for the water to zig-zag down?I guess that would solve the connection problem, using regular nails into the furring, but I have to watch the hidden gaps in the plank sheathing...---mike...Madison Renovations
Cambridge, Mass.
If you have that much water zig zagging behind the cedar, you better get saavy on installing cedar better. The point of rain slicker and the like is for air to be behind the siding, not copious amounts of water.Spheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations
They kill Prophets, for Profits.
http://www.benjaminobdyke.com/visitor/product/key/homeSlicker
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It is quite important when you have a watertight membrane like a foam immediately behind the cedars. That would keep them wet on the back side and they would not last as long, and you would see more cupping and curing.
The thicker you make the foam, the more trouble you will have with this system and with trimming the openings.
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There are a number of building wraps that provide a breathing/drainage plane for use under shingles. Cedar Breather, Rain Drop, RainScreen, etc. If you are painting shingles, I'd highly recommend using it.
In New England, we deal with long winters. The lack of a good interior vapor barrier along with walls that leak air can lead to the water vapor condensing and then freezing in the wall cavity. This makes a mess of the insulation and drops it's effectiveness. If the water vapor gets trapped in the wall, it can lead to mold as well as rot in the framing. Foam board on the exterior is essentially a vapor barrier on the exterior.
You're in a catch 22 situation. My approach would be to seal up the inside so air doesn't leak into the wall cavity. Use a vapor retarding paint on the inside. I would want 1" of foam board on the sheathing. The extra might help keep the temperature up and keep the vapor in a gaseous state so it will dissipate rather than condensate. Make sure the blown in insulation gets in every nook and cranny. I would use a rain screen product under the shingles so they can breathe. This will help them retain any surface coating and allow moisture that gets trapped underneath the siding to dissipate.
Beat it to fit / Paint it to match
That was my general plan, although I wondered if I needed the vapor-retarding paint. Wouldn't the foam board be an air barrier as well as a vapor barrier, and the cellulose would add more air barrier?I guess we do have the humidifier going, so maybe the vapor-retarding paint might be a good idea. I wanted to make sure trapped vapor could work its way back out.Also, would nailing through the foam board hold up? Ring-shanks or smooth?thanks,
---mike...Madison Renovations
Cambridge, Mass.
When you change the envelope of the house, be careful with your old environmental habits.You're going to end up with a tight envelope, so careful with the humidifier. You don't want to be driving excess moisture into the cells when they have no ability to dry to the outside.
madison....
couple things..
first... you are in a termite zone... so all your foam should be treated or don't use it
2d... the furring strips will work ,, but they will form dams just as you suspect
3d
nailing shingles directly on foam is going to take a very long nail ... you have to go thru the foam into the sheathing ... foam is not a nail base
4th
i hate foam on the exterior... it screws everything up... all the trim.. all the siding... the sill detail
if you want foam ... think about inside and furring on that.. or boot it and go with a mooney wall
5th what plane are you going to install your windows in...
will they go on the sheathing that is fastened to the studs ?
then you are going to apply 1 1/2" foam and then furr ? and then side
what detail to trim your windows ?
6th...... in a past life, we used to do foam on the exterior..... we did it like this:
studs
1" foam with blocking for corner , window , door, and soffit trim
1/2" plywood sheathing for a nailbase
siding and trim to suit
everytime i've gone back we find the foam infested with vermin of every shape and species
Mike Hussein Smith Rhode Island : Design / Build / Repair / Restore
Edited 3/23/2009 4:56 pm ET by MikeSmith
Edited 3/23/2009 5:00 pm ET by MikeSmith
Hi Mike,Good obsevations:1. termites: there is about 2 ft of concrete foundation below the sill line. I'd hope I would see termite channels if they came.2. furring: Instead of 1x3's, how about RafterVent by DCI products:http://www.dciproducts.com/html/raftervent.htmper an article by Mike Guertin (with link to it on their page)? According to the article, a two-ply vent is 3/8" thick. These are corrugated plastic strips that maintain a space but have vertical channels. This wouldn't form a good nailing base like 1x3's, but would allow a good air channel.3. nails: I realized the nails would have to be long to penetrate siding (1/4"), rain screen (3/8" or less if Home Slicker instead of RafterVent), foam (1-1/2"), sheathing , and then some for holding the wood (3/4"), so say 1/4 + 3/8 + 1-1/2 + 3/4 + 3/4" = more than 3-1/2" nail. hmm... ...and I want a thin nail so I don't split the old sheathing. If I went with s.s. ring shanks, I wouldn't need to get past the sheathing so much. I've seen some 3-1/4" s.s. collated ring/screw shanks.But I'm somewhat concerned about having weight at the end of the nail. I realize that since everything is pressed tight to the wall, most of the force would be shear force which the nails could hold. Maybe gluing the foam to the siding in addition to screws would help? Then RafterVent would hold by friction/foam compression in addition to screws, and that would move the fulcrum point to just behind the shingles. hmm...4. casing changes: Actually, this sills aren't in the best shape, at least on the south side, and the casings have seen better days. I tried stripping and epoxying one sill - 13 layers of tough paint (I'm guessing mostly pre-1977, if you know what I mean) except where it's chipping to bare wood. The epoxy couldn't hold through a few seasons of movement, so I'm ripping out all the sills, at least on the south and west walls, the same ones that need shingles. All casings will be removed and replaced, too, so I can be done with most of my peeling pre-1977 paint issues. So adding some inches at this time won't be too bad. 5. window trim: I'm keeping the old windows. The second floor were replaced with double pane inserts. I refurbished a few of the first floor 6-over-1's, which have pretty good weather stripping: sheet metal around the frame that fits into slots cut into the sashes. Very tight. And I've been replacing the sash weights with sash springs so I can insulate the weight bays.The side walls (north and south) are about 12 feet from the neighbors on either side, so there's not much visual impact. Nobody's going to look and wonder why the windows are set back so far. Plus there will be storms.The house is craftsman bungalow style; casings will be flat.6. critters: I was considering using wood blocking at the bottom of the foam as well as at the casings and corners, and some synthetic cloth and/or screen to cover the rainscreen vent holes somehow. Perhaps a row of CedarVent Plus on the blocking.---mike...
Madison Renovations
Cambridge, Mass.
since your goal is energy conservation.... i'd spend the money and labor on replacing the windows and forget the foam sheathing
you'll save money and labor and energyMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
Good call Mike!
>since your goal is energy conservation.... i'd spend the money and labor on replacing the windows and forget the foam sheathing.interesting point. I'll explain the windows below.However, since I have to replace the siding, this will be the once in a lifetime (hopefully) chance to upgrade insulation on these walls. I can decide to upgrade windows later, but nobody rips off good siding just to improve the insulation. So I figure, this is the only opportunity to upgrade, no?I'd imaging the thermal bridging of the 2x4 walls would detract from the limited cellulose I could stuff in the cavities. Adding R-10 foam to the exterior, which adds a full air seal, should help with the radiant cooling effect.The one consideration is whether this is a good time to be adding such a large cost to the project. I could just add the cellulose and use simple siding techniques. With foam, I need to be careful about a lot more.BTW, the attic is fully foamed with high density PU in the rafters and 2x4 cavities, so we're not leaking out the top.Almost all the windows are double glazed plus storms, although only the ones on the new additions are low-E casements. The second floor replacements admittedly aren't the best. First floor has some old double-hungs for to match the style - craftsman bungalow with lots of old stained woodwork. Three of the single-glazed are fixed windows with diamond muntins. You can't match the old glass with new windows.The double-hungs have zinc weatherstripping set into a groove on sides and top/bottom of the sashes, and an interlock where the sashes meet in the middle. See an article:http://www.kk.org/cooltools/archives/002028.phpwhich has the link to the supplier Accurate Metal Weatherstrip Co.http://www.accurateweatherstrip.com/While not a complete seal, there is neither rattling nor drafts. I have been contemplating making inside storms to make a triple glazing with the outside storms. (Yeah, triple tracks are leaky, but they do help somewhat with wind wash.)---mike...
i think we're getting a look at your project thru a keyhole....what are you doing inside... wiring ? plumbing ?are you gutting rooms ?did i mention that i hate foam on the outside ?Mike Hussein Smith Rhode Island : Design / Build / Repair / Restore
>what are you doing inside... wiring ?It's some finish trim left on the inside. I need to have the outside painted, but the shingles are weathered, splitting and cupping on one side (south) and a dormer; in decent shape on the opposite (north), and replaced on the other two. Since the siding is coming off, I wanted to stuff the cavities with cellulose, but I was originally worried about vapor migration. With no insulation on a loose house, the cavities stayed dry.Adding high density cellulose, I guessed, might cause condensation problems. According to Building Science Corp., if I put enough foam on the outside, the temperature will be high enough to prevent condensation in the cavity, and it will keep the framing warm (again, less likely to condense) plus remove the thermal bridge.But cedar shingles against foam board seemed to be a problem, so I needed a rain screen/drainage plane/air gap to keep the shingles happy. (preprimed red cedar, if you're curious)I'd like to improve the energy effeciency when necessary other work is done, but not more than makes sense.It seems that there are risks of foam-boarding the outside the wrong way, so I'd need to be careful if I do it: enough insulation value, watch corners and casings with blocking, full seal, nails. I think I covered everything except cost, and now bugs, as you have brought up. What else am I missing with foam?I could either do what I need to with foam and get an R-23 wall, or forget about it and leave it as a loose wall with no vapor barrier (other than interior paint) but R-13 minus the thermal bridging effects.Then I'd like to forget about the exterior for a while.---mike...
did i mention that i hate foam on the outside ?
Yes, you did.
But would you consider returning to your previous method of foam under 1/2" ply sheathing if the vermin/infestation was a non-issue?
With sheathing to the exterior, wouldn't that take care of the siding attachment and trim issues you raised?
Not that any of this makes a difference for Madison....
Jim x 3
no.... you need too much blocking ... where ( which plane ) do you mount your windows ?you need blockig all over... any place you want to attach soething... if you don't ave blocking you run the risk of crushing the foam... taking things out of planei've worked on several homes where they just did foam and sheathing... no blocking everything sagged over the years .... including the windows nope.... i don't want any exterior foam..... it is self defeatigyou need the blocking and the blocking is a thermal bridgei'll stick to dens-pak cells and mooney walls if i want super insulation orspray foam the stud baysMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
>i've worked on several homes where they just did foam and sheathing... no blocking
>
>everything sagged over the years .... including the windows seems like a lot of weight away from the attachment point of the nails - sheathing, windows, and siding. I'd expect it to sag.I think foam could work, maybe if glued to the sheathing, but I'm an engineer, and therefore, concerned that engineered solutions need maintenance to continue to perform. It seems like there is too much to get perfect, and a lot that could go wrong if not perfect.I just know I need to keep cold air from blowing into the rooms, particularly around the outlets, but I don't want to invite mold. On the gutted parts, I did high density spray foam and cross strapping to reduce the thermal bridging, but I didn't do anything to keep the framing warm. Hope that's okay.If I just stuff with cellulose on the not-gutted parts, with only a Class III barrier (paint) on the inside and tar paper on the outside, will the vapor escape before condensing in the cavity? #15 tar paper is considered permeable - maybe use two layers before Home Slicker or CedarVent? Should I let my siding guy decide, or specify one or the other?Building Science Corp. indicates cellulose only would be okay. thoughts?---mike...In practice, the difference between practice and theory is larger than it is in theory.
I have to totally agree with Mike Smith. No foam on the outside. Spend the time and money sealing out air infiltration and on attic insulation, possibly windows.
>Spend the time and money sealing out air infiltration and on attic insulation, possibly windows.Attic roof and walls are high density spray foam, 5" rafters, full in the stud bays. I cross-strapped the ceiling and end wall to reduce thermal bridging. I think I've done what I could on the attic, so I'm moving to the next area.I can feel cold air coming in around the outlets, so I want to seal the walls, but without gutting the inside. Additionally, I'm stripping off the failing siding from one side of the house, so I have exterior access to whatever I want to do. Ideally, I'd gut and spray high-density foam everywhere (air and vapor barrier, eliminating cavity currents, and improving structural stability), but that's not part of the plan.The question is what, if anything, should I do while the siding is off and I'm redoing the sills and casings?Windows are actually in pretty decent shape, all things considered, but fall a little short of modern standards.---mike...
Caulk or otherwise seal any gaps between sheathing, and foam up any gaps under the sill. Sometimes the sheathing is not nailed very well at the plates, so you may need to re fasten it there. I like tar paper under siding, especially for cedar shingles. I think the air from your outlets may be entering through holes drilled in the top or bottom plates. Those must be sealed up. Sometimes the holes are 1" diameter which lets a lot of air enter the cavity.Also seal the baseboard to the floor if possible.I found a tremendous improvement in my house when I went to a condensing furnace, and blocked off the old flue.
>Caulk or otherwise seal any gaps between sheathingWell, the sheathing is pine planks from 1925, meaning huge gaps in the sheathing. That was the point of the foam board; to seal up the gaps.I guess I could take my Great Stuff Pro gun (which lets you dial in a really consistent bead size) and seal it all up. Maybe that's a good compromise.I'm hoping that dense-pack cellulose will prevent drafts within the cavity - I'm just concerned about moisture/mold. I'm not planning on opening up the cavities to seal everything. Maybe I could foam what I can reach through small holes in the sheathing.>Also seal the baseboard to the floor if possible.I guess I could caulk it. Won't be a complete seal with the leakage through the hardwood floor, but might cut things down a lot.>I found a tremendous improvement in my house when I went to a condensing furnace, and blocked off the old flue.I got rid of the old flue entirely allowing a small closet on the first floor, a repartitioning of rooms on the second (to balance an oversized room with an undersized one), and an unobstructed large vaulted attic space.I guess the best thing to do would be to remove the sheathing, spray high density foam into the cavity, then cover with insulated sheathing panels. I think that's more than I can do, so I'll probably not do much.---mike...
You may want to lay on a layer of plywood or osb sheathing over that side.
last thought... someone I know mentioned a closed-cell, low expansion foam for retrofit applications. That would cure my air & vapor concerns while getting a decent amount of insulation. I thought foam was too risky for blowing out my old plaster & lath, but my friend said there was a new product with low expansion.I want a high-density product that stops vapor, not open cell or loose like icynene - then I could skip the asphalt sheathing layer. Actually I could put up an inch or half-inch of foam to break the thermal bridges without fear of trapping moisture.Any experience with retrofit spray foam for enclosed stud cavities?---mike...
p.s. current plan is: strip shingles; stuff cavities with dense-pack cellulose; nail and glue asphalt impregnated fiberboard to existing plank sheathing; foil tape seams; one layer of #15 felt; HomeSlicker; new preprimed (all sides) red cedar.
Edited 3/27/2009 6:06 pm ET by MadisonRenovations
I'm not too familiar with retrofit foam, but I have heard of low expansive formulations. I think it is going to be cost prohibitive unless you insulate the entire envelope because there is a lot of labor in a small job. You can blow in cellulose yourself very reasonably, and it does a very good job of stopping airflow.I like the rest of your plan.
I decided against retrofit cavity spray foam because I'm not sure the technique has been fully fleshed out. So I went back to my painter/siding sub and the result was:original quote for siding work (not including window casings and sills) on one side wall, for 3 progressively smaller stories totaling about 800 s.f. minus 1 square's worth of recently replaced siding he was going to leave, then Tyvek and preprimed red cedar shingles with weaved corners: $6700When I asked him to strip everything, add 1/2" asphalt fiberboard (taped for a complete seal), felt, and HomeSlicker including venting top and bottom, and add a square's worth of the same to a small front dormer: $13,500 not including the materials cost for the fiberboard or HomeSlickerWTF?I asked him to detail why the doubling in price and he gave a list that wasn't itemized for cost.So recall that my plan was to stuff the cavities with dense pack cellulose, which isn't considered an air barrier, and I want to limit air migration through the cavity since I'm not doing an interior gut. I didn't trust Tyvek to hold up the air barrier, even when taped, so that's why I wanted solid fiberboard (more vapor perm than ply or OSB). The existing sheathing is old planks that have shrunken to leave large gaps between boards.This new price seems insanely high for 900 s.f. of siding. agreed?Am I going overboard with the fiberboard? Can I just go with a housewrap instead? I was thinking maybe Typar - seemed a bit stronger and higher air resistance than Tyvek, but lower perm rating.My original thought with the fiberboard was that I could glue it to the sheathing to help keep down air migration between the existing sheathing and my air barrier.Since I'd be adding felt and/or wrap (possibly two layers) behind the drainage mat, could I use cheap, thin hardboard as a seal, since I don't need it for structure?Any thoughts on getting the cost down? I will be shopping around if that's his price.Also, where do I get asphalt fiberboard and HomeSlicker in the Boston area?thanks,
---mike...
1/2" hardboard ( asphalt impregnated ) sounds d-u-m-b... i'd use 30 lb. felt with grace at the corners & windows... but Typar would be all right too then .... if you want a drain plane... i'd use cedar breather
just remember you have to detail how the cedar breather goes under the trim
with all of these LAYERS you do need to pay very special attention to the flashingswhich layer the flashings go to....
it should be sheathing.... flashing....paper/membrane.... cedar breather... trimbut the flashings have to protect the trim..... and they have to be flashings..... not funelsMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
also.... you are way overthinking this air infiltration thru cellulose problem....between dp cells..... gun foam.... caulking and blower door testing / candles
you should be able to control air flowMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
MikeSmith wrote:
> you are way overthinking this air infiltration thru cellulose problem....perhaps I am, and I need to hear that. I read somewhere that dense pack cellulose is not considered an air barrier. So, then what? (and my mind went wild :-)I just know there have been many cases of moisture/mold problems, and since I only want to do this once, I want to get it right... or at least right enough :-)---mike...
http://www.buildingscience.com/documents/reports/rr-0501-guide-to-insulating-sheathingWhat a great article this is....thought it might help.http://www.nationalfiber.com/uploads/universe/docs/wysiwyg/documents/National_Fiber_Cellulose_Without_Vapor_Barriers.pdfthis also will help as I have researched the same issues as you have.Let me know your thoughts
The other Mike wrote:
>i'd use 30 lb. feltBut isn't that the opposite of what I want? Since I'm only using a class III vapor retarder (paint) on the interior, with cellulose in the cavity, I want to stop air movement into the cavity (good air barrier) but let any vapor that does get in escape to the exterior (high vapor permeability) as much as possible. According to Building Science, #30 felt is semi-permeable (1-10 perms) and not an air barrier.My concern with housewraps is that the seams need to be sealed with tape to ensure the air barrier, and I figure the adhesive will fail over time, which would let moist air seep into the wall cavities. (bad) Also, whenever I see Tyvek, it's flapping around, which seems like it would allow air to travel behind it to the weakest points in the air barrier system. And because it's so thin, it seems that all the penetrations from the shingle nails could compromise the air barrier because it won't seal against the fasteners.Georgia Pacific's asphalt impregnated fiberboard has a rating of >15 perms. When used as sheathing, it needs to be taped to make the air barrier, but I thought that nailed against my plank sheathing, I could space the gaps over the solid parts of the planks, so I would still have most of the air barrier when the tape fails. I would then felt over that.Maybe instead of the board, I could use tall sheets of housewrap and use something like CedarVent (furring with air channels) on top at the seams at each floor level to give mechanical pressure to keep the seams closed.>if you want a drain plane... i'd use cedar breather
What's the difference between that and Home Slicker, which is the mat sold by the same manufacturer but for walls instead of roofs? Do you know where to get these in the area?>but the flashings have to protect the trim
I had been reading about how caulk isn't good for protecting the edges where siding meets the casings, because wood movement is more than caulk can move. Of course I'll have the top drip edge, but how do you do the side casings and edges of the sills? For the casings, I guess I could extend the wrap under the casings then use some adhesive metal tape (like Pella window tape - that's great stuff) on the wrap and up onto the casing up to the back edge of the siding.---mike...p.s., since I had the Lstiburek document open:Materials that are generally classed as vapor semi-permeable to water vapor are: Plywood, OSB, unfaced expanded polystyrene (EPS), fiberfaced isocyanurate, heavy asphalt impregnated building papers, the paper and bitumen facing on most fiberglass batt insulation and most latex based paints.Materials that are generally classed as permeable to water vapor are: Unpainted gypsum board and plaster, unfaced fiberglass insulation, cellulose insulation, unpainted stucco, lightweight asphalt mpregnated building papers, asphalt impregnated fiberboard, exterior gypsum sheathings, cement sheathings, and "housewraps."
Edited 4/3/2009 11:05 am ET by MadisonRenovations
Normal living activities inside the house generates a lot of moisture. Ever sleep in a tent? This moisture laden air will permeate through you interior walls. This is what causes most of the water vapor in the wall cavities. If there is enough of it and the temperatures are low, it will condensate and freeze. Since foam board will not let this air escape as well as a building wrap, it could cause problems. Older homes don't have interior vapor retarders and it's a major job to install them. Vapor retarding paints along with sealing air leaks can mitigate that moisture getting into the walls. 2x4 walls limit the amount of insulation you can put in. The foam board will add needed R value but if the cellulose gets wet, you aren't achieving your goal. I've seen a few houses that have had a non permeable building wrap placed under the siding. These were unusual cases where someone used a weird product, probably trying to save money. In the worst case, the wall cavities were moldy and the framing was rotted. This has made me very leery of anything that approaches an exterior vapor barrier in cold climates. It can be the opposite in hot climates where the moisture comes from outside and hits cooler conditioned air. You can't completely stop the moisture but you can reduce it with vapor retarders, we don't call them barriers anymore. Since your envelope, with the foam, will limit the exit of moisture laden air, it's best if you can reduce it's ability to get into the walls in the first place.Personally, I'd prefer to place the XPS under the drywall on the interior. This can't be done in your situation unless you are going to gut the interior. Cedar shingle siding is one of the better choices for keeping the weather out but it also keeps any moisture that gets behind it from getting out. The rain screen products provide the needed breathing room. When you nail the shingles over the foam board, you have to be careful to nail in straight. You will need a longer nail but it doesn't have to be ringshank. The typical tripple coverage you get with shingles won't be blowing off the house in Cambridge. I'd just use a longer galvanized box nail. 7d or 8d type A gal box would be fine at 2 1/4" - 2 1/2" respectively. Use stainless for any place a nail will be exposed, like your last row, under windows, etc.Beat it to fit / Paint it to match
>This moisture laden air will permeate through you interior walls.I thought the majority of moisture penetrating the interior surface was caused by exfiltration of moist air, and that if you keep the air from moving through the walls, then you keep *most* of the moisture from entering the wall cavity, and that as long as the humidity level is not constantly too high, that the vapor can escape to the inside as long as there is no vapor barrier.With dense-pack cellulose and foam board, the exfiltration should be near zero, no?The problem with nonpermeable house wrap with old-standard insulation (fiberglass batts with paper and/or separate vapor barrier on the inside is that the batts usually don't fit well enough to be a good air barrier, and then moisture gets into the cavity and can't get out from either side.Building Science Corp. recommends blown cellulose and exterior XPS. The idea is to keep the framing from being on the cold side which would cool the cavity. Their complete wall assembly, from the outside layers to inner ones: siding, airspace, XPS, cavity insulation, but they use 2x6 studs and I have 2x4's.
During the winter, in cold climates, moisture is generated indoors by human activity. The water is in a gaseous state. As this air passes through the wall, the temperature drops and the water vapor condensates and turns to liquid. If you open up a wall cavity, you will often find that the insulation is frozen, generally close to the sheathing where temperatures are lowest. Interior vapor retarders help prevent the gaseous water from permeating through the wall. You also need to seal up any place where the air can get through. The worst case I have seen was 2x4 construction, 4" of fiberglass and a non conventional building paper that was not permeable. I've never seen that type of paper before. It was like a dimpled kraft paper with a poly sheet backing. All the insulation was moldy and the framing was rotted. There was no interior vapor barrier. Since XPS will act as an exterior vapor barrier, I would want to stop moisture from migrating into the wall cavity as much as possible. It's difficult to install an interior vapor retarder without gutting. A vapor retarding paint is the only practical choice. I'm talking about 2x4 walls with blown cellulose in a cold climate, not anything else. The XPS will add thermal performance but it's not enough to keep the warm in and the cold out, completely. You don't want moisture condensing and reducing the insulations effectiveness and you don't want it to accumulate and possibly cause mold and rot. Beat it to fit / Paint it to match
hammer1: thanks for engaging with me. I want to be sure I understand what would be going on.you wrote:
>As this air passes through the wall, the temperature drops and the water vapor condensates and turns to liquid.Why would the air pass through the wall? If there is a good air stop, there would be no pressure for air to move. This would assume a complete seal, such as continuous foam board taped at seams. With the resistance of the plaster surface, there is no reason for air to move into the cavity. Now if I also blow in high density cellulose (not loose fill stuff) so that the cavity is stuffed, there would be even less reason for air to migrate into the wall cavity - no internal air currents that might drop air out the bottom of the cavity and suck warm moist air through the top, which would cool and sink to the bottom, repeating the cycle and drawing more moist air into the cavity.I recall reading somewhere that with spray foam in the roof rafter bays, there is no moisture problem without an internal vapor barrier because air has no reason to move into the bays so it tends not to. There is some minimal movement by diffusion, but nothing like being carried the air exfiltration with leaky insulation, like FG.With the case you presented, could it be that the building paper did not form a sufficient air barrier? Certainly the FG wouldn't. If you staple/nail through building paper to secure the siding, it won't seal around the holes the way I would expect foam would. FG could let the moist air travel within it, eventually letting it hit a cold enough spot, since it could get all the way to the uninsulated sheathing.What I was thinking was that moist air couldn't travel, well, anywhere, with high density cellulose. It forms a solid barrier all the way from interior wall surface to exterior wall surface. Furthermore, the studs shouldn't get as cold, with the foam thermal break on the outside. Wood does conduct cold, but doesn't it have something like R-1 per inch?I'm afraid that putting a vapor paint on the interior might seal in any moisture that penetrates the semi-barrier. You can have a much higher vapor gradient from room-to-cavity (forcing moisture in) than the other way (considering the exterior side is vapor sealed).Again, the point is that most moisture in cavities, from my understanding, is because warm air is leaking out of the building, and carrying moisture with it. When it hits the cold, it condenses and gets stuck a water in the cavity. FG doesn't stop air movement, and traps water. If you stop the air movement, most of the moisture is stopped. Some minimal amount moves by diffusion, and it should have an easy way to get out of the cavity. If the exterior is sealed, it should have an easy way to leave on the interior.Am I way off?---mike...
This article is better at explaining vapor and air infiltration retarders than I am. It's reflective of my personal experience and both the insulation techniques and consequences that I have seen develop since the 60's. I think it will answer many of your questions. I put a space after 'advice' to keep the link from running off the page.http://www.oldhouseweb.com/how-to-advice
/best-practices-vapor-retarders-and-air-infiltration-barriers.shtmlIn New England, blowing cellulose into 2x4 wall cavities isn't going to provide the necessary resistance to conduction loss. Conduction and water vapor travel together, so to speak. Your interior walls are permeable. The interior is like a hot balloon, constant pressure being exerted against the walls. I would try to keep interior water vapor from getting into the walls. With proper flashing, new siding and a rain screen, I wouldn't be as concerned with weather getting in. The one odd factor is the XPS over the sheathing. A double vapor barrier can be trouble. My thought is that the XPS won't be that effective as a barrier. Therefore, I'd use an interior vapor retarder in your situation.Beat it to fit / Paint it to match
After much research, I figured I'd provide an update. I decided to go with 1.5" of foil-faced polyiso on the exterior, with dense-pack blown cellulose in the cavity. (...and a drainage plane under the cedar shingles)The science says to keep the interior side of the sheathing from reaching dew point. I'll give a quick example with round numbers that are close to what I'm doing.The dew point for air coming from 70 deg, 35% relative humidity is about 40 deg. That means that however the insulation is done, you don't want the moist air to hit 40 deg or it will become liquid water. If it stays in vapor form, it can move about. You can get a graph for dew points vs. RH at a temperature from wikipedia:http://en.wikipedia.org/wiki/File:Dewpoint-RH.svgFind the slanted line that represents interior RH, find the interior temperature along the top of the graph, follow it down until it meets the slanted RH line, then draw a horizontal line to the right of the graph. The number on the right is the dew point.So my goal is to keep above 40 deg, using and average exterior temp mid-winter of 20 deg (25 deg for similar Chicago according to the Building Science doc RR-0410: Vapor Barriers and Wall Design:http://www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design )Some definitions:
Ti = interior temp = 70 deg (note: humidity 35%)
To = outside temp = 20 deg
Ts = desired minimum temp of sheathing = 40 deg (dew point)
Rc = R-value of cavity insulation = 13, figuring 3.5" @ R-3.7/inchThe minimum exterior sheathing insulation value to hit Ts at those conditions
Rs = Rc * ( ((Ti - To)/(Ti - Ts)) - 1 )
= 13 * ( ((70 - 20)/(70 - 40)) - 1 )
= 13 * ( ( 50 / 30 ) - 1 )
= 13 * ( 5/3 - 1 )
= 13 * ( 2/3 )
= 8.67An inch and a half of R-7/in foil-faced polyiso would be R-10.5One interesting thing is that since the walls are only 2x4, I need less outside insulation than for a 2x6 wall, because there is less of a temperature drop in the cavity (less insulation). This assumes that the wall temperature will equal the room temp, which may need air circulation to maintain the wall temp.Another interesting thing is that by putting foam on the outside, since the studs have even lower R-value, they will be even warmer so less likely to condense, all the way to the sheathing. In this case, the temperature at the sheathing near the studs would be 55 deg, which supports 60% RH at 70 deg interior.The recommendation is for no interior-side vapor barrier, to let it breath. If I do a good job sealing the polyiso, then I should be able to keep moist air from flowing into the cavity by convection, keeping reserve moisture-capacity in the cellulose.On the other sides of the house where the siding is not being replaced, stuffing with cellulose would only use a class III vapor barrier, which is a coat of standard latex paint, or special textured class III wall finish.thanks for the discussion and leads,
---mike...
Madison Renovations
Cambridge, Mass.
If you want a good air seal on the polyiso, use tape. If you want a good insulative seal too, gap the sheets when you hang them by 3/8" or so, then gun the gaps with canned foam. Shave off the cured excess, then tape the seams.Cells are great (in a good way) for being able to hold moisture until a drying session rolls around. I agree that if you're using polyiso on the exterior to omit a barrier on the interior.Foam and cells...they go together like peas and carrots. With polyiso on the exterior definitely furr it out, screw/nail the furring to the studs, and hang the siding off the furring strips.
>With polyiso on the exterior definitely furr it out,Is that because polyiso has less compressive strength than XPS? It seems to dent easier. Or do you recommend furring for any foam? That's getting pretty thick: 1-1/2" foam plus 3/4" furring. Plus it's a pain to add all those strips. What about a drainage mat, like Home Slicker (1/4") or one of the Waterway products (though the one listed for wood is 7/16" thick?---mike...
Firring is because he is shingling.
>Firring is because he is shingling.Make that because I am shingling. :-)But adding that much strapping, having to go around windows, etc., will add a lot of labor, plus it adds a lot of thickness on top of the foam board.I guess I don't really know how much the final time and materials costs compare. These mats are probably a lot more expensive than 1x3's, but it's probably less labor to install.The question is whether the foam, rain screen mat, and shingles can be supported by long nails into 80 year old sheathing.Any thoughts?---mike...
I wouldn't but..
Mike, one problem with that approach is that xps foam has a perm rating less than 1. It's a vapor retarder. For that matter, so is OSB. Use a vapor retarder inside as well and there's no way for the wall to dry. Using xps on the outside should raise the temperature of the back of the sheathing. In many climates, that's enough to prevent serious condensation problems.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein (or maybe Mark Twain)
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"Everything not forbidden is compulsory." T.H. White, The Once and Future King
Andy,I've moved off 1" XPS to 1.5" of foil-faced polyisocyanurate. Only Class III vapor retarder on inside, that is, latex paint. Can't escape that. Problems are more practical, like how to get so many the layers attached to the outside of the house (foam + rainscreen + shingles).---mike...
Pressure Treated lattice is a curious option.