If you had the opportunity to buy an entire semi load of foam board, which would you choose?

2" 4x8 XEPS $6,250/552 sheets
2" 4x8 ISO $6,250/552 sheets
2" 4x8 EPS $2,100/552 sheets

Plus $1000 for shipping

I was originally thinking of using xeps or iso, but at almost 3x the cost of eps is it worth the extra cost?

I plan to install 2" below the basement and garage slabs, 2" on the exterior or interior of the basement foundation walls (still undecided), and 2" on the exterior of the house. I know the eps is a lower R value, but is it worth the extra $4000?

You know the answer.

Is this from the insulation depot? If not disreguard this, but, I have bought from them on several occasions and with some discussion and prying I found that 2 of their warehouses were within reasonable driving distance and as long as I bought a "skid",  (that is a pile of whatever type of insul. stacked about 94" tall to fit in a std. van trailer) that they would sell it that way rather than a full tt load. I only have good things to say about them so far. Now it's a crap shoot if they have what you want in a nearby warehouse. Hope this can help. And get the iso.

You don't want ISO under the slabs or on the exterior below grade.

Next question is which one gives you the most R/$. XPS is ~R5 per inch while EPS is ~R4/inch.

XPS is only 25% higher R value than EPS, but in the prices listed above, it cost 300% more than EPS. You do the math.

Posted By tbm878 on 03 Mar 2012 11:11 AM
If you had the opportunity to buy an entire semi load of foam board, which would you choose?

2" 4x8 XEPS $6,250/552 sheets
2" 4x8 ISO $6,250/552 sheets
2" 4x8 EPS $2,100/552 sheets

Plus $1000 for shipping

I was originally thinking of using xeps or iso, but at almost 3x the cost of eps is it worth the extra cost?

I plan to install 2" below the basement and garage slabs, 2" on the exterior or interior of the basement foundation walls (still undecided), and 2" on the exterior of the house. I know the eps is a lower R value, but is it worth the extra $4000?

EPS is has 80% of the R value of XEPS, so for the sub-slab stuff where thickness is of no consequence you can double up the thickness using EPS and get 1.6x the total R for 2/3 the money.

On the basement wall you can also use 2x total using EPS, but put 2" on the exterior, 2" on the interior, which gives you a thermal break, against the thermal bridging of the foundation wall to the footing if you run the sub-slab foam right up to the interior wall-foam (floating the slab.)  You then also have a thermal break on the exterior for the foundation sill.  (You should still insulate the interior side of the foundation sill & band joist for maximal thermal break though.)

Fiber-faced iso can be used on the interior of the foundation wall, but not sub slab, and not on the exterior of the foundation wall. Foil faced goods can be used on the interior of the foundation wall ONLY if you have a membrane or metal capillary break between the footing and the foundation wall to block the wicking of groundwater, and between the foundation wall & foundation sill (to protect the foundation sill.)

Lining up the exterior first-floor sheathing foam with that of the foundation foam requires some planning of where you put the foundation sill relative to the concrete, but it's not rocket science to put 4" of exterior foam outside the sheathing, staggering 2 layers for better air sealing- just leave 2.5" of the foundation top exposed on the exterior side of the foundation sill.   Even with a 6" foundation wall this works just fine with 2x4 framing.

If you'd planned on 2x6 framing, consider this: 

The whole-wall R of 2x6 with fiber insulation runs ~R14-R15 depending on framing fraction. Add R12 for 2" of iso you'd be at ~ R26-27  

The whole wall R of 2x4 framing/fiber is ~R10, and adding R16 for 4" of exterior EPS is R26. 

Same wall thickness, cheaper foam, same R.

The R value of iso at 25F average foam temp has to be derated to ~R5.6/inch, so 2" of iso gives you really more like R11  at the cold temperature extremes. So in a cold /very-cold climate the above stackup is more like R25.

The R value of EPS at 25F average foam temp can be upgraded to ~R4.3/inch, so 4" of EPS gives you more like R17 making the above stackup is really more like R27- a modest performance improvement over the 2x6 + 2" iso situation.

Dana,

I really appreciate the time you take with your posts. They are very informative, and very helpful. Thank you very much! Most of what you have said in your posts over the past week or so are starting to make sense.

A couple more thoughts....

Would it be worth the small extra cost to install the extra 2" (so 4" total) of under slab insulation in the basement? I am not using radiant floor heat, and i do see the basement being finished at a later date but not for a good 10 years.

Also, how about the garage floor? I had planned to install 2" of foam under the garage floor slab. I do plan on installing a small natural gas unit heater at ceiling height to make the winter temps more bearable if i'm working in the garage. I do not anticipate spending a ton of time out there in the winter months. Would the garage slab insulation be worth it in the end (total cost of about $500)? Oh, and should i insulate the exterior 4' garage foundation walls too?

Posted By tbm878 on 05 Mar 2012 11:21 PM
A couple more thoughts....

Would it be worth the small extra cost to install the extra 2" (so 4" total) of under slab insulation in the basement? I am not using radiant floor heat, and i do see the basement being finished at a later date but not for a good 10 years.

Also, how about the garage floor? I had planned to install 2" of foam under the garage floor slab. I do plan on installing a small natural gas unit heater at ceiling height to make the winter temps more bearable if i'm working in the garage. I do not anticipate spending a ton of time out there in the winter months. Would the garage slab insulation be worth it in the end (total cost of about $500)? Oh, and should i insulate the exterior 4' garage foundation walls too?

The breakdown of how much sub-slab R is "worth it" depends on several factors: 


*Your location/climate/subsoil-temp

*Your per BTU fuel pricing & mechanical systems efficiency

*The cost per R per square foot of the foam

*The relative R-value of the soil type

Buying reclaimed goods your foam costs are dramatically less than the ~10cents/R/square-foot of virgin-stock EPS, more like 2-4 cents/r-ft, depending on the deal. The installation labor may be ever so slightly higher than with virgin stock due to more sorting and ding-repair, but probably not even 10% more, so figure the installed cost of the foam will be something like half.

For virgin-stock foam and average utility costs the sub-slab foam that makes some longer-term economic sense can be found on  the right-most column of Table 2, p10 of this document, sorted by the US climate zone number, found in the left-most column.   If you're paying half as much for the foam and you're heating is something like propane or oil (not recommended) going 2-3x those valued might have a reasonable net-present-value, but if the heating is supplied by geothermal in a low-electricity cost market or condensing gas in a low natural-gas cost area anything more than the recommended values might have a long payback, even with half-price foam.  At 4x those values the lifecycle cost of virgin stock is probably more expensive than photovoltaic electricity driving mini-split heat pumps, (a criticism often leveled at the PassiveHouse folks for their high-R sub-slab recommendations by people on the Net Zero Energy side of the fence) but may still beat that metric when using reclaimed goods.

The subsoil R will vary by soil type and moisture content, distance to the water-table, etc, and it would almost always be cheaper to add another R8 in reclaimed EPS than it would be to do the site-testing to figure out if it was actually necessary.

So, long story short, if you live in US  zones 5 or higher doubling up the sub-slab EPS to 4" for ~R16 would almost always make some economic sense with half-priced foam.  In zones 7 & 8 there's probably a rationale for taking it to 6" (~R24).  But in zones 3 & 4, maybe not, unless you have expensive utility costs.  In zones 1 & 2  (and even the warm edge of zone 3) it's very design and site specific- sometimes "earth coupling" the house to the subsoil can reduce heating & cooling costs, but in very high-R homes PassiveHouse style you may still want some sub-slab R in those zones. The only way to tell for sure would be to model the whole house with site, orientation, and climate factors for it's estimated energy use with either a pretty good freebie like DOE2 or BeOpt, or the (relatively) low cost PassiveHouse tools.

It's worth insulating the garage foundation as well, but probably not to much more than in the table in the BSC RR-1005 document. (Note that the R-values are for "whole assembly" R, with all thermal bridging from framing factored in, eg:  A wood sheathed wood-siding 2x6 wall with R20 insulation between the studs and a 20% framing fraction comes in at ~R14, not R20.)