yikes, the journey continues..   It seems that most people on this board read all of the forums, so I'm not going to start from the beginning.  Long story short, debating between stick framing & SIP construction.  We have decided to skip ICF's for the basement & do a traditional poured basement with 2" foam on the exterior with an insulated slab.  I really WANT do to SIPs but it's starting to look like I may need to seek out alternatives.  Our builder is very affordable, but has never done SIPs.  If we order the panels & his crew installs, I'm not counting on any labor savings, since they've never done them before & will probably be slower.

We want(ed) to do geothermal as well.  I found out recently that the well-drilling costs are going to be significantly more than I expected, so we may be in a position to choose between SIPs & geothermal.

so here's my question.  We're using 2" foam board on the basement walls.  Can we use that on the above grade walls also?  My builder said he typically sprays 1" of foam in the stud cavities then fills w/r19 batts.  The problem with this is that it doesn't eliminate the thermal bridging issues on the stud spaces.  I was thinking that, in theory, an inch or two of rigid foam on the exterior and the r19 batts on the interior may be a better approach.  I think that board is r5/inch.  Could I do 2" thick?  that would theoretically give me r29 and better protection against bridging, right?  I know it's not a true "whole wall" r29, but I would think it would get pretty close to the whole wall r23 of a 6" sip.

Are there any flaws in my logic? 

Yes, rigid foam on the exterior is a good idea in that case. My builder offered to do either SIPs or stick-built/spray-foam with rigid foam exterior to cover thermal breaks caused by the studs.  I ended up deciding to go with SIPs.  Hope this helps.

If I can ask, did you ever get head to head bids on the two scenarios?  I could see the prices getting up to or over the price of the sip walls once you add in the foam board, jamb extensions, etc. 

In my case, the prices for 6.5" PU-SIPs and 2x6 studs with 6" of 2lb spray-foam (including rigid foam on exterior) were almost the same.  I couldn't guess in your case, since the materials and location are bit different.  I'd recommend you shop around and get quotes.

jerkylips,

I believe you can still get the 30% tax credit for the cost of geothermal... something to keep in mind. This may still allow you to do SIP's AND geo.
As far as stick framing, the use of rigid foam on the exterior is a great way to eliminate/minimize the thermal bridging (in studs, headers, bond) like you mentioned. I see no reason why you couldn't continue the 2" rigid foam from the poured concrete foundation, all the way up. Are you planning on siding or brick on the exterior?

Also get an estimate from Superior Walls for your foundation. What you may find is it is more cost effective going this route since it already has a r-5 insulation built in, compared to a poured concrete basement having to add rigid foam on the exterior... plus if you ever decide to finish your basement, you will save money and time too since you can hang your drywall right on the Superior Walls and won't have to build your 2x4 walls around the perimeter.

Last house I built was 2x6 walls with densepack cellulose, Huber ZIP sheathing, taped (doesn't need building paper), 2" XPS (Styrofoam) with the joints taped & foamed, 3/4" strapping**, Hardi Board clapboards. Windows were installed outside the foam. **We used plywood scraps around the windows so it could be wider, & more solid for attaching windows. Did not install extension jambs but drywalled in to the window & installed wood stool caps (windowsills). I think, and experts who have seen the house agreed it was/is an excellent way to build. Energy Star rated five stars +. Still cheaper than SIPS at least on that house - SIPS were the original goal which had to be dropped to save $$. I do think SIPS are an excellent method & realize that different houses have different costs.

There are two potential parts of the job that can take time - install the foam prior to lifting the wall if you can so you're not messing with that on a ladder or scaffolding, and be careful to install the windows in the center of the RO so furring the opening for drywall is easier.

I am not a fan of filterglass, but spray foaming the walls will cut down on the infiltration factor which will increase the performance of the batts.

One last thought - install a metal flashing - or "termite shield" between the foundation insulation & the wall insulation - also helps to keep out carpenter ants which love to burriow through & nest in foam.

Jerkylips, you're certainly getting a lot of very professional responses to your inquiries. Having said that here are some observations from my perspective.

1. I still question how a building system that is comprised of multiple "parts and layers" that all have to be assembled in a precise manner in order to be effective, can cost less than a SIP system unless your labor is FREE, or VERY
CHEAP. Even then, I'm not sure that it's enough less, in most instances, to result in a better overall value.
2. Any contractor who is skilled enough to properly install one of these complex "stick" framing systems should be able to pick up the SIP installation process very quickly and easily. There are many very good SIP
installation videos on You Tube as well as on the major SIP manufacturer's websites, eg www.r-control.com Furthermore, I don't understand why any contractor who plans to be in business for the foreseeable future, wouldn't want
to get aboard with a proven "green" building system that continues to gain market share year after year. I've also told contractors who are new to SIPs, but would like to get involved, that they shouldn't expect their clients to "pay
their tuition". Some will agree with this theory, some won't.
3. There is no doubt that SIPs are more cost effective with simpler designs but when someone emphasizes the "B" word (budget) in connection with their project, why do they want to build a house with 42 corners and a complicated
roof design? (which is also more expensive when "stick" framed, particularly with one of the "multiple parts and layers" as so often discussed in these forums) I may be wrong but I'm thinking that, for most of us, the days of
investing huge $$ on elaborate exterior designs, so that others can "ooh and aah" as they drive by, are waning. This is not to disparage the importance of good designers and architects but in my mind it will, and already does,
in many instances, affect their role in the process.

OK, enough proselytizing for now, good luck in your "journey"

To help us all out, what size is the house, how simple is the exterior wall detail, and what kind of exterior siding are you wanting? Posted elsewhere on the forums is the double stud house I just finished, which had used 2 3/8" foam between the 2*3 studs to accomplish the thermal break. We would have used the foam on the outside as you mentioned, but since we were using T-111 siding, it was actually easier to throw up more studs than to attach suitable furring through the foam, especially since the foam we found dirt cheap was of irregular thicknesses, and since T-111 in this situation adds to the rigidity of the structure, which was an added bonus. We actually "built the house" with just the outside studs, and added the inside studs later. I know double stud is not for everyone, but in some situations, it can make a lot of sense, especially since the exterior construction details are just like standard construction, as in no furring strips or fanciness around the windows/doors.

just my 2 cents, watch out for 2" foam on the ext, if you install furring (required in my cold climate) 2" + 3/4" furring = 2 -3/4. if you also install osb sheeting (for structural rigidity) you end-up at ~3-1/4. The longest nail that you can use in a regular nail-gun is 3-1/4 and suggested min. stud penetration is 3/4. This can create unexpected labor costs. Dow SIS and tyvek faced poly-iso boards (john manville) both ~r7/in might be a better fit.

mforget:
We used long tourque head screws (GRK, Fastenal) to fasten the strapping through the foam in to the framing. Not as fast as a nail gun, but you don'd need a lot of screws for each piece. Since you do need to hit the framing, screws are better in that you can "feel" when the srew entes the stud.
Bob

thanks again for the replies. I should have the first draft of our plans back by the end of the week so I can post the details at this point. Until then, here's what I can tell you. We decided to start slightly smaller & if the room sizes seem to small, we'll scale up. We were told it's easier to do that than start too big & go down. We're aiming for around 1850 sq feet to start & will be somewhere betweeen 1850 & 2000 when all is said & done. It will be a pretty straightforward split bedroom ranch, but it will have a covered porch on the back corner. Basically a 60x30 rectangle with about 12x10 bumpout on the back side and an attached 3 stall garage. I posted a sketch of the floorplan somewhere else on here - I'll see if I can find it & re-paste it. We are trying to do the house in a craftsman style & I found a roofline that seems to fit the style & should be fairly simple/affordable to build. That is shown below. thanks!

Have you explored what is sometimes called a hybrid steel SIP? Check out Thermasteelcorp.com. Cost is about $5.50 per sq ft. Easier to install because they are lighter than OSB SIPs and much easier to wire (using a hot wire) especially if code requires conduit.

looking for a little more help.  I'm trying to find a way to make SIPS work but I need to be realistic.  I threw together a spreadsheet of some alternative methods of insulating.  At this point I'm only looking at the walls, because even if we do go with sips, it looks like trusses will make the most sense for the roof.

here are my assumptions -

R19 batts in the walls
stud space = 15% of wall area
R value of stud space = R5

I know this doesn't take into account air infiltration but I calculated R values as 85% of the wall area at the stated R-value + 15% at R5 to come up with the "real" R-value.  When I considered exterior foam board, I added the stated r-value of the board to what I came up with.  As an example, a "normal" r19 wall would be..

19x85%=16.15
5x15%=.75

"real" r-value of the r-19 wall is 16.9

If I add foam board to the outside to protect against bridging & be an air barrier, that is added.  Assuming EPS is r5/inch, 1 inch of EPS added to the "16.9" wall would make it r21.9.

Throwing together different options like an inch or two of spray foam on the inside, either EPS or XPS on the exterior, I came up with a couple conclusions.  I think that having the exterior foam board would be a must if I don't go with SIPs.  On the spreadsheet below, I highlighted what I think are the 2 most realistic options.

I don't know how much more expensive XPS is than EPS, so that could be a factor.  Also, adding 2" of foam increases the wall thickness to 8", which causes some extra expense with doors & windows.

Does this look reasonable?  I'm trying to find the closest comparison to SIPs when I start looking at costs.


thanks!!

The technically correct way to average R values is to convert to U values, then average, then convert back. For example, R-19 batt =U- 0.0526, R-5.5 stud=U- 0.181818, 85%*.0526+15%*0.2=.04471+.0272727=U-0.07198=R-13.89. Your estimate is close enough, but just wanted to show you the technically correct way for future reference. When finding the R value for a whole wall, don't forget the R-0.5 for sheathing, 0.5 for drywall, ect. Even the film of air on the inside adds 0.68. Trust me, the manufacturers that advertise a certain whole wall value haven't forgotten them.

http://en.wikipedia.org/wiki/R-value_%28insulation%29 is a decent reference, there are more complete ones on the web.

Option "9" with the XPS looks like your best bet. If budget is a concern, your could use factory seconds XPS, which should save you 30% or more on foam. Another option is downgrading the wall from 2*6 to 2*4. Using 2*4 and and 2" XPS, you would end up with a wall as thick as just a plain 2*6 wall, so no extra costs for trimming around windows/doors, and about the same R value as option "12" with the EPS. You should do a heat loss calculation for your whole house, to see what upgrades are reasonable by finding out how many BTU's are leaving what parts of your house on a cold day. You will find strong and weak points, because R-30 walls are pointless unless the rest of your houses R values are in line with them. I am assuming you already know about energy heel trusses.

I thought xps was r5 and eps was around r4 an inch?

I would avoid bat insulation first and foremost. Your actual performance value will be much lower than textbook. If you want a system that performs similar to ICF for SIPs you will need to use foam.

interesting point.  I've read something similar, but it had to do with air moving through the batts or getting wet.

My logic was that with the foam board insulation on the exterior, it would eliminate the air infiltration, allowing the fiberglass to perform as stated.  I thought about blown in, like cellulose, but decided on fiberglass because I shouldn't have to worry about settling.

Let me know if my logic is "flawed"...

thanks for all of the input - very good info to have!  I do know about raised heel trusses.  It sounds like depending on how we insulate, they may or may not make sense, but we'll see.  I know that our builder has a good relationship with the insulation contractor, & both are very open to sitting down & discussing options.

Of course, I'm hoping this is all academic.  My ultimate goal is to find the option that's most comparable to SIP & hope that with all of the "extras", the costs are similar.

I do not mean to imply that your logic is flawed. If done properly you will eliminate most of the air infiltration. However, the fiberglass will still settle over time and this will still allow convective currents to flow around within the cavity. The only place bat perform as stated are in the laboratory where the test is performed in a vacuum -- no convection at at about 75 degrees -- a temperature where you will not require conditioning. THere is a very good video showing this out there on one of the SIP sites. BYW - when you do select your SIP make sure you have minimal if any actual wood or steel the connects one surface of the wall to the other -- similar to a stud.

Dense pack cellulose has a similar R rating as filterglass without the infiltration problems, and is far less expensive* and greener than either fiberglass or foam (*Except that it should be installed by a professional). EPS is the standard designation for expanded polystyrene - the white coffee cup foam which has an R value of 4 per inch. XPS is extruded - and therefore denser - polystyrene - like Styrofoam which has an R value of 5 per inch. Polyisocyanurate is a type of urethane (or vice versa) - typically a yellowish foam with aluminum foil on both sides which has a higher R value - used to be called R-8, then R-7.5; now typically rated at about 6.5. EPS is used in the majority of SIPS and ICFs and is usually the cheapest per R value. Winter Panel and several others make polyiso panels.