Hello everyone, since this is my first post on this forum I’ll introduce myself first.. my name is Brian and I’m a design/builder in the northern part of Maine.. it’s cold here (9200 HDD) with virtually no CDD. I am in the planning stages of building a short term house for myself that I will treat like a model home, where I can demonstrate some advance building practices and I can experiment with some I’ve never tried.
Anyway here are my thoughts and plans…
I’ve tossed around numerous ideas for the shell of the building. I’m not thinking sips and the insulation will be Dense cell. I like the modified larsen truss wall, but around here the farther you stray from the “norm” it becomes a much harder sell, and it requires some special techniques (especially for insulating). But if I had a client that wanted that I wouldn’t hesitate. Same goes for double framed walls. I’ve thought about the “mooney” wall but 2×4 construction certainly isn’t enough and most builders around here already build with 2×6 with 1″ XPS on the outside.. personally I don’t think XPS on the outside is a good idea. While a 2×6 mooney eliminates the XPS I doesn’t give you any additional R. I played around with a double mooney, strapping on both sides, but that raises a lot of structural issues and really seems to complicate framing.
My latest thought is to expand the mooney. Frame up 2×6 24 oc using OVE and applying 2″ strips of XPS to the inside edge of the studs, then horizontal 1″ strapping over the top ( I know I will need some longer fasteners, haven’t exactly figured out what I will use, wish they made 4″ collated screws). This leaves an 8.5″ cavity and adds about an R13-14 to the studs. The reasons I’m thinking this is a good idea for me are; same amount of time for framing, doesn’t result in a much thicker wall than the norm here (2″), eliminates XPS on the outside, or a continuous XPS on the inside for that matter, and it provides a true R value around 30 – not counting sheathing or finishing materials (R27.5 to R30.7) depending on R of cellulose (3.5 or 4/inch?)
Any questions/comments/thoughts? I’ll post some more of my ideas later.. sorry about my first post being so long. :
Edited 3/26/2008 12:07 pm ET by egdc
Replies
Welcome to BT,
You're in good company. Even a few crusty curmudgeons from your neck of the woods.
If you haven't noticed, we've been kicking the same ideas around in the Super Insulated homes thread in the same folder. Lots of helpful ideas and comments from designer/builders of super-insulated homes, with years of experience.
BTW, Mike Smith, our Mooney wall expert, uses R3.7/inch for cells calcs.
Thanks for the welcome.. I hope I'm not a crusty curmudgeon yet. I may be little stubborn, but I'm not old and I hope I'm not crusty : )Yeah I probably should've posted over there, just chalk it up to BT forum newbie. I tend to use the 3.5 more.. I'd rather be on the conservative side when it comes to R values.
This is nearly the same idea I had posted yesterday. For my home up here in Wasilla AK. (10K HDD) Riversong thought that it would work. I was thinking of a couple of different scenarios. One would be Icynene foam full depth of the 2x6 cavity to the face of the horizontal strapping. Option 2 was adding 1" to 2" of exterior blue board sheeting (presenting a whole new problem of attaching the siding and window trim.) There is nothing here that time and money can't fix but I will be limited on both.
Re collated screws. I looked at http://www.mcfeelys.com/product/1037-FPT/10-x-3-78-ProMax-NoCoRode-PLUS and don't see any collated either in that length. One at a time would be a pita but could be done if you have enough time. The difficulty is in hitting the studs. This would also degrade the insulation to some degree as the steel would conduct cold to the studs. "Riversong" has some data on that.
A third method is to apply external (over the outside foam board) sleepers, then there would be a thermal break between the siding screws, also allowing standard collated screws to be used. Time to install the sleepers would be less than trying to install the siding over the exterior foam boards.
I'm trying to stay away from xps sheathing on either the inside or especially the outside for a couple of reasons.. These are my opinions, it doesn't let your walls "breathe", any moisture that gets in can't get out, I don't like the problem it creates with finishing details (outside trim/siding, don't like hanging drywall directly over inside), and I think its somewhat a wasteful use of resources. Granted it probably saves more than it uses, but if I can substitute some of it for more "eco-friendly" options I'd rather do that. Yes I will still use some, but the strips will only cover about 10% of the wall area instead of 100%. As far as the screw issue I can tack the strapping on with 3 1/2" nails, and use one 4" scew to secure (probably torx). Couple of guys with impacts wouldn't take that long. On a 32' stretch of wall if you strap at 16 oc there would be 119 screw (17 studs x 7 screws per), I would think 3 screws/min easy, so that would take 40 min. I don't understand why it would be difficult to hit the studs? I hadn't thought of the screw degrading the insulation value of the wall, I haven't seen anything about it being detrimental or any different than nailing strapping on the inside of any wall. Re the icynene foam.. no one in my area installs it, so I haven't used it myself.. It would be cost prohibitive in my case.I agree money and time can solve most problems.. and I like you will be limited on both.