I’m looking for choices of wood for use as a sill plate. Here’s the situation:
Concrete foundation, 10″ wide. No openings for doorways requiring a header.
Double-framed wall above (large insulation cavity).
2″ XPS board insulation outside the foundation wall (with protection).
Sill will overhang foundation 2″ to cover top edge of XPS, so that
the sheathing & siding will just overhang the XPS & protective covering.
I-joists mount on the sill. Mfg info says minimum 1.75″ bearing required. Then add the thickness of the rim board (1.125″ there).
One choice for sill would of course be a 2×12 PT, dressed size 1.5″ x 11.25″. I’ve wondered about using a layer of 1.25″ rim board, 11.875″ wide, which would give me either an extra 5/8″ wall cavity for insulation or that much more joist bearing. Code usually calls for PT wood for the sill, or wherever in contact with concrete. But if there is sill seal between wood and concrete, there is no contact. What does code say about choice of wood when there is sill seal? I can’t find it.
The other thing I wonder about is the reduced thickness for nailing purposes. A rim board would be fine for nailing the ends of I-joists to the sill, but mfg info says to use 10d nails for toenailing rim board to sill. Toenailing will be at an angle, so maybe the penetration into the sill wouldn’t be more than straight down through the I-joist with an 8d. What’s your opinion on this?
There are other details of the wall construction above that I haven’t presented here. For the moment, I just want to focus on choice of sill material – what code allows and what thickness is really needed, for both code and practical structural reasons.
Replies
Dick, a picture is worth a 1000 words and I think a small section view of your situation would elicit the best responses. I've taken digital pics of the plans and posted them here with success.
Anyways, I have one concern that would apply if you use any sill: it sounds like you intend to install your structural rim board at the outside edge of the cantilevered (I think you are cantilevering the sill plate out to the outside edge of the 2" foam) sill plate. Structurally, I oppose that because the I Joists will ultimately bear the load that sits on the outside of the deck area.
In theory, your idea of isolating the sill plate from the concrete is easy to do. I would check with your building inspector to see what he would accept. A lot of BIs don't think too far outside of the box and it can get very difficult proving to them that a sill sealer will be suitable as a substitute for a treated sill plate. I'd opt to put two separate 2x6 treated sills with a space rather than argue with a building inspector about the need for treated lumber. Of couse, that raises another issue if there are tie down (anchor bolts) issues.
Bob's next test date: 12/10/07
Thanks for the reply, Jim. I hadn't wanted to post a pic yet, thinking it would only lead the conversation astray from the main issue of material allowed for a sill.So, here is a .pdf showing the assembly at the sill. This latest version has the outer wall implemented somewhat in the manner of a modified Larsen truss. The inner wall bears anything above, including roof and any second floor. Outer wall holds up just the sheathing, siding, and windows. Wall frames are tied together with 3/4" plywood or OSB at any top plates, so max height of a stud would be for a 8-9 foot ceiling. That gusset shown probably isn't needed with 2x4 studs in both walls. I haven't added any annotation or dimensions yet, as I am still playing around with the assembly. It is to scale, however.
Excellent pic.My concern was the structural bearing, so your drawing and explanation alleviates that. The secondary wall assembly amounts to a non bearing situation and the partial bearing certainly will carry that load. Your critical bearing surfaces are those that carry the ijoist. Your rim joist is solidly bearing and no problem. If you don't have the minimum bearing for your I joist, you will have to carry them with hangers. I wouldnt' advise that. I'd slide the sill plate in the 2" and run the foam up to the top of the sill. Sliding the sill that amount will give you plenty of bearing for the Ijoist and the "furring" will also have enough bearing because technically, it isn't changing. You will have about 1 1/2" of bearing for that "furring" wall in either situation.Alternatively, slide the sill and rip a 2" filler with wood. Bob's next test date: 12/10/07
Thanks for the comments, Jim. I have thought about sliding the sill over to be flush with the inside face of the foundation. As you suggest, I could rip a 1x to 1.5" and nail it over the outer edge of the sill to fill the space. Or I could make the sill a 2x10PT and slide the XPS up to the bottom of the bottom plate of that non-bearing outer wall.The only issue I see there is that nailing the sheathing to the sill would be affected. If the foam goes up to cover the outside of the sill, then the sheathing is nailed to the bottom plate of that outer wall, which in turn is only nailed to the sill. If I try to nail to the sill, I've got 2" of nail through foam arguably trying to provide shear strength (unless I beef up the inner wall to provide rack strength). Your thoughts?Also as noted before (was it?), I guess my fallback, and maybe not such a bad idea, is to set a second set of sill anchor bolts into the pour and bolt down two 2x6PT sill pieces, with a 1" gap between them.I ran this by the BI up there. We haven't finished cycling emails on it yet. I'll post what he finally says later.[I have to proof my posts better before letting 'em fly]Edited 2/14/2008 5:56 pm ET by DickRussell
Edited 2/14/2008 5:59 pm ET by DickRussell
Why aren't you running the I-joists all the way out and sitting both walls on top of the I-joists?
Joe Carola
Is it possible for you to run the rigid insulation up and over the top of the wall and then up again to the top of your rim joist? Lower the first gusset to floor level and set your outer wall sole plate directly on the foam. that wall is non-bearing so it doesn't need a lot of support. Sheathing would hang down past the horizontal piece of foam. The foam can support that much weight in compression if you get the right foam.
They can't get your Goat if you don't tell them where it is hidden.
Joe, as I mentioned in #9, the idea was to put a lot of insulation outside of the rim. So the rim bears the inner wall, which bears all the load above, including roof. Running joists and rim to the outside and having the load bearing wall be outside just makes the inner wall non-bearing, just for holding sheetrock away from the outside more - bigger cavity. More thermal losses through the joists, and the rim then has to be insulated from inside. Lots of permutations are possible. I'm open to all ideas.dovetail, in your suggestion, where is the rimboard - still well inside? Or is it near the outside, as Joe suggests? Then which is the bearing wall? If it is still the inner wall that is bearing, what is the load path? down past the i-joists via crush blocks?
I may have misunderstood your post. I thought you had decided on the inner wall bearing. Inner wall is load bearing, load is carried by a 2x rim and the joists in walls perpendicular to the joist, by a double rim or crush blocks under those walls parallel to the joists. Rim is set only far enough out to provide the bearing for the joists.
Exterior foam is carried up the wall across the top of the foundation and up the exterior of the rim. No added sill width is needed, or if one is concerned about the foam bearing the weight of the windows, sheathing etc. a wider sole plate on the outer wall could be used.
Cavity left outside of rim and foam is filled with blown insulation. Problem I see with my solution is that it separates the sheathing from the bearing wall. BI may quite reasonably find that to be a no go and require shear to be provided in/on the bearing wall.
They can't get your Goat if you don't tell them where it is hidden.
I think that if the sill does not come in contact with concrete you should be able to not use pt. BUT, the bi in your area may not accept. It is too insenifficant not to use pt.
Dick,
Just a thought here, How about bringing the rim/I-joist connection out to overhang the 2"? Then use 2x6 for your exterior(bearing) walls. 2x6's will give you 3 1/2" of bearing. I 'm no engineer, but I'm sure someone at the lumberyard or from the truss Co. could do a calc. of this situation for you.
It seems to me that your current design (with 2x4 exterior non-load bearing wall) allows for only 1 1/2" of full bearing, and even carrying just it's own weight, that seems like an insufficient bearing area. Not to mention the additional cost of the extra thick ( approx. 13" ) double wall construction.
As to your original question, I think your local BI will decide that for you, so I'd probably just go with the PT and wait for another time to "fight city hall" so to speak, you don't want to make any enemies before you've even broken ground.
Geoff
I agree that in the end it's up to your BI. They are the ones who interpret the code, and I don't really believe in getting into arguments with them since they would then probably start picking my work apart and make my job harder. I once asked if I could frame a basement partition wall bottom plate with white wood covered with ice and water shield (I thought I had seen that in the code somewhere) and was told no. So I quit researching it.
At any rate, I wouldn't be comfortable with rimboard because it is an OSB product which doesn't do well with moisture nor does it edge nail well so the wall sheathing to mud sill connection would not be that great IMO. Yeah, chances are you would come back in a 100 years and that house would still be standing there, but a mere 5/8" does not seem worth any risk.
I built a super insulated double wall house last winter. Your rim detail is interesting. Be interested in seeing your other details as well.
You make a good point about edge nailing into an OSB product used as a sill. I hadn't considered that. I'm not really in a strong wind-prone area, but still I'd like the structure to be really sound. Thinking ahead is pretty cheap, so I'm "building" this thing in my mind as I work out details.None of what I have done is particularly original. It's the result of a collection of ideas from others. The idea of setting the rim/joist connection inward, so that the insulation cavity is outside the rim, came from Gene Leger back in the early 80s.I heard back from the BI. He wants to see PT as the sill. Given that, I guess I'll have the second set of sill anchors set into the foundation, then bolt on two 2x6PT, an inch apart, so I get the sill under the outer wall over the XPS, and the inner sill flush inside for more bearing of the i-joist ends.The BI did add that he had no particular problems with the wall structure I sent, and added that new ideas and materials come along all the time. He seems open to doing things differently from ordinary, but added that I ought to run everything by an engineer (which will be done anyway). He has said he'd be glad to review drawings as they become available. He'd rather do that than have things changed farther down the road.Geoffrey, I've arranged things as shown in the diagram so that I have adequate insulation outside of the rim. That avoids having to fuss with insulating/sealing, covering for fire protection, etc from inside the basement level. At that junction, the only thermal bridging is through the sill itself, and that will be about R12 if a single 2x12, or more if a pair of 2x6 separated by an inch of cavity insulation (I may foam that 1" gap).In an earlier design, I did have 2x6 outer wall studs, but 24" oc, and that wall was to hold up the roof. For ease of assembly, my present thinking is to have the inner wall bear all load, then tack on the outer studs one at a time. This revision in thought comes from reading the thread on the modified Larsen truss and some PMs with Riversong. It's just another way of achieving a double wall cavity, with R40, and minimal thermal bridging through wood. It's just a matter of which wall bears what load, how shear is transferred, and how easy the assembly goes up.I did a very rough estimate of cost for material and labor to add the outer wall. Yes, it's an extra cost (and my estimate was very, very crude), but it ballparks the cost and tells me that it will be very small compared to the whole house. Heck, many people add just that in frills when buying a new car. But that's another issue.As to the bearing on 1.5" of the outer (non-load-bearing) wall, when I run the calcs for typical stud-grade wood, that 1.5" is far more than required.I may make further changes as more detail comes together, but this is where I am at the moment. I still have to finalize the interior layout. I suppose the wife is gonna want a kitchen somewhere in there, but that part's dull stuff.
I've heard of using the inner wall as bearing before. Are you sheathing the inner wall or the outer wall? Seems like the bearing wall should be sheathed for resistance to lateral loads, but I don't really have any engineering background--It just seems a little funny that one wall would be load bearing and the outer wall would be a shear wall. I think from a construction standpoint, the method you are suggesting sounds labor intensive.
It just seems a little funny that one wall would be load bearing and the outer wall would be a shear wall.
Not at all. The inner walls are taking only vertical loads (gravity and uplift), while the outer wall is taking the lateral loads (wind & seismic).
I think from a construction standpoint, the method you are suggesting sounds labor intensive.
Not compared to many other superinsulation systems. It takes a little more wood and time, but with pretty conventional framing methods - goes up pretty fast. And it makes insulating and air/vapor barrier simpler as well as eliminating most of the thermal bridging.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
I was the one who suggested using rim board for sill...
I wouldn't be comfortable with rimboard because it is an OSB product
...and, depending on manufacturer, is available in LVL - which is what I recommended.
Too bad the BI won't allow it.
But I think the two PT 2x6 sills is the answer, since this satisfies the BI, offers a thermal break, fully supports and fully anchors both walls, and offers "grounding" to the outer sheathing.
One thing to keep in mind for any double wall system is that load paths won't necessarily be where you plan them to be. If the outer wall is connected to the roof system, then some roof load will be transfered through that framing, particularly if the inner (ostensibly bearing) wall is interrupted by a sawn lumber floor system which will shrink in height while the outer (uninterrupted) wall will not. This could transfer additional roof load onto the "non-bearing" wall.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
I built a super insulated double wall house last winter.
What kind of wall system, insulation air/vapor barrier system did you use? Can you post a cross-section?
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
We used a double 2x4 wall, 9" thick. We used fiberboard sheathing for the most part at the request of the owner and architect. I used separate 2x4 plates. I boxed the window RO's with 1x10 ripped to 9", so the window could be foamed to the 1x and the air barrier sealed to the 1x as well. Rim joist was conventional (14" rim board and I joists). The outer wall was bearing, which allowed us to shell the structure fairly quickly. I also wanted to avoid squash blocks all the way around. We treated the inner wall as a partition with separate plates. I made a mistake in that I should have run a piece of plywood tying the top plates together--I wound up having to fit foam in that space to prevent convection out of the wall cavity.We used dense pack cellulose insulation in the walls, with a 6 mil poly vapor/air barrier caulked to the floor plywood. Rim joists were spray foamed which provided a continous air barrier from the foundation (ICF) or wall below to the floor plywood. There were several flat roofs which also got 14" of dense pack plus sloped EPS foam on top. Also used Thermotech windows (canadian made). The owners are thrilled with the energy performance of the house. They tried turning the heat off for a day that topped out in the single digits above zero-the house was actually still over seventy when they got home from work! I called them in mid November to see how their heating system was working, and they hadn't even used it yet, although my heat had been going for several weeks. Pretty impressive.
Hmmm...
Several problems with that kind of construction. It doesn't eliminate the aggravation of insulating and airsealing the rim joist area (as it would if the inner wall was load bearing). And it doesn't eliminate the need for squash blocks because some (or maybe most) of the floor load will get transfered to the inner wall which probably doesn't have a load path to ground. This is the most common engineering problem with double wall structures.
1x window boxes aren't as stable as plywood and the expansion/contraction with humidity changes can break the bond with the foam.
And, unless you offset the inner and outer studs, there would be only a 2" thermal break between them, for an R-16.4 at the studs instead of the R-34.2 in the cavities.
Double stud walls are problematic, which is why I abandoned them years ago and went to the Larsen Truss system, which is what DickRussell is modifying for his system.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Why is insulating a rim joist aggravating? Do you not have access to a good spray foam contractor? Also, how is air sealing handled with your method? Are you caulking your mudsill to the foundation, your rim to the mudsill, and your plywood to the rim? That isn't aggravating?You don't have to fit the inner wall tight if it just functioning as a partition. I left a bit of settling room. The foam is between the window and the 1x. Any shrinkage, probably negligible, would occur in the width not in the thickness. It would be no worse than (probably better) than conventional framing methods. I have taken out enough foamed-in windows to know that that foam doesn't pull apart.No reason you can't offset the studs. I did.The rim joist detail is interesting and has some value. But it is labor intensive. Two things are really going to cost you there--framing stick by stick from the outside and sheathing from the outside. Even with a lift (which isn't always feasible on the sites I often build on because of the terrain) it would be a lot more work--instead of standing on a plywood deck next to a pile of 2x4's and sheathing putting your walls together, you would be dealing with these materials off of scaffolding.
Why is insulating a rim joist aggravating? Do you not have access to a good spray foam contractor?
I don't use foam other than in rigid sheets for for foundation and subslab insulation (where nothing else will do) and in a foam gun for windows & doors and I rely as little as possible on subcontractors. Saves me much much time and aggravation.
Rim joists are known to be one of the most problematic areas for thermal bridging and air/vapor sealing.
Also, how is air sealing handled with your method? Are you caulking your mudsill to the foundation, your rim to the mudsill, and your plywood to the rim? That isn't aggravating?
I use the air-tight drywall system, which requires acoustical caulk at all those framing connections. Aggravating? Not at all. Particularly when it eliminates the need for a vapor barrier - which IS aggravating and almost impossible to install continuously.
You don't have to fit the inner wall tight if it just functioning as a partition. I left a bit of settling room.
And that's the rub. How much is a "little bit" and is it enough to prevent the "partitions" from becoming a short circuit for the load path?
The foam is between the window and the 1x. Any shrinkage, probably negligible, would occur in the width not in the thickness. It would be no worse than (probably better) than conventional framing methods. I have taken out enough foamed-in windows to know that that foam doesn't pull apart.
Wide sawn boards are not nearly as stable as plywood.
No reason you can't offset the studs. I did.
Good. That makes a better thermal envelope. But I've seen them unstaggered.
The rim joist detail is interesting and has some value. But it is labor intensive.
Yeah, his system is more labor intensive than mine. Since I don't use an exterior wall, but rather exterior truss chords with no exterior sheathing, I can nail up my prefabbed outer chord from the inside decks. Goes up real quick and easy.(http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/LarsenTruss.htm)
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Why don't you bring your rigid insulation up an over the top of the foundation setting your truss on the insulation? Doing so would create a complete thermal break.
Edited 2/15/2008 10:26 pm by dovetail97128
If you mean set the sill back 2" and run the exterior vertical foam board up to the top of the sill, there are a number of reasons I don't.
First I can live with the rough sawn 2x12 sill as a thermal bridge - it's still R-15 and it's the only significant bridge in the entire structure.
Second, I place the vertical foam board - both exterior and interior slab edge - in the grade beam forms and pour the concrete to them so I don't have to attach them later.
Third, the entire grade beam, including exterior vertical foam board, is capped with sill seal and copper, as capillary break, air seal, and insect barrier.
Forth, as soon as I install the sills, I staple 1/2" hardware cloth to them and parge the foam board with block bond to protect it from both UV and physical damage.
Fifth, even though the outer frame is technically not load-bearing, I prefer to have a full 3" bearing on the foundation, attachment to the rafters, and gusseting 24" oc for a secure and stable exterior.
This is a system I've been figuring out for 20 years. I'm open to suggestions, but I think I've got it pretty much where I want it. It works, it's easy, it's inexpensive, it's cost-effective, and it creates one of the most thermally-efficient envelopes of any wall system with minimal use of manufactured materials or petro-chemical foams.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Thank you for the detailed explanation of your reasons.
They can't get your Goat if you don't tell them where it is hidden.
so how do you airseal your rim joist with airtight drywall?
I just answered that:
Are you caulking your mudsill to the foundation, your rim to the mudsill, and your plywood to the rim?
I use the air-tight drywall system, which requires acoustical caulk at all those framing connections.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Which is the drawback of the system in the real world. For example, we frame houses in the winter--requiring framers to break out acoustical sealant when it's near zero is just not that practical. (Plus they will be wearing the stuff on their clothes, gloves, tools, etc.) Nor is it practical to get a sheetrock hanger to caulk before hanging a sheet. It's frustrating, I know, but subcontractors tend to have tunnel vision--they see their own trade to the exclusion of all others. I'm not saying the drywall as air barrier approach doesn't work. It's just not the panacea you claim it is.If I may digress here, sure, we can all sit around and dream up ways to build the perfect house but what is it going to cost? There are better ways to go than conventional framing, obviously. But budget is always an issue. I am in an area where new construction rock bottom prices start at $150/sq. ft. Customs easily top $200. And whether you are a framer, rocker, or whatever, you have to hustle hustle to bring a project in within budget. In an environment like that, if your new and improved methods aren't cost effective, noone is going to be able to adopt them.
Which is the drawback of the system in the real world.
You're quite right - the reason we're going to hell in a handbasket is because of what we call the "real world", which is nothing more than the sum total of our personal and collective choices.
I'm fortunate to live in rural Vermont, which has never quite been admitted into the "real world", and I've committed my own life to remaking the world into something that might still be around for my grandchildren.
For example, we frame houses in the winter--requiring framers to break out acoustical sealant when it's near zero is just not that practical. (Plus they will be wearing the stuff on their clothes, gloves, tools, etc.)
Yup, framing in winter makes everything more difficult and more expensive. Which is why I don't. Aint fair to me, to my tools, or to my customers. And it's much more difficult to do quality work.
I call butyl acoustical sealant "Black Death" because - in any weather - it gets hold of everything. So I keep some mineral spirits and rags near by. But you can also use silicone rubber ADA gaskets made specifically for this system, some of which are also great for sealing around doors and windows when your low-expansion foam gun is frozen. http://www.conservationtechnology.com/downloads/BuildingGaskets.pdf
Nor is it practical to get a sheetrock hanger to caulk before hanging a sheet. It's frustrating, I know, but subcontractors tend to have tunnel vision--they see their own trade to the exclusion of all others.
Yep, this is another "real world" problem. Not to mention coordinating a dozen subcontractors and trying to keep everything on schedule and within budget. Which is why I've adopted the "old-fashioned" approach of being a master housewright, who does everything from foundations to finish. Keeps me busy, avoids scheduling problems, and I don't have to deal with that tunnel vision thing or communication snafus.
Since I build only what I design, and I design a house to be a highly integrated system, rather than a collection of parts, by doing most or all the work myself (with a carefully supervised crew) I can make certain that everything is done to plan.
I'm not saying the drywall as air barrier approach doesn't work. It's just not the panacea you claim it is.
But I never claimed it was a panacea. Years ago, when I realized that I couldn't rely on the insulation subcontractor to install a proper vapor barrier and started doing them myself, I discovered how difficult and time-consuming it was to create a continous air/vapor membrane (gaskets pre-installed around rim joists and over upstairs partition top plates, caulked at bottom and top plates and around doors, windows and electric outlets...). So, when I came across the Air-Tight Drywall Approach - and learned that it was air transport and not vapor diffusion that was the prime culprit - I quickly adopted it, since it was much easier and less time-consuming to do a quality job.
If I may digress here, sure, we can all sit around and dream up ways to build the perfect house but what is it going to cost? ...if your new and improved methods aren't cost effective, noone is going to be able to adopt them.
True 'nuff. But how do you define "cost-effective"? If you mean will it add any cost to a project and make it more difficult to "sell"? Sure it will, but it then requires more informed "selling". The house I just completed was HERS rated at 46, meaning it requires 54% less energy than a house built to the International Energy Conservation Code and 34% less total energy than a 5 star Energy Star home.
It's been well documented that such an incremental increase in efficiency tends to add approximately 5% to the cost of a house. If the house is mortgaged, the each month's payments (mortgage + utility) are less than a "cheaper" house. So the payback time is zero. That should be a pretty easy sell, particularly when banks will routinely offer a larger debt-to-income ratio to a buyer of a documented energy-efficient house.
If we can assume that energy costs will rise at 10% a year (a conservative estimate), then the house I just built will completely pay for itself (the entire cost) in 20 years just by the energy savings.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 2/16/2008 2:52 pm ET by Riversong
I think it's obvious to all by now that there is no one method for achieving something better than merely Energy Star. It's a matter of what works for a particular builder, the weather region, materials available, and what the homeowner wants to achieve. Many systems have been presented here and elsewhere, and I applaud them all.Attached are some dwgs I finished over the weekend.WallA2 is like what I posted earlier. Now it has the pair of 2x6PT sills, and I dropped the gussets. Also I added some annotation.WallB shows the wall/foundation assembly where joists run parallel to the foundation. Someone may wish to comment on the need for lateral support of the rim board supporting the inner wall in this shot. As I read the sketch on the ilevel.com document http://www.ilevel.com/literature/TJ-4000.pdf
(p.10), they seem to show just the rim at the outside, sitting on the sill. Presumably the subfloor holds the top of the rim laterally, and the toenails hold the bottom, so it can't topple. I think I read somewhere here or elsewhere that at least some BI want to see lateral support from the first i-joist parallel to the rim.WallC shows the joint between walls where a second level is above the first. A 3/4" sheet of plywood connects top plates of inner and outer walls. It provides firestop and shear transfer.Fire away.One minor thought I had while looking over what I had drawn at the sills. The bolting has to be somewhat away from the center of each 2x6, since there is wood over parts of each sill. How close can the bolt go to the edge of the 2x6? Or is it better to just bore the wood plate or rim where it sits on a bolt?
Looks like a good system. Like you say, there is more than one way to skin a cat..double walls, exterior foam, sips, etc. I do think that framing the outside walls is going to add quite a bit to the labor. It is always fastest to frame and sheath walls laying on the deck. You might be able to frame the outside wall on the deck, but you would then have to slide it down and stand them up on this little step, which would be pretty hairy unless you have a crane on site. Maybe you would be better off if you brought the outer first floor wall up to the height of the top of your rim, and then your floor plywood could run out and catch that outer wall and serve as your fire block. Then at least, the second floor outer walls (which are really going to be the costly ones in terms of time) could be stood up by conventional framing methods. Since the only real gain is more insulation on the rim joist, I'm not convinced it's worth the extra labor, but then some people don't mind spending money for energy efficiency even if there is long payback period and that is OK too.Also make sure you have plywood between the top plates at the top of the walls too, to prevent convection currents from blowing into the attic. You probably know this, but your drawings didn't show the top of the wall.On walls parallel to floor joists, we are required to run blocking at 4' OC to restrain the wall below. Especially on the foundation walls which have backfill pushing against them.As far as bolting your 2x6 sills, I would think if you were within 1.5" of the edge it would be OK. Are you adding insulation to the foundation on the inside? Seems like 2" of foam is pretty minimal considering the rest of the house.
"Maybe you would be better off if you brought the outer first floor wall up to the height of the top of your rim, and then your floor plywood could run out and catch that outer wall and serve as your fire block."Interesting thought. I guess there's no reason why that outer wall at the lower level couldn't run to the height of the upper floor deck. I'll sketch that up next opportunity (probably the weekend).I have been thinking about the sequencing of things, and the tradeoff between how hard something is to implement vs. what is really gained. Gotta be able to build the darn thing!I haven't yet sketched the top of wall where it reaches attic level. That would have a top plywood plate connecting wall top plates, as in the other sketch. For every area where someone questions something, I need a sketch.On the blocking of the rim out from the adjacent parallel joist, is there a code reference for that, or some other source you can point me to?Putting more than 2" XPS outside would get unweildy. The house will be set into a hill, with ground up to within 18" of sill in back and half the sides. Halfway forward, foundation will step down to about half height, with ground similar distance below sill. Front, facing a lake, will be totally framed, looking much like slab on grade there. The foundation wall in front will be insulated also, of course.I have been thinking that the inside lower level finishing would have another 1" of XPS and finally drywall in the finished area. That would give R15+ on the foundation, most of which is below grade.
What is your plan to protect the XPS that is exposed above grade?
Jon Blakemore RappahannockINC.com Fredericksburg, VA
I'll probably use either pre-covered panels or apply a coating such as Tuff-II (http://styro.net/FoundationandICFCoatings.html) over regular 2" XPS.I'd like to hear of alternatives that go on easily and aren't too fragile (weed-whacker sensitive).It's not in a termite-prone area.
Edited 2/19/2008 2:31 pm ET by DickRussell
Sorry I don't have a code citation. I get it from the engineer who does our I joist plans. Our BI's ask to see the blocking, so I have just assumed it is in the code. Maybe not though.I agree that more than 2" of ext foam is pretty tough to detail well. I guess I thought that since you are going with a superinsulated design, why not superinsulate the foundation somehow. Thinking about this further, if I was the carpenter on the job, I'd lag some temporary blocks to the foundation somehow, frame and sheath the exterior walls on the deck, and then slide the wall down against the temp blocks and stand it up. Then stand up the interior wall. That really wouldn't be too bad in terms of time. Have to custom cut every stud, but if you set up a saw stop that really doesn't take that long. One worry I had with the double wall I built was that the interior stud is not restrained by sheathing and having one side warm and one side cold, it might be more prone to warping than a conventionally framed building. Haven't seen any evidence that that is the case, but something to think about.
I was talking with someone else about this, and I came up with the following:The outer wall frame can be sheathed on the platform with the bottom edge of the plate cantilevered 13.5" out over the edge of the platform. This is the diagonal distance from the edge of the platform to a point 1.5" in from the outer edge of the sill where the inner edge of the plate will come to rest.Metal straps can be nailed to the platform as usual, and out to the bottom of the plate, as usual, except that the straps will be longer. As the jacks raise the wall, the straps constrain the movement both laterally and vertically, and the wall pivots on the edge of the platform. Once the plate touches the sill, pivoting is on the sill. The metal straps remain tight while the wall frame continues its climb to vertical. Later on, the straps can be cut from inside the structure, to avoid thermal bridging, if that's a worry.This can be used for any wall section that is assembled on the platform and stood on the sill below platform level. For any outer wall section that is not on the sill but is over a framed wall at the lower level, then as you've suggested the wall below can be framed to come up to platform level, eliminating the issue of standing a wall to a level below the platform.I'd be interested in any numbers you'd care to guess at as to extra manhours per so many feet of wall for adding the second wall frame. I guess it would depend on how bad the window framing was. All that would have to match, of course.
I build pretty simple affordable houses, so if your house has jigs and jogs, beam pockets, etc, my numbers wouldn't necessarily apply.With that caveat, three of us can stand up the walls on a basic 30x28 box in about 10 hours. That's with sheathing and tyveck on (some corners will not be sheathed) and 8 foot walls with a few windows. I'd guess that standing the second set of walls would take another 6 hours or so. You would have to string the outer wall straight and then add your tie plywood. They'd go pretty fast because they don't have to be sheathed. The windows on the double wall I did weren't a big deal--I just laid the inner wall plate next to the outer wall and transfered the location of the king studs to the inner plate. I framed the window openings a bit big since you tend to lose some room when you install the box in the window. (I like to anyway to insure good room for a foam gun). Actually sounds kind of interesting. I hope I get to try another double wall some time.