1" foam board sheathing over residential 2x4 framing

[stevefitz]

On Apr 14, 04, thread 507-92407, Boo1 recommended 3/4 or 1" exterior sheathing to suppliment the R value of a 2x4 residential wall, and add a thermal break to the wood. I am interested, but I wonder how the finish veneers would be supported. -like cultured stone, or siding.

 

[Ron]

The most common surfacing for foam is synthetic stucco. Be careful with the waterproofing details. Make sure it is a drained system, not a barrier system.

Fake stone can be done, but might require supplementary tieing because of the weight.

Siding (HardiBoard) or similar can be used with special fasteners.

 

[ToadJones]

if foam sheathing is used directly on the studs, vinyl can still be hung without any problems. More rigid sidings like cedar or Hardi-board may look a little more wavy with out a solid substrate as they are nailed tight and vinyl is nalied loosely or "hung".
If you are placing the foam over top other sheathing like OSB, there will be no problems other than for some sidings like cedar, it is preferred to have a small gap behind so the material can breath.
The stone applications I have seen on my jobs where adhered over CDX plywood with a layer of "chicken wire" and a parge coat. I am not sure that the chicken wire and parge could be done over foam and in the case of stone you will certainly need a solid sheathing either way.

 

[SLTA]

We build panels with 5/8 CDX, 1" foam board, then vinyl or hardie every day. We use longer nails and nail according to the manufacturer's recommendations, and haven't had a problem with waviness.

 

[dhengr]

Stevefitz:

Exterior foam board is O.K., and good for its thermal break and wind barrier features when properly detailed and taped, etc. However, there are issues; is it permeable or impermeable and is their a vapor barrier behind the interior sht. rk.; i.e. will the wall dry out in one direction or the other, or neither, if it gets wet, and they always do? This whole issue is climate and region specific.

What provides the lateral stability for this structure, OSB sheathing does a good job, let-in diag. bracing (steel or wood) might work, 4x8 foam insul. doesn’t?

Ron is right on the money, re; stucco, but I would add, re; cultured stone, that because of its weight you need a brick ledge of some sort, plus masonry ties back to the framing. You want a proper water resistant barrier (WRB) behind any of your siding systems, and some air space too, so the siding can dry from behind.

As Toad says, vinyl can be hung directly, if nailed very carefully, it tends to be self draining. Most other sidings work better, and last longer, if there is an air space behind them. The best way to do this is with a WRB over the sheathing or foam insul. Over that you apply furring strips at each stud, and then fasten your, all edged and back primed, siding through the furring strips into the studs. There are several variations on the fastening methods, but generally you screw the furring strips to studs, to tighten the layers up, then nail your siding as usual to the furring strips.

Don’t forget good flashing and WRB application details at all openings, etc. And, remember that this added ext. wall thickness will alter jamb extensions and trim details around openings.

 

[racookpe1978]

Bookmarked for later. (South wall of house needs refinishing.... am considering foam over 2x4, then plastic sheeting and cedar (horizontal) over everything.

North GA region.

 

[dhengr]

racookpe1978:

Forget the sheet plastic thing until you study more, that’s not very good terminology. Good house wraps, WRB’s shed water and resist wind, they will not hold water (as in plastic), rather they are permeable so that they breath and the wall will dry to the outside. Don’t forget the lateral stability issue, which 4x8 foam sheets won’t address. In northern GA, you are near the dividing line btwn. predominantly heating and less humid conditions which call for the vapor barrier on the inside (warm side); and the humid predominantly interior cooling conditions of the south which call for the vapor barrier on the outside (warm, humid side, again). You want to keep the water vapor out of the wall, and keep it from condensing on anything in the wall cavity. And yet, let the wall dry in one direction or the other. I would want to study local custom and conditions a bit more, and I don’t mean ask the guy down the street who has a hammer and pick-up and calls himself a contractor.

As an aside: given what you used to do, I met THE Admiral (Rickover) a couple times in the 70's. I designed and built a bunch of transport equipment for hauling his new and spent fuel. Mostly special heavy duty railcars, shipping frames, saddles and the like. We worked with Westinghouse and GE who built the casks and containers, and for the Army who bought the ground transport equip. for the Navy, and we all had Hyman Rickover looking over our shoulders. That was really interesting design and fab. work, although it was a constant battle with nuclear people who thought we should design, fab. and tolerance to the same stds. they used on reactors and casks and the like. I suspect you’ve seen some of our equip. since you spent time in the shipyards.

 

[ToadJones]

dhengr-
You seem to have a good grasp on a lot of things so I would to ask you your thoughts on something.
Over the years that I worked building homes (in the mid west), often times insulation contractors would use wet blown-in cellulose in the walls but were adamant about not having a vapor barrier. They jsut wanted to make sure the insulation dried out for a week or so prior to drywall installation.
Also, I have almost never seen an insulation contractor place a vapor barrier on a ceiling prior to blowing in the attic.
Over the years, I have literally heard every variation on this topic you can imagine.
What are your thoughts?

 

[dhengr]

Toad:
I sure don’t have all the answers, I just keep my eyes and ears open, and try to read btwn. the lines on every piece of info. any of these product manuf’s. or installers provides. I have a jaded eye, they are selling a product and short of balled-faced untruths, they can say anything. I make an effort to understand what went wrong when I see a problem area, not just say ‘replace it in kind,’ and repeat the original error. And, I’m not always sure I get it right, but I sure do try to get it better than it was. I do spend a fair amount of my time investigating, commenting on, detailing fixes, testifying about building problems and failures and am always learning new things.

Regarding wet blown cellulose, and I’m in the Twin Cities, MN and northern WI, so most of my weatherization experience has been in the upper mid west. And, you are right, there are almost as many opinions and ways of doing these things as there are people doing that work. The story I got was that they could blow it denser and it didn’t settle as much with the wet/glue additive. Their claim was that if the cellulose was dense enough, there would be little air movement through it, and thus little vapor transport, therefore no need for the vapor barrier. And you’re right it needed a week to dry, so they sure didn’t want the vapor barrier applied then. This week caused scheduling problems with the sht. rockers. Most often they still had to come back and fill settlement voids at the top of stud bays, etc. But, what literature and testing I saw seemed to make sense. You still have to pay special attention (caulking, gaskets, tap over) at sub-fl./bot. pl., double top pls. to ceiling, rim joists/sill pl. and similar details or they become the leakiest joints.

I don’t know what sense it makes to put a vapor barrier behind the sht. rock on the walls, and then not do the same thing on the ceiling. You are just as likely, or more likely, to be losing hot air and moisture vapor to the cold attic as through the walls. Stud spaces around chimneys, plumbing vents, exhaust fan vents and ceiling light cans, dropped soffits, etc. leak a ton of conditioned air into the attic. Sealing these and the ceiling is absolutely essential. There is actually a fair argument for less insul. in the wall cavity, a vapor barrier outside the struct. sheathing and then most of the insul. outside the vapor barrier. This keeps most of the thermal mass in the conditioned space. That vapor barrier must be kept warm enough (balancing wall cavity insul. with ext. insul.) so that it never reaches the dew point temp. Then the wall cavity dries to the inside. However, EPS and the like are made out of petroleum, ya just can’t win.

The science is not new, thermodynamics, fluid mechanics, and the like give us most of the math we need to deal with the problem (understand, may not be the right word yet). We do know that vapor drive and temp. drive are from humid & warm to drier & colder. And, they and air movement, the transporter, are subject to pressure differentials inside to outside. But, lack of good sound mat’l. data and testing, and good consistent construction details as often as not are what shoots us in the foot. Codes and mat’ls. are changing so fast, we don’t learn from our just completed mistakes before we are making new ones. Codes said we needed more insul. and a vapor barrier (tighter houses) and now we are growing mold as if it were a cash crop. Every mat’l. manuf’er. wants the newest product to sell you, supposedly to fix this or that problem, you do the testing, and low-and-behold, you just built three new problems.

In my climate many people run their houses too humid during the winter and cause their own problems. Given enough money we could probably build a hermetically sealed box, with a few quadruple glazed port holes, for people to live in. But, no one would like those very much or want to pay for them. And, it’s a tough sell to convince people to conserve in small ways or change the way they do things in small ways, which if done by the many, could drastically reduce the problems we have. So, I still don’t have all the answers.

 

[racookpe1978]

Bookmarked.

Robt