DOW SIS vs XPS vs SIPs
Last Post 11 Oct 2010 07:36 PM by Bob I. 18 Replies.
Printer Friendly
Sort:
PrevPrev NextNext
You are not authorized to post a reply.
Author Messages
lzerarcUser is Offline
Basic Member
Basic Member
Send Private Message
Posts:423

--
05 Oct 2010 09:49 AM
Working on hashing out the exterior shell design of a new house.  Been going back to forth between multiple systems. Here are things I have basically narrowed it down too.  I am really not sure which is "best bang for the buck" system.  I want it to be good and tight, but not going crazy R40 walls either.  I want to be in the mid R24+ range, would love to it R-30ish. 
I have quite a good amount of construction experience, so I would be doing all labor and install.  So figure labor as 0 for all of these systems.

Option 1:  SIPs 4" urethane panel- R24, tyvek house wrap, about $4.00 /sqft  includes electrical boxes and conduit cast into the panel

Option 2: SIPs 6" urethane panel, R-40, tyvek house wrap, about $5.00 sqft   includes electrical boxes and conduit cast into the panel

Option 3: 2x6 framing w/ 1" DOW SIS, blown insulation, about R-24,  $2.8 sqft

Option 4:  2x6 framing w/ 2" XPS on studs, 1/2" osb at corners with 1.5" XPS on osb at corners.  about R-28  $2.70 sqft

Option 5:  2x4 double wall:  1/2 osb, 2x4, 1" gap, 2x4, plywood top and bottom connenting plates, blown insulation, R will vary on insulation amount, somewhere around R-28+
Also takes up room area:  $2.40sqft

I am also getting SIPs prices on poly panels, however for the 6" it is still only an R-24.  They are suppose to be coming in at around $2.80 sqft range.  Is the tightness and overall thermal properties of a SIPs still more efficient then an exterior foam XPS application despite the lower R value?

The R-40 SIPs would be ideal, however not sure if they fit into the budget.  Over a 2" foam option, they are about $5000 higher, plus possibly needing a rent a crane for erection, things like that.

I am currently leaning toward Option 4, even though it will require more work for siding and window locations.

Thoughts anyone?
Jesse ThompsonUser is Offline
New Member
New Member
Send Private Message
Posts:89

--
05 Oct 2010 01:31 PM
#4. Cheapest, most flexible construction, easiest window and door flashing details (equal to SIP).

But there's no reason not to go to R-40 if you are in any kind of heating climate. Unless you're building in a city and lot size is extremely limited, don't worry about the outside of your house getting bigger (design your rooms to the face of inside of wall). I have never heard anyone complain about having too much insulation in their house after it's built. Ever.

Buildings are so expensive to build, take the time and get it right, we don't know what the future is going to be. One clue, however, is that we've gone from empty 2x4s to insulated 2x4s to insulated 2x6s to R-40 and beyond in less than 30 years in our part of the world.
Jesse Thompson<br>Kaplan Thompson Architects<br>http://www.kaplanthompson.com/<br>Portland, ME<br><br>Beautiful, Sustainable, Attainable
lzerarcUser is Offline
Basic Member
Basic Member
Send Private Message
Posts:423

--
05 Oct 2010 03:40 PM
actually I was looking over my numbers, and the double stud wall assemble would be the cheapest, plus yield the highest R.  However I have also read it can have a higher moisture and air infiltration rate compared to the system with externally mounted XPS or similar.
Dana1User is Offline
Senior Member
Senior Member
Send Private Message
Posts:6991

--
05 Oct 2010 05:22 PM
If you're only going to ~ R40, a lower labor reasonable cost method would be to use 2x6 with wet spray cellulose + ~3" of iso over OSB sheathing would also yield about R40 with no increase in framing complexity. Per square foot cost on R20-ish iso is about $1.75 (substitute that for your XPS costs in your 2x6 w/ cellulose estimates). Fiber faced iso is sold as roofing insulation for flat roofs in the commercial space but it's becoming common to use it in wall stackups as well. The foil-faced wall sheathing versions are as-easy or easier to make fully air & vapor-tight as your exterior XPS variants. By putting at least half the R on the exterior you can go with the foil-faced stuff in most of the lower 48 of the US, and much of maritime Canada without running into condensation issues, since the entire stud stays above the dew point of the interior air for a preponderance of mid-winter hours, and doesn't need to dry toward the exterior. A double-layer of foil faced iso with FSK-taped overlapped seams is inherently pretty tight once you foam seal the edges. Fiber faced goods come in a variety of perm ratings, but most are under 1 perm. Foil faced goods are less than 0.1 perm. See: http://www.atlasroofing.com/tabbed.php?section_url=18&tab=31

Making double-studwalls air tight is a matter of air-sealing the exterior shell (complete with blower-door testing & verification) before insulating. With rainscreened siding and R40-R50 of cellulose you can use standard latex paints (class-III vapor retardency) on the interior for maximum seasonal buffering/drying, and it'll would even have a significant resiliance to some air leakage into the insulation from the interior (although you'd still be advised to make it as air-tight as possible.) Sealing the structural framing penetrations of the insulation can be a lot of detailing though, which is why some prefer Larsen Trusses, where the inner stud is structural. (Both have their issues.)
lzerarcUser is Offline
Basic Member
Basic Member
Send Private Message
Posts:423

--
05 Oct 2010 06:16 PM

Thanks Dana

What about blowing in 1" or so of spray foam first to the back side of the osb and then filling with cellulose in the double stud wall option?

Or perhaps spray foam with the 2x6 wall, and then reduce cost of the exterior insulation by changing to EPS instead of XPS?

Then again, I might be approaching a SIPs price at this point. 

Dana, do you have any strong feelings either way towards the SIPs?

adi43dUser is Offline
New Member
New Member
Send Private Message
Posts:87

--
06 Oct 2010 11:20 AM

I think your calculations are ok. the double wall stud is the cheapest per R value from all the options. you can find a more indepth analysis at

http://www.100khouse.com/2010/07/16...-assembly/
http://greenedmonton.ca/mcnzh-insulate-and-seal

I don't know where you are located but I've run some WUFI simulations on a similar assembly and I can tell you that at least "on paper" moisture acumulation is not a problem for Toronto - Canada given the detailing is done right.

I've also passed thru "SIP stage" for my project but after some research and budgeting and code restrictions in my location I've switched to double wall myself. I don't think is worth the effort to do a double wall if you do not want to superinsulate (>R40). superinsulation brings with it some other advantages beside lower energy bills. if you take into consideration a simpler mechanical system the price becomes even more attractive. for me for example another advantage of superinsulation is that I don't have to hook up to a gas line so I don't have to pay that service fee every month whether I use gas or not.

double wall is competitive by itself and when you add all the other advantages - at least for me - it was a no brainer

good luck
adi

http://torontonetzerohouse.blogspot.com/
Dana1User is Offline
Senior Member
Senior Member
Send Private Message
Posts:6991

--
06 Oct 2010 11:47 AM
Posted By lzerarc on 05 Oct 2010 06:16 PM

Thanks Dana

What about blowing in 1" or so of spray foam first to the back side of the osb and then filling with cellulose in the double stud wall option?

Or perhaps spray foam with the 2x6 wall, and then reduce cost of the exterior insulation by changing to EPS instead of XPS?

Then again, I might be approaching a SIPs price at this point. 

Dana, do you have any strong feelings either way towards the SIPs?


Flash & fill can work pretty well as an air sealing strategy, but it still needs to be pressure tested. (The notion that SPF will automatically always be air-tight in a real-world app is a fallacy.)

EPS is typically the cheapest R in rigid foam. Closed cell spray foam is typically the most expensive, and can be budget-breaker for high-R assemblies unless used sparingly & appropriately.

I'm somewhat middlin' on SIPs in general- the particulars can go from "really great!!!" to , "WTF...???"

 A co-worker of mine is living in an R25 SIP house installed about 20 years ago, and it's less air-tight than my 1923 stick-built antique, used ~35% more heating BTUs (despite mere ~R14 nominal center-cavity cellulose and R20 roof compared to his R25 EPS walls & R40 roof), has annual battles with ants in the exterior OSB, etc.  Newer SIP technology has fixed the ant & termite issues (pretty much), but actual performance depends on the quality of the installation, which can still be all over the place. 

SIPs can be a huge labor-saver on the installation though. I have a friend  living in a ~ R28 SIP house built about 6 years ago, and it went up in a heartbeat, and thermal performance is quite good.  There were some dimensional errors that needed to be "chainsaw corrected" on-site, but the contractor knew what he was doing, and it didn't change much.  The quality of the designer and the builder (as always) are critical to success.
lzerarcUser is Offline
Basic Member
Basic Member
Send Private Message
Posts:423

--
06 Oct 2010 12:11 PM
I've read that 2" of XPS can get rid of most moisture issues as well as be used as a great air infiltration break. However I am not sure if it is really necessary on a double stud wall option. 2" of XPS is nearly as much as the wood material itself. However it is also my understanding the EPS does not have near the same capabilities of stopping moisture and air infiltration. Is this a correct assumption?
I am basically considering the double wall system at 9"-10" thick giving me an R of around 33-37. Then possibly adding 1/2" or 1" of exterior XPS or EPS to the OSB. If I can limit it to around 1", that can ease the detailing requirements of 2" foam, not to mention the price. Is this a waste of money?
Or is exterior foam not really that necessary with this assembly and simply use Tyvek wrap or similar?
My design is to not use a large plate to connect the 2 walls such as a 2x12 or example. Rather I am either going to frame the exterior wall first, get it enclosed and sheathed, roofed, etc, then frame the interior wall. This should eliminate almost all thermal issues with wood to the exterior face. It would then leave a gap at the top for the attic insulation to form a continuous unbroken insulation flow from wall to attic. I need to check codes however to make sure this is allowed. It is my understanding since it is cellulous it should be fine.

I have read on the Building Science report that they recomend a moisture barrier to be installed on the INNER face of the inner 2x4 assembly before that wall is tipped up. What are the thoughts on this?

I have also ruled out SIPs.  Price vs other assemblies with similar R (and higher) just does not make sense.  Install costs are not a consideration since it is my time for each instance.
Plus the price of the crane, shipping, unloading, working with the electrician, on and on.
I framed a 2x6 9' wall 1100 sqft addition in about 2 days alone.  As mentioned, framing is more forgivable with other items such as foundation being slightly off, etc.  And the basement of this project happened to be 2" off from the plans....
Dana1User is Offline
Senior Member
Senior Member
Send Private Message
Posts:6991

--
07 Oct 2010 03:23 PM
2" XPS (or even Tyvek) is only as air-tight as the installer makes it. Whether 2" XPS "gets rid of most moisture issues" is a matter of the total wall stackup- it's vapor permeanance at the limit of what you'd want to have any appreciable seasonal drying on the side which it is installed, but it slows vapor diffusion into the assembly from that side.

Putting the vapor retarder on the interior-most edge of the interior studwall (essentially at the gypsum), would be the place where it's most vulnerable to damage. Putting it on the exterior edge of the inner studs (the side facing the exterior studwall, not the room) means you can run your electrical, etc without penetrating the vapor retarder. This requires that the insulation on the exterior side of the vapor retarder be sufficient that the vapor retarder's temperature stays at or above the dew point of the conditioned space air for a preponderance of the time even during the coldest weeks of the year. (The inner/outer R-ratio to guarantee this will be local climate dependent.)

In most of the lower 48 putting just half the R value to the exterior of the vapor retarder would be more than enough, so in a double studwall that's say 10" from sheet-rock to sheathing, putting the vapor retarder on the exterior edge of the interior studwalls and filling both sides with cellulose would put the vapor retarder 65% of the way inside the insulation layer, so it would stay more than warm enough. But in very cold places, say in Whitehorse, Yukon you might need to thicken the wall for more exterior R, or move the vapor retarder to the conditioned space side of the stud to keep moisture from accumulating.
lzerarcUser is Offline
Basic Member
Basic Member
Send Private Message
Posts:423

--
07 Oct 2010 04:02 PM
So basically if I follow this layout and assembly, it should result in a good, high R system....

http://www.buildingscience.com/docu...nstruction


Of all I have mentioned, this system would be the most cost effective, and even comes with the highest R value option.
jerkylipsUser is Offline
Basic Member
Basic Member
Send Private Message
Posts:359
Avatar

--
08 Oct 2010 05:22 PM
option #4 is very similar to what we did, with a couple exceptions. We did 1" xps and 1/2" on the corners (+ sheathing). We also did "flash & batt", spraying an inch of foam in cavities & then R19 batts. Factoring in the compression of the fiberglass, we realistically have R28 in the wall, but the foam definitely helps with the air sealing.

One other thing to consider is how you're going to insulate your attic. We did a flash of foam over the top plates, recessed lights, etc. - basically every corner & connection, to really seal that up as well. We haven't gone through a winter yet, but so far our utiities (while AC is running, etc.) have been lower than expected.

If I had it to do over again, I'd consider getting more of the insulation outside of the framing - that seems to be where you get the most bang for your buck. Something like 2x4 walls with 4" of xps outside. If you used a standard fiberglass batt in the cavity, you'd have roughly R37 with an excellent thermal break & not a super-thick wall (need to consider window opening sizes, extension jams, etc., if you go too thick)
lzerarcUser is Offline
Basic Member
Basic Member
Send Private Message
Posts:423

--
09 Oct 2010 11:43 AM
The problems I see with the really thick exterior foams is in the details.  2" is a hard enough detail to do, and 4" would be even more difficult.  Attaching siding and things too it would be hard as well.  Would require really long screws.  4" of XPS would also be expensive compared to other options.  at $1.56 sqft for 4" (2 layers of 2") that would make the price of just the foam $2250.  Compared to cellulous material, it is about $400 for the walls.  The 2x6 studs are just under 2x more cost than 2x4 studs.  So the double stud construction (16" oc ext. 24" oc interior) actually only is about a $200 difference between 2x6 walls.  Since I am framing it all, labor costs are not a consideration. 
I will not do extension jambs on windows.  Go with standard jambs, and use 1/2" osb to frame around the opening, with drywall returns and a wood sill.  SLightly more material costs there, but the osb would most likely be left over scrap from the exterior sheathing.
Bob IUser is Offline
Veteran Member
Veteran Member
Send Private Message
Posts:1435

--
09 Oct 2010 12:14 PM
I've done the exterior foam a few times - there is nothing difficult about it. I'd recommend sheathing - prefereably ZIP since it is easy to air seal. The foam only needs a couple of nails since it is held on by the strapping (1x3, 16"OC) which is srewed through the foam into the studs. You can buy excellent torque head screws by GRK, sold at lumber yards & some big box stores, or Fastenall stores sell their own brand. The hardest part is remembering to locate the studs through all the layers. If you're building the wall on the floor & standing it up, all the easier. One of the problems with double walls is insuring a 2" thermal break at the sill, floor joists & rafter plates. Again, not difficult, but needs to be part of the assembly. Drywall window returns are an excellent feature whichever wall method you use.
Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
jerkylipsUser is Offline
Basic Member
Basic Member
Send Private Message
Posts:359
Avatar

--
10 Oct 2010 12:16 AM
Posted By lzerarc on 09 Oct 2010 11:43 AM
The problems I see with the really thick exterior foams is in the details.  2" is a hard enough detail to do, and 4" would be even more difficult.  Attaching siding and things too it would be hard as well.  Would require really long screws.  4" of XPS would also be expensive compared to other options.  at $1.56 sqft for 4" (2 layers of 2") that would make the price of just the foam $2250.  Compared to cellulous material, it is about $400 for the walls.  The 2x6 studs are just under 2x more cost than 2x4 studs.  So the double stud construction (16" oc ext. 24" oc interior) actually only is about a $200 difference between 2x6 walls.  Since I am framing it all, labor costs are not a consideration. 
I will not do extension jambs on windows.  Go with standard jambs, and use 1/2" osb to frame around the opening, with drywall returns and a wood sill.  SLightly more material costs there, but the osb would most likely be left over scrap from the exterior sheathing.


To clarify my earlier post, if doing 4" of foam, I would do a single 2x4 wall, not double-stud.  If 2x4's are  1/2 the price of 2x6, you'll make up some costs there.  4" of XPS should be plenty of a thermal break - personally I think that and a double stud wall would be overkill.
lzerarcUser is Offline
Basic Member
Basic Member
Send Private Message
Posts:423

--
11 Oct 2010 09:48 AM
I really hate these forums! HA
Once you THINK you have something figured out, you go and continue reading and researching, then you are back to the confused state you started at as to what is the best, most cost effective system.

The question is, which will ultimately lead to a better wall system as far as not only R, but air and moisture infiltration, ease of construction, etc.

So 9" (+) dobule stud with blow in cellulose, 1/2" ext. osb, 1/2" interior gyp, vinyl siding and Tyvek OR

2x6 studs at 16" oc, 2" 250 XPS, no osb except at shear locations, 1 1/2" XPS over the osb at those location, blow in cellulose, 1/2" interior gyp., vinyl siding, no Tyvek. I do not think exterior osb is really needed on this system at all.
Bob IUser is Offline
Veteran Member
Veteran Member
Send Private Message
Posts:1435

--
11 Oct 2010 12:19 PM
With the double stud option, how are you going to handle the thermal bridging at the floor system?
You've eliminated the strapping from your XPS system, so how are you going to fasten the vinyl siding? Nail thru to the studs? I've seen that done & the foam becomes a pin cusion with lots & lots of air gaps around the nails where slight movement in the building has enlarged the holes. I think eliminating the sheathing, while it may be theoretically possible is a mistake structurally and mechanically. And exterior foam without strapping is also a mistake.

If you're going to choose one or the other as described, the double stud system may work better, although neither is as good as it could be.

"Wall grade" knotty cedar shingles with knotty cedar trim is almost as cheap as vinyl, will look better longer, and is "green."
Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
lzerarcUser is Offline
Basic Member
Basic Member
Send Private Message
Posts:423

--
11 Oct 2010 12:38 PM
For the floor system, one option I am considering is an extra wide sill plate on the top of the foundation. I need 4" of bearing for the trusses, and then 3-1/2 for the stud. I would run the outer stud past the floor and sit it directly on the treated plate.
The other option is to just spray foam it and live with the bridging in those locations.

I also do not see how strapping punctures the foam any less then the nails would from the siding. then again, it would create a good drainage plane. If the walls were framed, foamed and strapped laying down, it would not take much extra time at all to add the strapping.
I was also planning on installing the foam with a very generous amount of caulking on the studs prior to nailing it on. I am hoping this would also eliminate or come close to the air infiltration around the attachment points.
where the sheathing would be needed structurally, it would be used. Also I am not going with the 'true' advanced framing method of 24" oc. I will still go with 16" oc if I do not use osb behind the foam.
RoberthUser is Offline
New Member
New Member
Send Private Message
Posts:97

--
11 Oct 2010 05:23 PM

I think you can use the double stud wall and cellulose. This requires that you pay attention to air sealing. Obviously if you use a SIP most of the air sealing is provided by the SIP but at $ cost. So the trade off is do you pay extra for the SIP or accomplish the air sealing in another fashion.

There is also something called the modified Larson Truss that is used in place of a double wall. Like the double wall it addresses thermal bridging and also saves on lumber.

There are multiple ways to air seal.  I would consider sealing the outer shell.  Plywood or OSB are air barriers with the weakness being the joints. I think sealing from the exterior is a good approach. There is also the Huber Zip system, a coated OSB in which the seams are taped. 

You can combine  air sealing methods.  To air tight drywall approach can be used along with the exterior sealing. What is recommended it that a blower door test is down after the exterior is sealed but before the insulation and drywall are installed. By doing this you can find air leaks in the shell and fix them while you have access and can easily find.

If you have throughly air sealed then the only source of moisture is through diffusion which will be a very small amount. The good thing about cellulose is that is can store the moisture until such a time as it can dry.

The largest source of moisture in a wall comes from air leaks. The more air leakage the more moisture that is carried into the wall with a chance to condense. Take away the air flow and the only moisture- not including leaks- is from diffusion. As long as you don't include a poly vapor barrier the moisture will be able to reverse and diffuse back to the interior when conditions change.

SIPs have had problems too and you should learn about them and how to deal with their shortcomings. They can leak small amounts of air along the seams leading to condensation and failure of the OSB. There are ways to address this and is one of the reasons some recommend rigid foam on the exterior. of a SIP.  The exterior foam moves the dew point away from the OSB. If you decide to use SIPS learn the best ways to deal with air leakage at the seams.

Bob IUser is Offline
Veteran Member
Veteran Member
Send Private Message
Posts:1435

--
11 Oct 2010 07:36 PM
"do not see how strapping punctures the foam any less then the nails would from the siding"
Just count the fasteners. With the strapping you're doing one or two every 16" in each direction, with the siding, quite a few more (depending on siding)Also the strapping screws will be tight, vs vinyl siding nails which would be more liable to loosen up.
Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
You are not authorized to post a reply.

Active Forums 4.1
Membership Membership: Latest New User Latest: hudson2000 New Today New Today: 0 New Yesterday New Yesterday: 2 User Count Overall: 34707
People Online People Online: Visitors Visitors: 97 Members Members: 0 Total Total: 97
Copyright 2011 by BuildCentral, Inc.   Terms Of Use  Privacy Statement