How do SIPs compare to 'gapped' 2x4 stud walls (i.e., building an exterior wall using two 2x4 stud walls with a gap inbetween to create a thermal break) with spray foam insulation?  Looking for insight into costs, build time, build difficulty (both for framer and other trades), insulation values, etc. given a home design that is essentially a rectangle but has a more complex roof (multiple gables and dormers).

Hmmm, you mean something like this:



And, you don't even need spray foam(the foam sheathing seals).

However, you will be disappointed. If you were providing all of the labor(DIY), I could make a case for cost savings. However, if you are hiring out the labor, SIPS(6.5", 8.25" or 10.25") would probably be less to have installed. And, I guarantee that SIPS would give you a 'dried in' structure quicker. Oh, and when you price out spray foam, you'd think that you were buying the SIPS at WallyWorld.

JC - What's the going rate for urethane spray foam in the Springs area? Say 3-3.5" of foam on the walls and 6" of foam in the ceiling? I'm curious because I'd like to tell my general contractor that the SIP structure was cheaper than his proposed stick built. He keeps reminding me he would have been cheaper after we had the one week of roof assembly problems.

jboysen,

First of all, I think a gapped 2X4 wall will perform better than a 2X6 stud wall because there will be less thermal breaks. According to the Oak Ridge Federal Report, a 2X6 stud wall with R-19 batt insulation on 24” centers has a whole wall R-value of 13.7. Let’s say for the moment that gapping the 2X4’s somehow brings the whole wall R-value up to R-16. Again, according to the Oak Ridge Federal Report, a SIP wall has a whole wall R-value of 21.6. 21.6 compared to 16 is a substantial difference.

Labor will probably be about the same to do a 2X6 wall versus a gapped 2X4 wall, and the material will be less expensive, as 2X4 are less expensive that 2X6’s. Labor for SIP’s would be much less because they go up so much faster. For a 2500 sq ft house, probably 2 days for SIP walls and 4 weeks for stud walls.

A 2X4 gapped wall won’t be as strong as a 2X6 wall, as 2X6’s are stronger than 2X4’s, unless you place the 2X4’s on 12” centers. I’m not sure of the codes in your area, but I’d call and find out what they allow. A SIP wall is about 3 times stronger than a 2X6 stud wall.

Again, I’m not sure, but I think spray foam plus its installation is substantially more expensive than batt insulation.

SIP’s, on the average, cost 1-3 percent more than a stick built house. SIP’s go up faster, make a tighter envelope, are generally easy to assemble and have a higher R-value than stick built.

SIP roofs can be difficult if the proper design work isn’t done properly. Dormers can be tricky. If you have flat shed dormer roofs, it’s all pretty simple. If you have gabled dormers, you have to make sure that the homework is done. I’ve heard of people taking ½” Styrofoam and making a scale model of the panels for the roof to make sure every thing works. If a dormer panel is off, it can be modified at the site to work, but it takes a little time and effort. I like the flat shed roof dormers because they make more interior room in the house. Ask your SIP supplier if they use a CAD design program to draw the plans. There's a much better chance of success drawing with a CAD design program.

The Oak Ridge Federal Report can be found at: http://www.ornl.gov/sci/roofs+walls/articles/WallRating.html
Also, they call a SIP panel a Stress Skin Panel Wall.

Steve Etten
[email protected]
www.GrandCountySIPs.com

jboysen,

One of the most important factors in your decision making needs to be the heat loss numbers - if you have a relatively low to medium delta T then the differences between wall types will not make too much difference in heat loss rates, so upfront price and install time (capital outlay) become more weighty in comparison to fuel usage numbers.
If your design temperature is 55 below 0, like here in Fairbanks, then your delta T is in the 125 degree difference range and you better plan on foam (whether SIP, ICF with extra foam outside, or spray foam), because the extra capital cost is going to pay off much faster in fuel savings.

Every significant study of fiberglass batts has shown that its real R-value decreases with drop in temperature. This is a bad trend for anybody with high delta T's. Fiberglass batt is not the devil's gift to homebuilders, it is simply being replaced by better, faster methods. Why not use them?

Full ICF walls can be difficult because of bracing needs, concrete knowhow, can't fix it easily if you don't do right the first time, etc - but if you got the talent, it is an awesome system.

SIP walls are the same except that the manufacturing end is where it can get messy - bad support or lack of planning coordination can make a simple install a nightmare, but it is also superior structurally and insulatively to any kind of stick construction.

Spray foam is $1.10 to $1.50 a board foot up here - by the time you stick frame and foam cost gets prohibitive if you want an appropriate depth for cold climates.

Basically, the only stick frame construction I have seen that compares to ICF/SIP is laid out by the Cold Climate Housing Research Center - LINK
But all this is really just dreaming about possibilities if you don't do a heat/cooling loss design up front - you need to know within 10-15% what your yearly fuel cost would be for at least one wall type. Then compare the better systems and see how long it would take to pay off the increase in up front costs by reducing your heating cooling bills. There are many different programs to do this - I use AK Warm. A good program like this will allow you to put in wall, roof, underslab, against foundation, FPSF, or any other kind of insulation, at any thickness, and even different types - batt inside against spray foam with board foam on exterior - whatever!

It really comes down to the money - are you going to spend more now or later? I intend to live in my house for another 10 years at least - I spent enough up front to get heat loss down to 10Btu/sq ft at 55 below, but it will take most of the 10 years at $4.50/gal heating oil to pay off the up front cost increase. Anything beyond is gravy.

Find a program you can learn and run numbers. This is the part that most people skip, because it can be difficult. If you don't, however, you are just rolling the dice and anybody's guess goes about which system is best for you.

P.S. Whatever system you use, Tremco is cheap - R100 walls won't keep you warm if it ain't sealed up right...

Good Luck!

The last time I checked, and it was a while ago, it was something like $0.60 - $0.80 per board foot, 1" thick. So, a 3.5" 2x4 wall it would cost between $2.10 - $2.80 per sqft(installed of course!).

I have a client that built a quonset hut on his property. He had the inside of it sprayed with foam. As I recall, the foam cost as much as quonset hut kit.

I'm located in MN, so my temperature delta is fairly high. The house design is effectively a 35 x 65 foot two story rectangle with one gable roof going the long way with another gable roof at either end perpendicular to the middle (think of a big 'H' shape where the middle connector is as wide as the legs are long). Each perpindicular gable at the end has a shed roof dormer on the outside. Am I correct in thinking SIP construction should be fairly straightforward with this design? Even if doing a SIP roof?

JC, if I'm not DIY any of the construction, would a Super Wall (as illustrated with out spray foam, using batting instead) be cheaper than SIP? Taking into account labor AND materials? Seems like the Super Wall would have significantly higher R values as compared to SIPs.

The walls would be easy. A pic would help, but the roof sounds fairly straight forward also.


Cheaper? No. The labor would eliminate any savings. R-Value? That would depend on the size of the panel. We have done 2 projects this year with 10.25" SIP walls ~R-40. However, most projects out here utilize 6.5" R-25 SIPS.

What's an average price difference between a 6.5" SIP , 8.25" SIP, and a 10.25" SIP? I assume labor is basically the same?  What are the average R values for each of the three thicknesses?

I estimate them at $6, $6.50 & $7.00 respectively. That is a sqft price and includes everything but installation materials & shipping. But it is an estimate, and it should be a bit high. Labor should be about the same, but each panel size is heavier. R-Values are: 25.5, 32 & 40.

Includes everything but installation, materials, and shipping? What DOES it include!?   :)

Jelly,

I don't know what jc intended, but your two added commas radically change the statement.

Larry

I had assumed that Jelly was making a joke. Maybe I should have said 'Installation Supplies'. Never-the-less, the price would include fully fabricated panels, with most lumber factory installed(window/door wraps & point load members) and top & bottom plates.

Yes I was making a joke! I just read it that way the first time through and got a chuckle out of it.

Sorry Jelly.

Larry