What is the best wall construction in North Carolina?
I have clients wanting to build a LEED (hopefully Silver) home in NC and I keep going around about the best wall system to use that is energy efficient, cost effective and structurally sound. We (builder, structural engineer and I) have decided on a 2×4, 24″ O.C. staggered wall design that is off set 12″ between studs and 2″ front to back. Top/sill plates are 2×6’s. 5/8″ sheetrock on interior, 1/2″ OSB exterior with a double layer of TYVEK DrainWrap taped and sealed then spider lath and stucco finish (looking for low maintenance finish). We looked into 1″ foam on the outside and the structural engineer didn’t like the strain it puts on the connectors to hold the weight of the stucco. Staggering the 2×4’s is to reduce the thermal bridging. I was concerned about a vapor barrier and was thinking about adding 15# tar paper to the OSB. The lot is tight and the spaces inside can’t get any smaller so I was looking for an option that wasn’t super thick. Any advice would be appreciated.
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Replies
Sophie,
So just to be clear, you are thinking about a staggered 2x4 stud wall using common 2x6 top and bottom plates? I have thought about using this wall in the past. I came to the conclusion that this wall assembly has a very high wood to insulation ratio and would not perform that well (you are basically framing 12" on center). They are great for acoustical reasons, and decrease thermal bridging, but lack sufficient insulation.
Regarding your drainage plane on the exterior. The two layers of Tyvek Drainwrap is overkill. You are using stucco so I would suggest a bond break between the one layer of Drainwrap and stucco. For that I suggest 15# building paper.
Whatever Michael Chandler recommends:
https://www.greenbuildingadvisor.com/homes/nc-home-grabs-five-green-ratings
Sophie,
If I were investing in a double-stud wall, as you are, I'd want to go with thicker plates than 2x6s. Since you've invested in double framing, can't you use 2x10 plates?
I agree with James that asphalt felt plus Drainwrap is much better than two layers of Drainwrap. I would use #30 felt.
Finally, you don't mention what type of insulation you are using. If you end up with 2x6 plates -- a relatively thin wall -- you can improve the wall's thermal performance by using closed-cell spray polyurethane foam.
Sorry about not mentioning insulation- Icynene open-cell spray foam. That is why I thought I would be getting enough insulation for this climate. The spray could go all around the studs.
Thanks for the Tyvek/felt answer. Very helpful.
I don't really have the space for 2x10 walls. I am pushing the setbacks on the lot right now. Is it necessary in NC climate? Most articles I read recommending this are in Northern climates. With spray foam do I really need the thickness? Down here, getting installers to do more that 5.5" is hard because of diminishing returns.
Martin, back in October didn't you recommend open-cell foam over closed-cell in your article "Open Cell Foam Beats Closed Cell Foam"? Any reason you say closed-cell now?
Thanks.
The author of "Open-Cell Foam Beats Closed Cell Foam" was Jim Coler, not me.
For most purposes, closed-cell foam is better than open-cell foam — if you can afford it.
The January 2010 issue (No. 208) of Fine Homebuilding had a good article on high r-value wall assemblies. It does a pretty good job of laying out the world of options for these types of assemblies, some of which are double wall, some not.
It might be worthwhile to revisit the outboard insulation approach. If you need a better load path for the stucco/fasteners you could anchor to horizontal strapping installed over the housewrap and place the rigid insulation between the courses of strapping. You won't get a continuous plane of exterior insulation but you will still dramatically reduce the exposed conductive surface of the framing.
Another thing to consider: I realize your question here was about the efficiency of your wall, but have you assessed how the staggered stud design, and the relatively large amount of framing lumber it requires might affect other LEED credits? I'm thinking in particular of MR1 Efficient Framing. It may not be enough of a factor to affect your overall certification strategy, but I thought I would mention it.
Hi Sophie
We had Southern Energy Management run the numbers on the 6" wall assembly over the 12" and the savings really didn't justify the $4,200 added cost for our 2,500 sf house. Of course, they came to the same conclusion on solar hot water and the reality is that there is no payback analysis that can properly compensate for the real possibility that energy costs might increase in the next few years but it was interesting to compare the cost/savings of ICFs, Hebel, and double wall as referenced to solar hot water.
I vote for one layer of taped drain wrap and two layers of 15 lb felt. The felt wrinkles when the wet stucco hits it and creates an air space in front of the Tyvek to create your drainage channel. Be sure to write on the plans that the Spider Lathe should have the rubber gaskets run vertically and should be stapled 6" on center each way with all penetrations through the gaskets. (How would you do that w/ 1" foam?) If the lathers don't staple it enough the stucco guys will get very grumpy and fill it full of roofing nails and they won't take the time to put them through the nailing gaskets. We use the 1" crown 3/4" leg galvanized roofing staples but 1/2'' crown galvanized sheathing staples work well too. getting the weep screed detail right can be challenging as with the expansion joints and the out corner
Builders First Source will make up wall panels for you to that spec and can really save a lot of hassle on tight lots with dramatically reducing the lumber deliveries and storage by delivering pre-cut floors, wall panels and trusses directly from truck to the foundation with virtually no lumber piled on the ground except plywood and bracing stock. Nice bonus on a tight lot plus the benefit of less trash & theft. (same price as stick built though)
I think open cell is fine but I usually save it for roofs and bandjoists and use JM spider in walls. Pricing on all this insulation is wildly volatile, get at least three bids, you will be shocked at the variance.
I don't like 5/8" drywall in walls because I generally get 6 9/16" jambs on my doors and have a hard time finding them with 6 11/16" jambs required for 5/8" drywall so the trim guys mash the drywall and try to torture the trim to make up the 1/8" which looks bad.
We just recently completed a LEED Platinum home in NC - we explored many different wall types and insulation options - the best bang for the buck we came up with was 2x6 walls @ 24"OC, 1" DOW SIS sheathing and Climate Pro blown in fiberglass which is dense enough to be an air barrier - we ended up with an R-28 wall. I have a hard time justifying the cost of spray foam in walls - typically advise our customers that the true value in spray foam is in the attic doing a "hot deck". I cannot commet on the stucco/foam combination but can't imigine there is not an easy solution for that. Foam is an excellent thermal break. I always use felt instead of tyvek.
Joseph makes a great point about loosing your points going 12"OC in lieu of 24"OC. Also adding to the cost - more material and labor for a staggered wall.
Just for fun, I would consider doing the National Green Building Standard as well - no telling which will mean more 15 years from now (I believe the standard). Good luck with your project.
Wow! Thanks for the great feedback. Some resposes:
Martin, sorry for the misquote. Although I am curious if you would trade the extra wall for the closed cell foam since you can get the same R-value in much less space.
Joseph, I Iooked at the FineHomebuilding article. Great article but I find myself caught between a super thick wall or the exterior sheathing (or a SIP). Thanks for the assembly option.
I was concerned about the additional wood myself which is why I am trying to explore all the options.
Hi Michael. Thanks for the stucco details, very helpful. You used a double wall system on your recent LEED home for aesthetic reasons (and cost vs AAC etc) vs your typical 6" wall, correct? What do you typically do? Do you use exterior insulating sheathing? I went to 5/8" sheetrock only to avoid wavy walls on 24" O.C framing. I appreciate the tip.
Danny, thanks for the alternative to the spray foam in the walls, Michael too, we may use these options to keep the overall cost of the wall in line. As we "value engineer" the cost of doing LEED is hard to justify on a custom home (Michael's article was also very helpful with this issue). We have started talking about other programs. I will look into NGBS as well.
Sophie,
For the highest R-value with the least thickness, go with closed-cell spray polyurethane foam or SIPs insulated with polyisocyanurate.
Here's one SIP manufacturer that uses polyisocyanurate -- I think there are others too:
http://yankeebarnhomes.com/building-process/true-panels-compared-to-SIPs.html#TheTenAdvantagesofTruePanelsoverbasicSIPsare
Sophie,
I think your original idea of the double wall is a good one, just go with a 2x8 plate. This will allow for 7 1/4" of insulation, couple this with fiberboard sheathing, housewrap and whatever else is needed for stucco to keep vapor drive at bay. You could do a wet spray cellulose or the new spider-insulation and easily meet superinsulation levels for your climate. I have used this wall in MN and it works very well with 7876 heating degree days.
Let's talk about the roof.
Doug,
The rafters are 11 7/8"BCI60S @ 19.2"O.C with open-cell spray foam (at the moment!), 3/4" plywood, 30# felt and 5V metal roof. All the attic is in the conditioned space. Some of the smaller rafter spans are done with 2x8's. The subs I use do 5 1/2" of spray in the rafters. Thoughts?
Sophie,
Looks like a code violation to me, but I don't know your local code.
The prescriptive tables for the 2009 IRC call for R-38 insulation in climate zone 4 (northern NC) and R-30 ceiling insulation in climate zone 3 (southern NC).
If you are using 5.5 inches of open-cell spray foam, all you've got is R-19.8. That's not much.
Thoughts?
Why are you building a cathedralized attic with a very poor R-value?
It sounds like the 5-1/2" foam decision is based on cost.
Probably advice from a foam salesman
If you want affordable high performance ...forget cathedralized attics and spray foam.
Sophie,
You might want to read Joe Lstiburek's latest screed on stucco failures and scrap the Tyvek for a double layer of felt. http://www.buildingscience.com/documents/insights/bsi-029-stucco-woes-the-perfect-storm. Felt is almost always superior in performance than plastic housewraps, particularly Tyvek which has far higher perm than is necessary.
Martin and John,
I was concerned about the Icynene thinness as well but then I received documentation and a letter from the company saying:
Icynene® is a thermal insulation and an air barrier material. It provides improved energy
performance as a result of convective heat flow control. The air seal advantage of
Icynene® provides improved energy performance over much higher R-value insulations
that are air permeable. The blower door testing of air tightness of the houses insulated
with Icynene indicates an average air infiltration rate 5.5 times lower than a house
insulated with mineral fiber insulation. Based on measured air infiltration and the number
of energy analyses performed we have found that Icynene®, installed in accordance with
the manufacturer’s installation instructions to a nominal thickness of 3” in the walls and
floors and 6” in roof/ceiling applications, will perform better thermally than R-19 and
R-38, air-permeable mineral fiber insulation.
Sophie,
Sorry, I forgot about you for a day. The R-value for Icynene per inch is not much greater than standard density fiberglass.. I have personally declared 2010 "the year of speaking out". In my opinion, forget about Icynene, I think it is a bill of goods. I have asked them for monitored results for one of there homes to prove their claims and they would not do it. It is open cell and it can hold lots of water and it is not a stand alone air barrier, at least in a cold climate. Your roof design as you have described it is seriously flawed and I believe you are headed for trouble. Let the good folks on this site work you through some quality roof designs, I am a fan of vented roof assemblies and have no problem with blown cellulose or fiberglass for that matter.
Robert,
Thanks for the great article.
Sophie,
Any building inspector who accepts the Icynene letter as a basis for allowing you to install below-code levels of insulation is failing to enforce the code.
I intend to focus on the misleading aspects of the Icynene letter in an upcoming blog. Thanks for sharing it.
In the meantime, don't be tempted to install below-code levels of insulation because of the misleading pitches of a spray-foam salesperson. You need thicker than code minimum insulation -- not thinner!
It is not only the foam industry spreading the wrong "message".
A recent Spray Foam Article(July 09) in Fine Homebuilding Magazine is reapeating the same "bad logic".
http://www.finehomebuilding.com/how-to/articles/choosing-spray-foam-insulation-what-you-need-to-know.aspx?ac=ts&ra=fp
Sophie,
I live in a "spray foam" house with a cathedralized attic.
My home is rather airtight and it performs well.
A good air control layer is extremely important and spray foam offers an "easy" solution.
I am not convinced that spray foam is an evil product...but I do know that it is very costly.
The spray foam companies encourage low R-value installations in order to keep the price from becoming a total deal breaker.
There are other ways to achieve a good air control layer...once you have a good air control layer... then adding more inches of non-foam insulation is much more affordable.
I think you can only go so far with spray foam until it becomes too costly.
I see a similar problem with cathedral ceilings and cathedralized attics.
Cathedral Ceilings can be risky assemblies and they demand much more surface area than flat ceilings.
Spray foam is a typical "solution" for cathedral ceilings .... but then you really get hammered on the cost.....High surface area multiplied by High cost per square foot = Not-At-All-Affordable
If you can create an air control layer at the ceiling or the attic floor then adding inches(or feet) of non-foam insulation is relatively cheap.
Another problem with cathedral ceilings is the thermal bridging at the rafters and ridge beams.
I thought the info was questionable, which is why I have been going around with these issues and appreciate all this input. I have cathedral spaces because of clients various needs some within the living space some for attic storage. I have the house priced out with spray foam and so far the clients are ok.
Michael talks about 8" of foam in the rafters in his recent article. I am still only at R-28/29 depending on the literature. So it sounds like I should try to do thin (2"?) coat of closed-cell and then balance the cost by adding another 7" or so of non foam insulation. Yes? I would like a min of R-38.
How do you address the thermal bridging? It seems that is a big topic on walls but not on roofs.
Sophie,
One cathedral ceiling detail commonly used in the northeast is:
1. Install sturdy vent chutes, soffit vents, and a ridge vent.
2. Install rigid foil-faced polyisocyanurate insulation across the bottom of the rafters. The polyiso should be as thick as necessary to get your total R-value up to your local code requirements.
3. Install 3/4-in.-thick strapping over the polyiso, creating a 3/4-in. air space with a radiant barrier on one side.
5. Punch a few holes in the polyiso and insert your hose to blow in dense-packed cellulose insulation.
6. Seal the holes with spray foam and install drywall on the strapping.
You can use a vaulted scissors truss, you can install foam on the room side, you can install a foam layer on the roof. If the Icynene solution was as great as they claim, the production builders would use it.
Sophie,
If you are really trying to position yourself as a promoter of energy efficient housing I suggest you follow thes guidlines.
1. The wall R- value is the local heating degree days divided by 180
2. The ceiling R-value is hdd divided by 120
3. The tested air changes per hour @ 50 pascals (ach50) is 1.5 or less
4. You install mechanical ventilation with heat recovery
You wii be a hero in the eyes of your clients and new customers will find you
Doug,
While I have been promoting (and building) super-insulation for 30 years, I think it's fair to point out that Harold Orr's 180/120 R-value formula is suggested for net zero or passivehaus homes, if I'm not mistaken. That's an extreme that not everyone wants to, or needs to, meet.
Robert,
I first heard of the hdd/180 formula for walls from Harold Orr at the 2007 PH conference in Urbana, I added the 120 for attics but Harold may have suggested this as well. I had the distinct pleasure to join Harold along with a few others to view his slides on some of his breakthrough projects in Canada.
I feel this formula is borderline superinsulation with the 1.5 ach50, a very achievable goal for all builders. This would be in Minneapolis (7876 hdd) a wall of R-44 and a ceiling of R-66 and I would estimate a btu/sf/hdd in the 1 to 1.25 range, Passive house is about .6. If I remember correctly in a presentation given by Tim Eian, a PH in Minneapolis would be walls R-70 and ceilings R-110.
Sorry for the delay. I was a little distracted with the snow down here! Thank you all for the great input. hdd/180 or 120 is really pretty achievable in the South. I have got a lot of good direction but some details to still sort out. Thanks.
Sophie,
I quick comment on Doug's post - he is correct to recommend mechinaical ventilation and an HRV is an excellent option in MN - here in NC, it is more likely you will want to use an ERV unless you are high up in the mountains. Good Luck.
Thanks Danny. I have to make sure it is appropriate for this location.
For those interested in more information on the question of whether 5 1/2 inches of Icynene can meet code requirements in areas where the prescriptive code minimum is R-38, my latest blog explores the issue:
https://www.greenbuildingadvisor.com/blogs/dept/musings/it-s-ok-skimp-insulation-icynene-says
Im glad i stumbled onto this discussion. I think ill have to find a copy of that magazine to shore up my designs for the new house i plan to build this year. I too have planned on doing a double outside wall to beef up R values and eliminate thermal bridging caused by headers and the top plate.
My plan was to frame the outside wall with 2x4 by 24" o.c. using 2x4 as the top and bottom plates, separating the two walls by 1.5 inches so i can connect the two with horizontal framing (not sure on spacing with this or if its even needed), then staggering the inside 2x4 by 24o.c. wall the 12" inches, using a 2x4 for the bottom plate and foregoing the top plate by connecting the studs to the roof trusses thereby eliminating the thermal bridging at the top plates. the open space on top of the wall cavity would allow me to blow loose fill dry cellulose directly into the walls attaining an r value of around 25 and not having to use crappy fiberglass or expensive spray on insulation.
now i have to figure out how to address the issue of lack of adequate insulation where the wall meets the heel of the roof truss. It is so sad that most homes built today have less than 5 inches of insulation here. im playing with an idea of using longer trusses and making a "pan" ceiling on the perimeter of the house to get the 20 inches of cellulose i desire here.
Tell me what you think. Any questions comments or criticisms would be much appreciated.
Uriah,
You're trying to reinvent the wheel. Double-stud wall homes have been built in many varieties for decades (I've done several) and they are all problematic. You cannot tie the inner wall to the roof trusses, which are designed to be connected to framing at the bearing points only. There are also many varieties of "high-heeled" or energy trusses that allow full insulation depth at the eaves.
A better approach which eliminates all the liabilities of double-stud framing, which I developed during 30 years of experimentation, is the modified Larsen Truss wall (or Riversong Truss), which you can see at: http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/LarsenTruss.htm
i don't understand how your method isn't double stud framing. could you please enlighten me?
looked through your site and it was interesting to say the least. i liked some stuff a lot and will think about it. some stuff i consider sub-par. sorry. no outside sheathing? i really dont like that. using green unseasoned lumber? not using treated lumber for sill plates? 1x4s holding up your roof? again sorry but that would just not pass code here.
also your drywaller put a vertical seams in the sheetrock right next to the windows. this severely compromises structural integrity.
by the way bearing points can be moved in trusses with proper framing. this is a non-point as the outside wall is the bearing wall. that gave me an idea about swapping which wall is bearing. i really like the outside envelope wall.
and i have installed energy heeled trusses but the ones we get here are only 8 inches deep. anything else is special order and the prices go up considerably.
but anyway thanks the link was great and give me many things to think about.
Uriah - yer sort of Hijacking this thread, might be better to start a new one but I'd at least comment that double framing is pretty economical either way you do it, for one recent project a> it cost us less than $1/sf in added framing and $.60/sf for added insulation to upgrade from 2x6 energy frame to 12" dbl stud.
Sophie - it was great to get together for a beer with you this week. Thinking further on our conversation I have to disagree with the thought that the stucco would be too heavy to be supported by nails through the spider lathe and foam. Remember that the lathe must be fastened six inches on center each way. if you use 1" crown, 2" leg galvanized staples through the gaskets and then through 1" foam or even through 2 layers of 3/4" foam offset with 2 1/2" staples you would have very good support and a great thermal break. the issue then is just how you want to detail the Tyvek, tar paper and window flashings. My sense is that it would be easier to Tyvek the OSB and set the windows and doors at that level and then install the foam, tarpaper and SpiderLathe over that with applied trim and extra flashings around the penetrations. Worth thinking about anyway.
Uriah,
You are hi-jacking this thread. But to answer your questions and concerns:
Double-stud wall framing is a method of platform framing, using the exterior wall as conventional load-bearing structure and adding an inner wall for insulation depth. Both walls can be on the same, wider, plate and built as one unit. The two walls can be on separate plates for additional thermal break and ease and quickness of closing in the shell. The two walls can have aligned or offset framing.
There are several problems with this approach.
1) excess framing material (less if inner walls are 2x3)
2) ambiguous load paths - the inner wall will end up carrying much of the floor and ceiling loads without having continuity of load path to foundation.
3) does not eliminate the major thermal bridging at band joists, which are very difficult to properly air seal as well
The Riversong Truss creates an unambiguous load path, since the inner wall is load bearing, uses less lumber. Full-dimension rough-sawn 2x4s are 45% stronger than KD 2x4s in compression parallel to the grain and 78% stronger in buckling resistance. RS 2x3s are stronger than KD 2x4s in compression and much stronger in buckling. And, most importantly, it eliminates both the thermal bridging and the air sealing problem at the floor decks and ceiling, allowing the simplicity of the air-tight drywall air barrier system.
The elimination of sheathing is optional, but reduces the total wood. With full 3/4" wood shiplap siding, sheathing is redundant. Except in seismic or high wind zones, the IRC allows the use of metal shear bracing. The 1x4 ledgers are also code-approved and have been used successfully since the beginning of light frame construction in the late 18th century in Chicago.
Hanging drywall vertically is common practice in commercial work, since it eliminates butt joints and makes a higher quality wall surface. It is much easier to do this, however, when the full 2" studs allow for more fastening surface. Occasional seams near window openings do not "severely compromise structural integrity" - in fact, don't compromise structural integrity at all unless you're relying on drywall for your shear bracing. And joint cracking is not a problem with my 1-2-3 drywall finish system (Durabond, EasySand, premixed). Even with green lumber, I've never had a joint crack (I do all the DW hanging and finishing myself).
So, not only is there nothing "sub-par" about a system which outperforms almost any other, but it's been approved by a number of code officials in several jurisdictions.
Uriah,
Where are you located, your wall design will be much different if you live in south Texas or northern Minnesota. I love the double wall system. it is cost effective, easy to build and performs very well.
sorry for the hijacking and being a little crass. it was late and i apologize. i definitely will start a thread down the road. i still have lots of studying to do.
and to Doug. i live in North Dakota, land of brutal cold, gale winds and shoddy construction.
I grew up and built in NW Minnesota so I have a real respect for winter, I will watch for your post, double wall is the way to go in my opinion. I did R-50 walls in Fargo, at the time, 9,343 heating degree days.
Michael,
It was good to catch up and brainstorm- Thanks again. I am going to rebound these ideas off the structural engineer and the builder and see if we can get any closer to a solution.