Pretty Good Wall Design – Maybe
你好，
The attached file shows a wall design concept for a new house build near Minneapolis, Mn. I’m trying to design a wall that is both easy to build and has good performance.
I really wanted to bury the air and vapor control layers at the sheathing layer so the stud cavity only uses 2.5 inch thick rockwool batt (I’m firm on the rockwool insulation direction). Approximately 3/4 of the insulation is on the outside / cold side of the vapor barrier to prevent condensation on the sheathing. This also leaves 3 inches open for a utility chase. One big installation advantage is that fitting the insulation around wires and boxes is minimized since the batt is usually behind everything – no cutting needed.
The sheathing and control layers could be lots of designs. One is the Huber Zip wall system. Second is taped plywood plus house wrap or roofing felt. The third is peel and stick covering over plywood. Fourth is a liquid applied over plywood. I’m leaning towards number 2 for cost reasons.
There is one layer of 3.5 inch rockwool batt. Since there are no studs here the thermal break is good and moisture can move sideways if needed.
下一层是2×4螺柱，3.5英寸的岩石瓦特，再加上较高的perm天气屏障（即＃15屋顶毡浮装或房屋包裹）。这可以使风和雨水隔开，但可以使一些空气和水分通道。
Over this goes the 1×4 nailers. These create the rain screen vent behind the cladding, hold the insulation and WRB on tight, and act as a wide nailing surface for the siding.
The 2×4 studs are attached to the 2×6 wall at the top plate and sole plate – I haven’t worked that out yet. They are cantilevered off the wall and hold the siding (so only the 2×6 wall sits on the subfloor). The two outer layers of insulation (outside of the sheathing) are entirely on the outside of the walls and floors to get a good, continuous layer of insulation and air/vapor barrier.
I intend to have windows and doors attached to the 2×6 wall and sheathing just like a normal wall.
OK, so what do you think? I’m cautiously happy with this approach and how it balances performance, construction, longevity, and cost. All feedback is welcome.
Karl
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Replies
Just to clarify: Are you intending to cantilever the whole outside wall, or does it just extend down past the floor system and sit on the foundation too?
Build it.
You mention both 2.5 and 3.5 rockwool. Where are you going to get 2.5" rockwool? If you custom order it from Thermafiber, you'll pay a lot (and people have told me their product is not fun compared to Roxul). If you are talking about using the Safe N Sound, my understanding from comments made by others on GBA is that these non-thermal rated batts probably insulate at around R-3.3 instead of R-4.2 for the comfortbatts. Also, last time I ordered both, the SnS cost me more than the comfortbatt. Or was the 2.5" note a mistake?
Malcolm Taylor - I could do either one. Do you have any thoughts on those options? It is a single story house with a crawlspace and a standard foundation at 48 inches depth. I plan on insulating the outside of the crawlspace foundation anyway. Otherwise I was thinking of attaching the siding studs to the rafters with metal nailing plates (takes the vertical and horizontal loads) and attaching at the foundation wall with a pressure treated 2x4 on edge. That would be the bottom of the siding and below that would be XPS insulation on the outside of the foundation wall. If that's confusing I'll make a drawing.
AJ Builder - Thanks, I'll take that as a thumbs up! Much appreciated.
基思H-好问题。我计划使用Roxul Comfort Batts。根据Roxul网站的说法，舒适的巴特绝缘材料有2.5英寸的厚度可用于钢螺柱墙。我敢肯定这是一个特殊的订单，但我可以接受。我将不得不将侧面缩小1.5英寸才能安装在木钉墙中。
使用2.5英寸的原因是要在空气/蒸气控制层的内部维持绝缘值的1/4，以防止在非常寒冷的条件下护套上的凝结。我宁愿使用3.5英寸的厚度，但后来我在内部1/3，在外面2/3，我不确定是否可以。
我还可以将约5.5英寸厚的烤面包拆分至2.75英寸（我之前使用过泡沫板，所以没什么大不了的）。如果很难获得2.5英寸，那可能最终是我要做的。
Thanks all,
Karl
Karl,
I'd work hard to try and get both walls to bear on the foundation and avoid any gymnastics hanging all that outside it. - although not if it meant that the foundation wall width had to be increased (there's a puzzle for you!)
我也怀疑小优势丐ned using the 2"x6" inner framing is really worth the extra 2" of wall width and heavier framing just to minimize cutting insulation around the few electrical boxes and what little plumbing generally needs to be run in exterior walls. Especially as both the services remain safely inside the protected plywood air barrier. I'd be inclined to use 2"x4s and get my utility knife out.
As far as going with no sheathing on the exterior, Lucas Durant has done it on the house he has frequently posted here, and perhaps it is just the result of seeing the onerous flashing requirements necessary to keep walls dry here in the PNW, but I think it might take more forethought to get things right than most builders have.
Karl,
您想要一堵“易于建造”的墙，并且这是非常规的。您是否希望它为您“易于构建” - 所有者/建筑商 - 或为雇用的框架船员而言？如果您要雇用框架工作人员，“易于构建”通常意味着更常规的东西。
The most unconventional aspects of this wall are (a) the sheathing layer in the middle of your double-stud wall, and (b) the lack of exterior sheathing. This wall can be built, but some of the workers on your site are going to be scratching their heads.
如果您没有任何护套可以钉住这些元素，则必须仔细考虑固定固定固定要求和外部装饰的附着（角板，窗帘，门套管等）。
如果您没有任何护套可以将闪光灯连接到闪光灯，则对于闪光渗透（软管围嘴，外部电气容器，灯具）也可能具有挑战性。
As usually happens to me, I found the answers that I needed after asking the question - but not in the way that I thought (there is something buddhist or zen in there). One of the sites listed on the blogroll called nbsuperinsulatedhouse.blogspot.com has the details that I want in amazingly well done drawings. I'm going with the Larsen Truss approach shown in the construction details section. I just happened to click on that link an there it all is.
I also really like a lot of what is written in that blog. So there it is. Thanks for the input everyone - I'm off to do floor plan , etc. Gonna revive the Larsen Truss because maybe that Canadian carpenter did it right back in the days of disco.
Karl
Karl,
For more information on Larsen trusses, see关于拉尔森桁架.
有人可以评论装满绝缘材料的墙壁上的对流循环的效果吗？总的来说，当封装在所有六个侧面时，绝缘材料的性能更好。我想象的很大程度上取决于材料的密度，因此使用从玻璃纤维棒到刚性泡沫的光谱，矿物质羊毛的问题会有多少？即使隔热层内没有空气运动，其性能是否会大幅度降低，而表面在其与干墙之间的对流移动所洗涤？最后，鉴于内壁外部有太多的隔热材料，甚至会有任何对流循环，还是该腔真的像内部的一部分一样？
Martin - Many thanks for your help. You and everyone here have been great. GBA is a fantastic resource.
I have been looking through some of the Canadian research into wall condensation and it shows that the barrier layer can be much closer to the cold side than the 1/4 - 3/4. One recent paper is "Heat, air and moisture control strategies for managing condensation in walls" (archive.nrc-cnrc.gc.ca/obj/irc/doc/pubs/nrcc46734/nrcc46734.pdf) and if I'm reading table 2 correctly the control layer can be a lot closer to the cold side.
马尔科姆-我将th以来的猜测e temperature difference of all the surfaces in the cavity is very small then the convective looping is also very small and the wind wash effect is a negligable loss.
One other thing is that the open area might help drying the insulation. The moisture has a big open area to vent into instead of sitting in the insulation and captured between the drywall, sheathing and studs.
Also, the rockwool insulation is hydophobic. That might help keep vapor out of the insulation.
Part of the house will be 2 stories and to use 24" OC framing the 2x6 studs are required. That's part of the reason for the large open cavity - but I would like to use 3.5" insulation in the cavity if possible.
I would be very skeptical of the longevity of the house wrap or felt attached to studs 24" oc and unsupported by any form of sheathing. I plan a very similar wall design, with both walls 2x4 24" OC, but I'll use fiberboard under the house wrap. I also agree with the suggestion that the outer wall sit on the "foundation", obviously this requires that the "foundation" be of adequate width. (which is why I plan on a "double" walled wood basement) You'll need to "box" around windows and doors and carefully plan the "flashing" around them even though the actual attachment is to the inner wall. To minimize condensation risk it is best to keep the inner and outer walls studs inline
Karl,
Here is a link to Lucas Durand's blog which you may find useful:
http://ourhouseuponmoosehill.blogspot.ca/p/details.html
I second the motion, Lucas Durand has a great blog and great build.
Jerry - Agreed about the outer sheathing - I've added that now and thank you for pointing that out. I'm curious about lining up the studs and why that would minimize condensation - could you clarify that for me?
I went with the 2x6 because Wisconsin code only allows 2x4 at 24" OC for a one story and part of the house will be 2 levels. I thought that incorporating the open utility chase would be a good thing - any thoughts on that?
After further research it appears that I could go with 3.5" insulation in the cavity and 7.0" minimum on the outside ( 1/3 interior - 2/3 exterior). I will have to get an OK by the inspector (which code allows) since the vapor barrier is buried. If the inspector says no then I will have to find another way.
My current plan is for 100% plywood sheathing with taped / caulked seams and good quality house wrap. That should take care of the structural, air control, vapor retarding, and water resistance requirements. Any thoughts? Am I missing anything or a better way? I thought about 6 mil poly but current direction seems be be vapor open especially with a buried vapor retarder layer.
Malcolm and AJ - Thanks and I am looking at that blog now. I am continually amazed by the great work so many people are doing - very inspiring. His envelope design is very much what I was trying to do.
Karl,
I'll try to explain why aligned studs have lower risk of condensation on the sheathing. Consider the outer wall and inter wall insulation, the studs will be colder than the insulated spaces between them. With the cold strips of sheathing created by the outer wall studs faced by the warmest strips of the inner wall created by the inner wall's stud there is least risk of condensation but maximum heat transfer (a very small penalty, well worth the reduced risk of condensation).
FWIW, My plan is to sheath the inner wall with plywood applied over beads of acoustic sealant to the wall framing before the wall is "erected". Also a bead of acoustic sealant will be placed on the sub floor before the wall is erected. Wall corners will be caulked with acoustic sealant as well. With this method I see no reason for tapes or house wrap on the inner wall, the plywood itself is the intelligent vapor retarder and air barrier.
Jerry - Thank you, I understand your explaination.
The house wrap over the structural sheathing is in the code here and I would rather just put it on than fight. It seems like cheap insurance against water damage and provides a good drainage plane around window and door openings. The crews are accustomed to installing something over the sheathing.
我想我已经找到了我在阿拉斯加寻找的墙设计。Thorsten Chupp的北极墙设计（我知道他一直在这里发布）正是我想要的。我计划用岩石棒代替纤维素吹来的纤维素，并减少我气候的绝缘量。我喜欢蒸气开放设计。
Karl,
我想我正在以不同的假设进行操作。我的主要WRB不在所有的绝缘和框架之外，在我的外部饰面材料下的雨屏缝隙下。这意味着我所有的闪烁连接到此WRB。虽然内螺栓壁的外表面上的护套是结构性的，而我的主要空气屏障则不是WRB。我不希望由于雨水而导致的湿框架等。我可以看到您的方法会使闪烁更轻松，但可能会导致外墙的潮湿框架。我将与代码官员进行认真的讨论，因为该代码与您要建造的隔离墙无关。但是，如果他们坚持另一层房屋包裹不会造成任何伤害，但我仍然会在外墙外部实施主要WRB。
Karl,
为什么您如此开始使用Rockwool？这可能是一个语义问题，但我通常将这些东西与类似于石棉的旧隔热材料联系起来。我什至不再使用玻璃纤维，甚至不得不在现有的阁楼和墙壁改造中围绕它。所有这些无机材料对于肺，眼睛和皮肤都非常不利，并且具有高体现的能量，并且通常不太可重复使用。为什么不在泡沫中使用纤维素或吹干？
Curt:
//m.etiketa4.com/blogs/dept/guest-blogs/mineral-wool-insulation-isn-t-fiberglass
Jerry - Yes, I may not have been clear but there will be a WRB over the insulation (inside surface of the vented rain screen). I was thinking of having a second vapor open WRB over the structural sheathing but I'm not sure if that's needed - I suppose the outer WRB should take care of that.
Curt - I like the combo of fire and water resistance plus vapor openness and DIY friendly installation. Placed properly it should last 100 years or more and perform just as well as the day it was installed. It can also be easily removed and reused - remodeling friendly. I haven't seen any other insulation with that combination at a decent price and easy availability.
石棉可能不是最好的在任何一个类别but it scores high across the board and it has a century long track record for proof. Most of the other insulations have at least one big downside that need to be accepted (or ignored by the advertising department). Rockwool seems like that old, reliable material that just can't get the respect it deserves in the noisy world of selling the latest, greatest, whizz bang insulation material.