My latest thoughts for walls for my “dream house”
My dream house will be located near the northern tip of Michigan over looking the shipping channel. The back will face north and have lots of windows for the view, I know terrible for heat loss but that’s what the boss wants. The good news is the front, street side, will face due south and can hold lots of PV. I’ll strive for exceptional air tightness and aim for r60 ceilings, r40 walls r30 basement walls and r 30 below the basement slab (using perlite). I’ll incorporate a masonry heater but heat with a Mitsubshi hyper heat minisplit. I’ve looked at oh so many walls and, I hope, finally settled on “the one”. Double stud framed out of 2×4 16″ OC with 3 1/2″ between walls. I’ll plan to make both load bearing to accommodate my large roof and snow load requirement, discussed on another post. The exterior must be brick to please the boss! I’ll use Ambrico EZ wall, a thin (1/2″) brick glued to steel substrate panels. Under the panels is Greenguard DC14, a 1/4″ XPS product that has drainage channels on both sides, a low permeability to block solar driven moisture and add r1 to the wall. Under the DC14 is Tyvec covering 1/2″ structural rated fiberboard that adds r 1.3. The stud cavities are filled with R15 Roxul comfort bats the inter wall space is filled with r15 Roxul comfort bats for steel studs. The inner wall, is sheathed on it’s outer face with 1/2″ plywood and has an interior service cavity filled with Roxul r 15 bats.. The interior is 1/2″ drywall. I’ll use aligned studs for greatest condensation immunity in winter. The plywood will be carefully detailed as an air barrier and is the only moisture control layer. With a mid cavity r value of 47.83, ignoring drywall and air films. and a mid stud r value of 26.6 at 20% framing fraction, which I’ll try to beat by adopting principals of advanced framing even though I’m 16″ OC with double top plates, I calculate r 41.2 assuming pure 1 dimensional heat flow and 40.9 assuming the plywood is a constant temperature plane allowing unlimited crosswise heat flow in the plywood, like it had an aluminum foil layer. At 25 % the results are r39.83 and r38.6 And the wall is only 12 3/4″ deep including brick. The construction sequence will be a bit unconventional as the outer wall will be erected unsheathed and remain so till after the first round of air sealing testing ( which may begin with temporary ” plugs” in the windows. I’m working on details of the transition from concrete basement to wood walls trying for minimum heat loss economically. I’d like to consider the wall plan finalized so if you see that I did something dumb PLEASE speak up.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Jerry,
If I understand correctly, you are planning to build a double-stud wall with steel studs -- or maybe you are using wood studs but insulating the wood studs with a Roxul product designed for steel studs? If the latter is correct, you should be aware that the width of insulation used for steel-stud walls is wider than the width of insulation used for wood-framed walls. I advise you to use the correct width insulation for the type of studs you plan to use.
It seems as if your double-stud wall will be sheathed with fiberboard. You plan to clad the walls with steel panels with glued-on thin bricks. I'm concerned that these steel panels create an exterior vapor barrier. I'm doubtful that the Greenguard DC14 ("a 1/4-inch XPS product that has drainage channels on both sides") will provide enough air flow to get rid of all of the moisture that will condense on the interior side of those steel panels.
Jerry,
You are going through the exact same thinking I am. You've been building the house in your head for a long time, right? The difference is that you have actually built a home before. The building site sounds beautiful.
Lucy,
Yes I've been "building" my dream house, both in my head and on paper, for about a year now, I guess that's a long time. The house I built, when I was much younger and dumber, ironically actually incorporated double walls. But they were an inner vertical wall and an outer sloped wall to create a "Mansard" roof. While it created an impressive look with curved sloping mansard walls on the second floor it was a real PITA for the rough carpenter, ME, to build and it was an abysmal energy hog, . I built the entire double wall combination,using my handmade trusses for 'studs' ,including curved sheathing, and overhanging soffits along with curved top dormers for the windows, and even roofing "on the deck" and raised the assembly then completed the exterior corners on a ladder from outside. A process I'l surely not repeat!
Martin,
Thank you for looking at this! Regarding the studs, they will be wood, what a thermal disaster if I used steel! My plan is to use 3 layers of r15 Roxul, the outer layer will be the wood stud version the middle layer, which is between the two stud lines will be the steel stud product taking advantage of it's extra width to fill the gaps behind each stud. The inner layer will again be the wood stud version of Roxul.. In other words I'm planning to use steel stud insulation where there are NO studs. The steel panels are pierced to provide tabs to locate the bricks so they are full of holes creating a rather imperfect vapor barrier and the reason I'm using the DC14. In addition the panels are embossed rather deeply to, according to the manufacturer, provide a path on both sides for liquid water to drain. In effect I'll have the DC14's drainage path AND the panel's path next to each other on the inside of the panels. Again thank you.