Building New Home very soon. Have encountered an awesome circumstance where I'll be obtaining a nearly endlesss supply of Very Good Condition USED 4'x8'x2" Polyiso Insulation Panels FREE! Certainly there will be some small holes to patch and edges to trim where they will be removed from another massive set of projects but I'm used to recycling products when building my last couple of projects. The normal question is when "purchasing" insulation, where is the threshold of diminishing returns "BUT" in this case my only cost will be transporting the material from the old jobsite to the new and touching up slightly damaged areas, both of which in this case will be absolutely minimal expense plus I am the total labor source and enjoy the work. So, when its free, how much is enough for the basement under rat slab, underground walls, above ground walls, roof for a 1 1/2 story home? 1200 sq' main with 1200 sq' basement only heated when using the workshop, 500 sq' in attic heated occasionally for guests. Wood fired hydronic heat, stream power, supplimental solar. Unfortunately with this location, it's a north facing view with virtually no southern exposure around 3000' elevation in north west Virginia. Triple glazed outswing casement windows will be utilized. Mrs. Mule requires traditional looking home but is very understanding of deep window wells so I want to maximize this free opportunity to reuse what would most likely become landfill materials and use it well. If I frame 2x6 24" OC, fill with Roxul which adds nice sound control as well, wrap with 5 layers of the Polyiso panels, figuring an aged R6 per inch, that gives me a combined R83 in the walls, 2x10 Rafters at 16" OC filled with JM R30 fiberglass, topped with 6 layers of the panels gives me a combined R102. Feedback please.

1. don't use it under the slab - it absorbs water when it situations like that.
2. Look into Passive House modeling, or better yet, hire a Passive House consultant to review your plans and help you design the thermal envelope. There is an excellent PH consultant in Roanoke. For one thing, you have the potential of huge moisture problems so learn how to design the house so that isn't an issue; for another you'll need the right amount of ventilation. All in all this free insulation is a huge help in building, but work with the science to do it right.

Thank you so much for the reply. I built the hydro wood system for our current home and coupled it to preheat the fresh air exchange which runs automatically during burn-down to prevent any potential for smoke. We actually experience over dry air during the winter and have to run a humidifier but this home only has 2 overlapped, sealed and taped 1" insulation panels with 3 times the sq. ft. Are you thinking the extra thickness of polyiso and lower sq. ft. will change our humidity structure that much? Thanks again.

in well sealed and ventilated new homes the humidity should be fairly constant; the fact that your home experiences dry air could be an indication that it is not well sealed. Your new house can be better insulated than your old, but the insulation level is only one of several factors that make a superinsulated house work as it should; others include air sealing, minimizing thermal bridging, ventilation & others. One danger of not being aware of these factors is that you will be building a house that has the potential to trap moisture, since the polyiso doesn't breathe. That could under the right conditions lead to rot, high mold levels and other problems. http://www.greenbuildingadvisor.com is a good place to start.

It seems to me like this is overkill for your climate zone. You might as well just build a passive house, but for the Mrs. Mule.....

And, what do you mean by the "fresh air exchange" and what does it have to do with "potential for smoke"?

Thank you for the reply. Due to the ridge behind the to-be new home, passive house is most likely impossible as my understanding of following this forum and other resources that southern exposure is necessary - the Northern exposure we own is an incredible view but has no energy value thus I was hoping to offset this as much as possible with extreme insulation. When searching for this property we found others with southern exposures with no views – certainly we were Wow’d by this property but I wasn’t feeling very green until I realized the potential of the stream for generating virtually all of our electrical needs without fossil fuels thus offsetting the passive benefits to some degree. Not a perfect tradeoff but minimizes the need for a large array of solar panels. At some point, someone was going to purchase this piece of property – we’re simply working to efficiently be the best stewards of it as opposed to someone throwing up a McMansion and heating the world through inefficiency. Our current home has a preheated heat exchanger to expel indoor stale air and bring in fresh outdoor air - the incoming air is preheated off the outside hydronic wood stoves water reservoir. The reservoir is heated by short high temp burns of wood. This minimizes smoke and buildup for super high efficiency but the initial startup does give off some smoke odor. As a result I coupled the hydronic stoves blower relay to shut off the homes fresh air exchange unit when it fires. The home is so efficient and air tight that we only require the primary hydronic heat to operate when we’ve had several overcast days and nights into the teens, otherwise the fresh air exchange preheating is all that is necessary. The home is impeccably air tight. Even after 10 years I can rest the front door on the main floor in the closed but not latched position, go downstairs to the rear door, swiftly open that door and the front door will close and latch, granted it doesn't make the second seal tight latch without being there to give a little extra push. We were most meticulous in flex caulking every single area of potential hidden air leakage along with proper taping, staggering joints, spray foaming etc - makes a big difference when you do it yourself rather than hoping hired labor cares as much - lots of good hired labor available, simply stating that we personally made sure it was done to exacting standards.

First, 5-6 layers of 2" iso takes some VERY long screws to get to hang on the walls, and even at 6" (pretty much the limit of what's relatively easy to manage) the heat load is going to be dominated by the windows. Even though the foam has zero cost, the fasteners aren't zero cost, and

There's no point to going with 2x6 24" o.c. rather than 2x4 16" o.c. when you have the majority of the R-value on the exterior, with cheap/free iso. Going with the 2x6 framing rather than 2x4 adds only R3-R4 to the whole-wall value. If you instead went with 2x4 and made up the difference with a layer of iso you'd be adding R6, an R3 improvement over the 2x6 solution of the same total wall thickness.

In rough terms, allow R10 for the 2x4 wall (with R15 Roxul cavity fill), and R11-12 for the polyiso. ( If it's 2lb roofing iso and you're rating it for the heating season R5.6/inch would be a closer match to it's average performance than R6/inch.) So with 3 layers of iso you'd be R40+ and still have something that's buildable without needing to design mechanical supports (with thermal breaks) for the insulation. R40 whole-wall can be PassiveHouse or Net-Zero worthy in your climate if the rest of the design is up to snuff. R80+ is ridiculous, requiring a lot more design and labor for next to zero gain in envelope performance (half of "nearly nuthin' " isn't much.)

Similarly on the roof three staggered layers would be enough. An R100 roof may be necessary to hit Net-Zero or PassiveHouse levels in Saskatchewan, but not in VA. While R75-100 might be reasonable & buildable for attic floor insulation, the fastener lengths and keeping it air tight becomes very difficult with a foot of iso. Again, three layers is far more managable, and would yield about R55-60 whole-roof (after thermal bridging of the rafters is factored in), which is also

Low-density R30 fiberglass batts aren't great performers (not that you'd need much)- better performance (but still cheap) solution would be to compress low density R38s in there which would deliver ~R32-33 (center cavity) performance or R36 rock wool. With and ~R4/inch rock wool solution and 6" of iso on the exterior you'd be getting a hair over R60 whole-roof at the 99% outside design temp for heating.

To make even those amounts of insulation worthwhile you have to pay attention to the below grade wall & slab R-values. If the foundation is well drained there are no problems with using iso against the interior side of the foundation walls, and if the stuff is free, it's worth 2-3 layers there, keeping the bottom edges off the slab. At 2 layers it's possible to secure it to the foundation with furring & TapCons through-screwed to the foundation, but at 3 layers a studwall approach makes more sense. Putting an inch of XPS under the studwall plate, extending all the way to the wall concrete works fairly well as a capillary break for both the iso and stud plate. Turning the studs sideways allows you to use 2" of the studwall-plate depth for iso. If you wanted to you could cut/split batts to fill in the 1.5" depth between the turned studs, but it's not really going to make much difference if you already have R30+ of iso between wood & concrete.

Under the slab your cheapest option would be EPS, which runs 10cents/R per square foot if using virgin stock. If you have local vendors who handle reclaimed roofing EPS you can get it for about 2.5-3.5 cents/R-ft. R10 would be the absolute minimum that makes sense with R40+walls and triple-pane windows, but R20 isn't out of the question if using reclaimed goods at deep discount.

The other thing that makes little sense in high-R houses is zoned hydronic heating, whether wood-fired or otherwise. At the R-values I'm suggesting your heating season will be extremely short, and your cooling season much longer (but shallow- your loads will be low if you don't get much sun.) Your heating loads will be SHOCKINGLY low, even at the 99.9% outside condition, and even the smallest straight-ahead-simple woodstoves might be overkill. (A PassiveHouse near me in central MA has R100 walls, and the cooling season for them is from late-February to mid-December, and even during the mid-winter there are days when they need some cooling, easily had by opening windows. :-) At R40 it might have added 4-5 weeks to the heating season, not more.)

Sincere thanks to all with an extreme thanks to Dana1.

What are the polyiso panels faced with?

Thin layer of fiberglass, nice and itchy! Blue gloves and breathing mask worn with all work.

Most fiberglass facers come in at 0.5-1 perms for water vapor permeance, not that it is particularly relevant here. With your stackup the structural sheathing is well inside the major-R of the thermal boundary running near the conditioned space temp & humidity, and never needs to dry toward the exterior. Just be sure to avoid highly vapor retardent interior materials & finishes, Standard latex paint (2-5 perms) is great, foil or vinyl wallpaper (<0.1 perms) is not.

There is a modest improvement in overall performance if the facer exposed to the rainscreen gap behind the siding is foil rather than fiberglass, but it's an irrelevant factor for a high-R home where most the gains/losses are at the windows.

It seems like several layers of polyisocyanurate foam panels with foil facings all stacked together could present a problem in drying if any moisture found its way into the panels (e.g, window flashing problems).

Stacked foil faced iso has a long and generally good history in wall assemblies- I wouldn't sweat that one, even if it WERE foil faced.

Only "outie" windows (where the glass is roughly co-planar with the outer layer of iso rather than co-planar with the structurally) with optimally lousy flashing would accumulate any water at all, and even then it's not as if the stuff wicks up water readily- it would have to be subjected to fairly protracted periods of saturated-wetting to soak up enough to make a difference- it can dry out the same edge that it was wetted.

You're more likely to run into water loading of iso when the bottom edge is resting on a concrete slab that lacks sufficient sub-slab/slab-edge capillary breaks, as in a basement insulation retrofit.