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I can’t decide whether to use rigid foam sheets or plywood (or both) as exterior sheathing. I live in an area of very high winds year-round, so plywood sheathing seems like a good idea. Plywood sheathing also would provide a good nailing surface for the brick ties, since the exterior will be brick.
On the other hand, rigid foam insulation apparently prevents thermal bridging and adds extra R-value. The structure of my house will be 2×6, with fiberglass blown-in insulation. I live in the southwest, so my concern is not heating, but reducing the air conditioning load.
Or would I be foolish to use both materials?
Replies
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Bonnie I would go with plywood. The r value of the foam on 6" walls versus the strength of the plywood, to me it's no contest. And since You didn't ask I'm no big fan of blown insulation.For what it's worth. Skip
*Go with the plywood,or osb,and if the budget allows add the foam.As for your insulation, would blown in cellulose be better?Floating fiberglass particles scare me.
*Cellulose is safer, has a higher R value, and blocks radiant heat far better than fiberglass.
*Bonnie, et al. The 1977 Legre house, arguably the first double wall super insulated house in America, was sheathed with i-inch of Dow Styrofoam. The only bracing was let-in corner bracing which, a few years later, we learned does not work. After twenty years the house still stands, free of moisture or rotting problems. It has been subjected to dangerous high winds. By the bye. I'm not necessarily arguing for non structural RFBI sheathing. Just wanted to let you know how one house in Massachusetts sheathed with RFBI only, fared over twenty-two years in Massachusetts.If one opts for this method, use two full sized sheets of plywood or OSB vertically in each corner. The UBC--West coast code-requires that additional bracing be installed every 25 feet. Keep in mind that the interior gypsum wall board (GWB) provides some racking resistnace. The UBC code acknowledges this. Neither the BOCS or UBC says anything about metal bracing. GeneL.
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Bonnie,
I posted a message with a concern I have using rigid insulation with no wood product sheathing.
Along with the structural integrity issue, I am concerned about security. With no "hard surface" sheathing, intruders could cut through vinyl siding, rigid foam, and drywall.....never having to break a window!!
The national builder Ryan Homes seems to be big on this type of construction, i.e. rigid foam over studs (no OSB).
I am trying to find out results form long term tests of this method.
To your question, I am a traditionalist and OSB sheathing with fiberglass batts (esp with 2x6 walls) have shown to be a very effective insulation solution as well as structural.
Jerry
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Bonnie:
The brick ties should be nailed into the studs, not just the sheathing.
Jerry:
20 years ago I worked as a subcontractor on Ryan homes. Not sure what has changed since then, but from what you say, it sounds like things are about the same. What I am getting at is, at least back then, Ryan delivered very inexpensive square footage.
*Gene, if you say the let in corner bracing doesn't work, why is the house still standing?It sounds contradictory.And we have been using that sysytem here in MI for the last twenty years. And there aren't any leaning towers of pizzas here!Please elaborate.blue
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Skip: Why don't you like blown-in insulation? Fiberglass batts do not seem to be particularly effective at stopping air insulation because they never completely fill the wall cavity. Bonnie B.
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Bonnie;
I am a believer in Radiant Barrier insultation for reducing AC loads. Results are dramatic. You may want to look into this.
*Bonnie,I do not know where you are but I would go with Plywood or osb sheeting. FEMA requries this now because of experiences of wind loading during hurricanes in my area. If you use osb it does not have the nail holding power of plywood. Make sure siding is nailed into studs. If using vinyl siding staple it.Use batts to minimize shrinkage and setteling in your insulation. You dont want 6 inches of uninsulated space at the top of your walls 20 years from now.Do not use osb on your roof. Use 5/8 plywood. If you are in a humid area ventilate the hell out of your house.Good luckRick Tuk
*Maybe dumb Q: Why isn't (or is it?) T&G OSB or ply used on roofs? I know some people use those Simpson clips, most seems to just stick a 16d nail in for spacing. Doesn't this cause problems when the completed roof is walked on, esp. at low pitches?
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Reinhard: Thanks for your tip. I just checked into the availability of blown-in cellulose. It is available, but not common here. Do you think the fiberglass fibers would still be a threat once the drywall is installed? Bonnie B.
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Blue. No it is not contradictory. I've written and posted on Breaktime several in depth articles on wood corner let-in bracing. I wrote an article on it called To Brace or not to Brace for the May 1999 issue of Building & Remodeling News.
Codes require let-in corner wood bracing to with stand 5200 pounds of racking force. A considerbale amount of actual testing of braced walls shows that the let-in brace will break when less than 2/3 of the 5200 pounds of force is appliesed to the wall.
So why doesn't the wall collapse? Because the ultimate strength of the wall is dependent on the studs. The worse that happens is dyrwall and plaster crack, doors and windows stick and become hard to open and close. This is also true of metal bracing such as manufactured by Strong Tie. They do not recommend the use of their bracing only, and say it is to be used only as a temporary brace until the wall is structurally sheathed. GeneL.
*Andrew yes t&g ply can be used on a roof deck. The sheets still get spaced like any other sheet goods. The reason why it's not more common? $$$$$$$$$$$
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Speaking of tongue and groove. Floor decking panels say to space them. We always butt them tight with a sledge. Which is correct? How to you space them. Same question for wall sheathing...
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Not so much that it is a "threat" as it is inferior to cellulose, as Ron notes. If someone were to market cellulose aggressively -- for example, a pink version -- it would be more common. Note how much direct consumer marketing Owens-Corning does.
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I've seen a lot of guys skip the spacing on OSB roof sheathing and in the fall when frost forms on the roof you can see every seam in the whole damn roof. Somone else asked about the layout problem when spacing OSb sheathing, I've always just cut a sheet back to the next layout and kept on going. I think that this issue was gone over not too long ago in another posting which delt with 2x4 vs. 2x6 wall thickness, you may want to check the archives for that posting.
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Andrew:
The plywood clips are used (and required here) with 24" OC rafter (truss) spacing. If they are left out, local inspectors will make you block every plywood or OSB joint on the roof!
*Most wood or wood composite sheet goods,(decking,sheathing),are sized for spacing.The allowances are made so panels still hit 16" centers and have the spacing for expansion and contraction. In the real world does anybody do this? On a roof sometimes ,on a deck rarely.How do I space them?By not bangin em' tight with a sledge hammer!
*Although I know OSB isn't popular with a lot of people here, we use it for sheathing , subfloor and roof decking.I prefer 3/4" OSB to plywood for subfloor, as long as it doesn't get wet repeatedly. As far as roofs, around here the H-clips Andrew asked about are mandatory even for 16" centers. They do help and also space the sheets automatically. OSB might not be perfect, but we haven't had any problems with it so far. Pricewise it's not nearly as cheap as it used to be. I might be nervous if I was putting three-tabs over OSB, but I worry more about uneven rafters than joints in the OSB. Around here in the morning, when the sun is at a low angle, you can see every rafter on the cheap tract houses.
*Bonnie, The blown in insulation settles like the crackers that come packed by weight not volume. Some serious caulking and expandable foam should help the infiltration. The batts can be fitted pretty tight. I'm not hip to the cellulose versus fiberglass r value competition.The detailing around wall penetrations makes a sizable difference.Good Luck! Skip
*Nick,I also prefer 3/4 T&G on floors. And 1/2 on walls. But we have learned manylessons from storms. I saw a lot of shingles and metal roofs With nails and staples pulled out of the osb. It was easier to patch because there were no nails to get in your way. On plywood and plank roofs the shingles may have been ripped off but the nails were still in the wood.Rick Tuk
*Blue,The drywall contributes to the bracing more than most people realize. By the way it is easier to break a window than to cut thru vinyl siding.Ron
*The storms, particularly hurricanes, sure do the most relevant research work! "My" hurricane demonstrated the need to properly nail off sheathing, for example. I would not be eager to live in a foam home. OSB seems to be fine for wall sheathing, and really not overbuilding.The most common reason given here for switching from ply to OSB on roofs has been a the builder's bitter delamination experience from rain during the exposed phase of construction.On floors, I was surprised a few months ago that the overwhelming response here was for T&G OSB (yes, 3/4 or better). OSB's nail retnetion isn't important with flooring or siding where the fasteners go into the underlying studs/joists.
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Main reason T& G is not used on roofs: $. More to buy,
harder to install. Clips work because not too much traffic
up there and pigeons don't weigh that much. Also, how big
would a tongue or groove be on half inch or (less?) diameter
plywood/OSB?
Bonnie, I do agree with Andrew and Skip that the strength of
Plywood or even OSB for wall sheathing is unquestionably a
bigger plus than any r-value gain or sealing help that you
would get from foamboard. If you indeed have high winds,
you'll have a lot less worry, and a lot less cracked or
"moved" finish materials ie drywall.
Yes a lot of builders are using OSB for floors but not this
one. I don't even like it on roofs, the pigeonload
notwithstanding!
MD
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*Adam S: Yes, I was planning to use a radiant barrier in the attic because I live in a dry desert climate and heat gain is the biggest problem. However, I also wanted to use a radiant barrier in some of the walls, especially walls facing south and west; however, I haven't figured out how to accomplish this task. I need to keep the a/c air in the house and try to keep the hot sun out of the house. Bonnie B.
*Rick: I live in the desert southwest, very hot with regular strong winds and lots of dust.The builders in my area use osb; however, I do not want an average house, I would like a well-built house. I will make a note of your advice on using plywood over osb. I plan to have brick siding with a cement tile roof, so the question is how to best attach these materials to the plywood, considering the high winds here.I wanted to use blown-in insulation rather than batts in order to fill all the cracks in the walls so that expensive a/c air does not escape. Bonnie B.
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Bonnie: If you want something better than an average house and you plan to use a brick veneer you might want to check out a recent report regarding masonry wall failures. The report I read was in the Journal of Light Construction (Sept 99 issue) quoting one Clayford Grimm article in Southern Building (I have never seen that mag) and another Grimm article in The Construction Specifier. Even though you are in a relatively dry climate you may want to specify stainless steel brick ties. Grimm reports that common galvanized ties have rusted through in 10 years. The bulk of the JLC report on Grimms' findings have to do with the effect of windstorms on brick walls. Water and poor workmanship can combine to make a masonry wall much weaker than most codes anticipate. The workmanship issues include unfilled morter joints, using bricks that are too dry, over-sanded mortar, fat mortar joints and creating wall with bows. Add that to rusted out brick ties (remember a previous post that they should be fastened to studs) and brick walls over a certain height could end up crushing someone. I haven't had any experiences with this myself but thought you might want to look into it. BTW the JLC is hosting a building conference in Vegas on October 22 and 23 with a number of workshops and exhibitors booths. You may be close enough to drive there. http://www.jlclive.com Thor
*A detail on ply that doesn't get mentioned much -- the number of plies. 4-ply 1/2" plywood is much better than 3-ply 1/2", flatter and stronger. I have some 5-ply 1/2" that I ripped out in my basement; I keep it around for odd jobs where I can use its sturdiness. 4-ply is a couple bucks more than 3-ply, though lately prices seem to have gone up (be nice if they went down for a change).
*Thor I thought Your post was interesting in light of a recent experience of mine. A house I was working on showed a sag in the lintel over the garage door. The new homeowner got a mason out for an assessment. The lintel was put in 1/2 assed but when they started to repair it the whole wall above showed brick ties that had rusted, in some cases in half, and there were several stress cracks along the wall. I've seen a lot of houses with these cracks and wonder if it is for the same reason. They rebricked that area but there are stiill three sides sitting there.
*Andrew; you've metioned 4-ply plywood before. I've never seen or heard of this, and I'm wondering if it is an American thing. My understanding of plywood is , it always has to be balanced (a core + even numbers of veneers, alternating in direction: 3, 5, 7, 9 etc.) Definitely for interior stuff,it has to be balanced to meet the specs of the millwork orgs (AWI in the US, AWMAC in Canada), and last time I looked at the Canadian Plywood Association, everything had to be balanced. Is this stuff for a specail application or something?
*I won't suggest I'm a plywood expert. The APA ("P" no longer means plywood) agrees with you that plywood "always" has an odd number of plies: http://www.apawood.org/products/plywood.html.However, I'm fairly confident of my ability to count to four. Some builders offer 4-ply as an upgrade, e.g. http://www.jadehomes.com/misc/9000upgrades.htm. It's nothing special, even Home Depot sells this stuff (you gotta look way up high to see it sometimes); I don't know if 5-ply 1/2" is still available in the states. The face plies are in the same direction as each other, I should pull some apart to see what's inside. Anyway, it is definitely superior -- the sheets are flat, significantly sronger, and hold nails much better. The interior ply on 3-ply is just garbage. I plan to use 4-ply -- at a staggering increased cost of about $50 -- on my roof.Anyone out there have thoughts on this? This came up quite while ago, in the archives.
*Cesil, just don't bang them tight. It does requre a little more finesse, but once you get used to it, it's actually easier.And be sure to leave the 1/8th on the sides too!And yes, space the osb roof panelts too, both ways. The clips will not space it enough, you gotta kick it with your foot and staple it in one motion.blue
*Thanks for the reply Gene.Next question: if this is not passing codes, why are the local inspectors allowing us to use the metal braces, and foam?And some of these guys are the pickeiest people that I've ever encountered! We're not out in hicksville. They attend conferences, seminars, and read all kinds of code type literature to come up with new ways of torturing us oldtimers!But metal t braces always pass!blue
*They say there sizedfor spacing, but I've never senn any that acrually were.blue
*Thanks for the info, Andrew. I wasn't doubting you, I've just never seen it.
*Be glad to send you a piece! I wonder what the Post Office would think of a plywood scrap with an address and a stamp on it? I'd have to write "do not bend" all over it...
*Not to be an alarmist or accused of being Chicken Little but I would have a tough time sleeping (at night or on the job) if I had "seen a lot of houses", had at least a suspicion that a masonry wall MIGHT crush somebody and I had done zilch. Perhaps a copy of one of those magazine articles could be handed to Ms. Unsuspecting Homeowner and prevent a tragedy. Send one to your local building inspector--wouldn't it be great to show THEM that a higher standard should be required? The article I read (JLC) said that SS ties were REQUIRED on walls over a certain height in Canada, Sweden, Germany & Switzerland. In what part of the country have you seen these problems (could be expansive soils or some other problem)? Thor
*Thor over the years I've seen a lot of houses with what we call stress cracks. This was my first experience of seeing a probable cause and effect. Many houses get pointed up and I had come to the conclusion, perhaps erroneously, that this was a given. Now I'm not sure. Masonry is usually set on an adequate foundation and tied in other ways besides the ties. I don't know how emminent the danger is. Seems like this would have come out by now. Look at the scramble with EIFS houses. Next stop? Stainless steel nails for all exterior sheathing? Skip
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Thor: Interesting idea. . . the use of stainless steel ties. I had not questioned the bricklayers about the ties, but I did check and found that stainless steel ties are available locally. (Cost: $.05 vs. $.01 for galvanized) I am aware, however, of some problems with brick veneer walls due to poor workmanship.
I did find the Journal of Light Construction on the Internet and so I could possibly get the back issue with Clayford Grimm's article about masonry wall failures. I am very interested to read about the effect of windstorms on brick walls.
Your referral to the Journal of Light Construction proved to be unusually useful to me, since this month's issue featured an article on insulation and moisture problems for cooling climates, information which is incredibly scarce.
I decided to build my own house so I could make all these construction decisions myself. Bonnie B.
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Bonnie,
Since you're using brick for siding, the only thermal bridge will be the ties. There will be
an air space between your wall exterior and the brick interior.
With 2x6 walls and blown in insulation, you shouldn't really need the RFBI thermally
speaking. From a structural standpoint, you'll have a strong outer skin of brick to
absorb much of the wind, so with bracing at the corners, you don't really need to worry
about using plywood.
Unfortunately you've gotta use one or the other...
shop price for your wall sheathing and make sure it's well wrapped with 15# felt prior
to brick installation. Brick ties should definitely be nailed to studs. The strength
difference between a crappy mortaring job and an excellent one can be as much as
60%. Don't skimp on the contractor.
I've never done a blower door test on blown-in fiberglass vs dense packed cellulose
but from what I've read about the two materials, it seems that cellulose is the way to
go. It does need to be dense packed (2.5 psf I think?) or it will settle over time.
Where in New Mexico? I lived in Jemez Springs for a winter once...
Dan
*BonnieI highly recommend plywood or OSB over rigid foam board. It give a better nailing surface for your exterior siding and adds to the structural integrity of the house. On all the homes I build, I push for Icynene foam for insulation. It costs a little more, but it seals all the gaps, crevices, etc. that never seem to get insulated and let air in and there is no thermal bridging. The payback is about 2 1/2 years in savings on your utility bill. A house we finished 6 months ago was 1150sf, total electric and their highest electric bill was $72.00 (this is in the OKC area.) I believe the web site for Icynene is icynene.com.Vince
*How much do you pay for the Icynene in $ per sf per inch? My perception is that it is more than a little more expensive compared to fiberglass or cellulose or even foam sheets. Was quoted $1.75 sf for 2x6 rafter depth here.
*AndrewThe local dealer here charges $1.30 - 1.40 sf. The up front cost is more for the icynene, but here are some of the advantages other than the cost savings in the long run:* Four functions in one; combines the functions of building wrap, poyvapor/air retarder, caulking, professional air sealing and insulation.* Time saving; a 3000sf home can be insulated in one day.* Insulates and air seals at the same time* No sound or order transfer* Eliminates major causes of building envelope problems; controls air leakage around doors and windows, roof decks and wall joists, I-beams and soffits.* Virtually eliminates air-borne moisture movement, providing an effective barrier to air and moisture movement under wind pressure.* Eliminates moisture entrapment; resists water absorption and wicking.* Eliminates special air seal/vapor treatment; i.e. house-wraps, tapes, sealed electrical boxes, heated plenums and the frames involvement in installing an air barrier.The heat flow reduction through 3.5 inches of icynene is 92.2%, through 5.5 inces is 95%.Again, I'm not an icynene dealer, but they have me sold on it and it is the insulation I prefer and recommend.Vince
*For that typical 3000 sf house (17% of starts now are 3000+ according to NAHB), what kind of check would you be writing to the insulation people? 15k? (Just getting a sense of scale.)Thanks for sharing your experience. Getting the whole house done in one day must be EXTREMELY attractive, esp. since you're getting much more than the typical sloppy fg installation. I discussed with our Icynene installer the possibility of 2-3" of icynene over cellulose blown behind Tyvek for the roof to save a few $. R-30+ Icynene is VERY expensive. (Semi-cathedral roof, we're trying to escape ventilation and Icynene is an approved method here.)
*AndrewJust got finished rough framing a 3,500sf home with 9 & 10 ft sidewalls and approx half the house had a cathedral ceiling, 17ft inside (floor to ridge was 22ft). Homeowner went with icynene and his bill was $9,700.00. He had the underside of the roof sheathing sprayed rather than above the ceiling. The pitch on the roof was 9/12. The earlier house was approx 1,200 sf with a 6/12 pitch roof, 8ft side walls, gable ends and it cost $3,200. It is total electric and her highest electric bill was $72.00 this past summer.Vince
*Thanks! Numbers make it easier to grasp.
*Dan. It depends on the type of corner bracing. Wood corner let-in bracing is a NO no. Better method is two full 4'x 8'sheets of plywood, OSB or whatecevr the code allows, in each corner, installed vertically. The Uniform Builing Code (UBC) requires additional bracing every 25 feet of wall length.Tp prevent cellulose settling blow-in at 3.5 lbs/cubic foot density. GeneL.
*Vertically! That's what I've heard. But why why why? I assume it has to do with nail pullout strength? See confusing thread entitled Rick Tuk "Sheathing Direction?" 10/26/99 6:15am
*andrew d et al. Keep in mind that regardless of plywood orientation, a plywood panel supported at all eldges is always stronger than panels with some edges unsupported.If you do not block your exterior walls at the 4-foot mark, then you must apply the sheathing vertically if you wish the highest shear value. Otherwise apply the sheathing horizontally. Note, however, that this rule does not apply to roof sheathing. This is because roof sheathing is subjected to loads that are both parallel and perpendicular to the long direction of the plywood. To get greater shear value, increase the plywood thickness or nailing. Hope this helps. GeneL.
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I can't decide whether to use rigid foam sheets or plywood (or both) as exterior sheathing. I live in an area of very high winds year-round, so plywood sheathing seems like a good idea. Plywood sheathing also would provide a good nailing surface for the brick ties, since the exterior will be brick.
On the other hand, rigid foam insulation apparently prevents thermal bridging and adds extra R-value. The structure of my house will be 2x6, with fiberglass blown-in insulation. I live in the southwest, so my concern is not heating, but reducing the air conditioning load.
Or would I be foolish to use both materials?