GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Fiber-faced polyiso for continuous insulation

don_christensen | Posted in Energy Efficiency and Durability on

Been reading GBA for a while, although this is my first post:

When using fiber-faced roofing polyiso for continuous wall insulation, can you install the polyiso outboard of the WRB?  A large recycler in my area (usually) has reclaimed roofing polyiso at a reasonable price (4×8 sheets at 2 inch thickness, $0.52/sq. ft.), and reclaimed material seems sort of ‘green’.

Please consider the following assembly, inside to outside:
– Drywall with latex paint
– 2×6 framed wall, dense-pack cellulose cavity fill
– 1/2 inch plywood sheathing, taped for air-tightness
– Water resistive barrier (vapor open membrane – housewrap, drainable?)
– Windows installed at sheathing plane
– Fiber-faced polyiso, 2 to 4 inches
– 1×4 furring strips, screwed to framing
– Cladding (cedar, engineered wood, maybe metal)

I’m looking for a reliable, proven assembly using locally available materials wherever possible.  I see advantages in locating the main air barrier, WRB, structural sheathing and window flanges all at the sheathing plane, advantages like simplified flashing details and a somewhat protected air barrier.

1) Will fiber-faced material withstand the occasional incidental wetting it will likely see, or does it need to be protected by the WRB?  Moving the WRB to the outside may mean moving the windows out too.
2) Assuming outside is OK, do you need drainable housewrap, or something to provide a small gap, between the sheathing and polyiso?
3) Is fiber-faced more vapor-permeable that foil-faced, enough so to permit some outward drying?
4) I live in a somewhat termite-prone area (just east of Denver, CZ 5B, 6100 HDD, 15 in. annual rainfall).  Does that make exterior foam insulation of any kind a risky idea?  Note – There is also a local distributor for Rockwool Comfortboard, but that goes for $2.30/sq. ft. at 2-inch thickness, 4 times as much as the reclaimed roofing polyiso.  I’m also interested in fiberboard (Gutex etc.), but I’d rather get that from a source closer than Europe.  I hear GO-Lab in Maine is expecting to go into production next year.

FWIW, an acquaintance gave me a few leftover sheets of 2-inch roofing foam that look an awful lot like R-Max Multi-Max FA-3 or equivalent (see attachment).  I left a broken piece outside for 2 years (still out there).  It’s been rained, snowed, hailed, and UV’d on, and it still looks pretty much intact – no cracking, swelling, warping, or delamination.  The fiber facing is starting to look a little ‘furry’, but otherwise it still looks usable.  Attached Multi-Max spec sheet lists Water Vapor Transmission as < 1.5 Perm, although it doesn’t say at what thickness.

Thanks everybody.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. onslow | | #1


    The fiberglass faced iso is a bit scratchy to work with, but as it is put on roofs in huge quantities, I imagine it would prove to be stable with minor wetting events, though not pooling. Both roofs and walls are not supposed to be seeing much (if any) bulk water anyway. I believe that the fiberglass surface is to provide secure adhesion of hot roofing. You have your test of just how tight the facing is bonded already done. The material isn't likely your biggest worry. Deciding on the WRB location is much discussed on GBA. I will not rush in on this point, I made my wall decisions based on a very different final cladding. More further on.

    FWIW, I used new fiber faced roofing polyiso in two locations on my build. I am at 8,000 ft about 300miles west of you. I used half of the stock on an inside garage walls which is not subject to water, but is subject to mice. Despite 5/8" fire rated sheet rock, the little buggers have found a few corners that provide a foot hold. Not sure if termites wouldn't be an improvement. Less smelly when they die.

    The other sheets are on the roof of a smallish bump out which has 4" nail base over the polyiso. This decision was driven by the fall-off of R value in iso when very cold. The nail base is intended to keep up the iso value and the total profile within safe R percentage of above deck insulation. No issues yet. For your location I think all polyiso will be fine. I suspect that, while snowy and windy, the winters are a bit less intense over on the front range and the summers hotter which favors the polyiso.

    Both termites and carpenter ants are here on earth to disassemble fallen or sick trees. Detailing your foundation sill against termites might be most necessary. I think they need consistent moisture to thrive, so damp sill plates, door thresholds and similar would be their preferred environments, not the foam. Copper and stainless steel barriers are sold as termite guards. I don't have an opinion on them as I, fortunately, have never dealt with them. Do not plan on cast concrete patio or door stoops that kiss the foundation and match the floor height. That sort of feature is begging for hidden invasion.

    The foam might be more appealing to carpenter ants. Ants gallery the wood for a nest. They can go get water elsewhere if needed and don't need the little mud tunnels to stay safe while fetching it. I have had past homes that got invaded around a door jamb. A soggy door jamb bottom was their cue to nest in the "rotted log" . The moist jamb was a nice community well, which allowed them to construct a huge nest.

    The nest occupied the entire first bay next to the door framing. The fiberglass insulation was re-formed into a magnificent antfarm only without the glass sides. The wood got chewed too, but the main city was in the insulation. Point being, they didn't seem to mind biting the stuff. Polyiso might seem as convenient. Others will have stories too.

    Detailing the siding rain gap against intrusions and protecting the bottom exposed edges of the iso needs to be pretty tightly done. Every manner of critter will be looking for access to the nice 3/4" gap behind the siding. Perforated stainless or Corovent at minimum for the rain gap. This gap is where you might expect bulk water to make it in. With good overhangs and proper trims, it is unlikely that water would drive all the way to sheathing. Still the WRB question has many answers and locations.

    Generally, "outsulation" means drying will be to the inside. I did air gap my foam, however. The sheathing is entirely covered in Henry Blueskin which I like a lot. No need to tape the plywood first. It functions as air and weather barrier all in one. Sticks well even when cold. The windows are "middies" that are mounted on window bucks that stand about 3" proud of the sheathing. Detailed taping in conjunction with the Blueskin seems to be performing very well. I can't say that the method I used is repeatable in your cladding style. I have had four years to peak for leaks since the interior work took much too long. (only one more window sill to go!) So far so good.

    My home is clad mostly in EPS. A formed steel tray provides a drain path from the gap at the sheathing layer outward and protects the bottom edge against mice and bugs. The foam over the sheathing is about 6 1/2 " total. A two coat stucco over that creates a faux Santa Fe look. The walls are capped at the parapet walls, so there is no air flow behind the foam, which is held off the Blueskin by about 3/16" to provide the main drain plane. As noted, all drying is intended to be inward, with the lowest sheathing temperature expected to be around 45-50 at the most extreme design point. I went with R-19 batts in the stud bays because I am gun shy about cellulose settling. It might be good to check whether dense pack will shift your R ratio into your sheathing plane. Good air sealing is of course a must.

    Setting flanged windows on the sheathing or to the foam face will entail careful detailing and integration with the WRB. Others will be better able to chime in here. There are many details available in the GBA library as well as articles to go over. Getting crews to lap the WRB and tape seams correctly will be a headache to anticipate. The new ThermalBuck product might be a good answer to bringing the windows and WRB layer forward in a clean way. The faster and more repeatable results could outweigh the costs.

    Do consider how you will trim the inside of the opening. I went with large radius corners on drywall on three sides which proved fussy, but the goal was a faux-dobe appearance. The sills are deep, so the cats and plants enjoy them. Think about window drapes and shades now, before you commit. If you are tempted by the Euro-style windows, think about what happens when you swing or tilt the window. Also make sure you have blocking for window treatments in place before drywall crews show up. They move very fast.

    Hopefully, I haven't strayed to far from what you asked. It is certain that I have carried on too long.
    Do check for expansive clays and keep radon in mind when planning your foundation.

  2. Expert Member
    Akos | | #2

    The roofing polyiso here is part paper/fiberglass faced. If it gets wet it tends to taco, I would not mount it in a way that it will get wet.

    The material itself is somewhat vapor open, most data sheets don't specifically state the perm rating but it is somewhere around 1 for 2". This makes it a good candiate for exterior insulation as it does allow a bit of drying.

    Whenever I've used fiber faced polyiso, the WRB vent on the outside. This means if you want to use flanged windows, you need to build some bucks to bring them out. This is not hard as it is just a some 2x lumber riped to the thickness of the foam and screwed right to the perimeter of the rough opening.

    With flangless windows, you can mount them without buck further in. You need a bit of flashing tape origami to bring the WRB back to the window. I've done it this way, and it is not hard just have to be careful.

    With any type of insulation, you need protection from critters. This means perforated J trim on the bottom to protect the foam and the rain screen cavity.

  3. don_christensen | | #3

    Roger, Akos, Thank you both for your thoughtful replies and advice.

    Roger, you brought up several issues that are also on my mind, especially the prevalence of swelling soils in my area, also mice and bugs. At my place, there is a thin skin of topsoil; below that is hard clay, going down as far as I have dug or seen. I haven't had any foundation cracking or movement, but I know soil conditions can vary over just a short distance around here.

    I had a paragraph in my original post about protecting the bottom of the rainscreen gap and bottom of the insulation boards, but I took it out to keep my post from being too long. Good points however, and well taken.

    Your project sounds really interesting and unique. It's good to get another perspective from a nearby, at least mountain west, builder.

    Akos, I am inclined to follow your advice about sequencing the layers if I use the fiber-faced polyiso. I follow you on the window bucks and different flashing details for 'outie' windows. I have seen other successful examples of this assembly. I trust your experience here.

    Overall, though, I think you have mostly talked me out of using the fiber-faced material in this application. One sticking point seems to be how you keep the WRB in place until you get it held securely by the rainscreen strapping. wind-free days are rare around here. Maybe something adhesive, like Henry Blueskin or Siga Majvest SA, would stick to the fiber facing, especially if primed around the edges? Whatever happens, I don't want to compromise durability to save a few bucks. On the whole, I am leaning back toward Rockwool Comfortboard for exterior insulation, which, besides being vapor open, non-absorbent, and inherently drainable, is more bug resistant and fire resistant than foam (although protecting the bottom is still important).

    Looking at other assemblies too. Now along comes the 'Tournament of Wall Assemblies' article - now I'll never decide!


  4. Expert Member
    Akos | | #4

    I think before going down the insulation rabbit hole, best find a contractor that is willing to work with you. Find out which walls they are comfortable building and design around that.

    Here, exterior rigid foam insulation was unheard of until it became code, now all the sudden it is "simple". So the lowest cost / high performance wall is take a code min wall and just bump up the exterior insulation a bit.

    No point in coming up the "perfect" wall assembly and not finding somebody willing to build it, never mind the know how to build it well.

    Exterior rigid mineral wool would be a challenge as it is only ever used in commercial buildings.

    Also good to do a bit of heat loss and energy cost modeling. Sometimes very high R value walls are just not worth it.

  5. onslow | | #5


    I certainly wasn't meaning to scare you off the recycled polyiso, just advise about possible invaders to plan on defeat. I think the cost of a bit of extra detailing is well worth the savings over rockwool. My second home was a mashup of 30' 50's and 70's remodeling. Some walls had no insulation, some had pink batts, some had what I grew up calling Rockwool, the loose fill version of todays Comfortboard and batts. The mice enjoyed tunneling through all of them. The Kimberlite paper insulation in the attic I removed immediately upon buying the house. Oddly the mice left that stuff alone. Good thing the mice didn't smoke, the stuff burned like crazy.

    The EPS I have used is considered prime ant farm real estate by many, but I am gambling that the detailing I have done will prevent ready access. Plus I am bit drier at ~11" of rain a year. One thing to consider with the Comfortboard is the potential for over screwing the battens which makes the siding go wavy. This has been addressed by others, but still needs to be noted. Details for keeping things true and level, as well as how to guide the crews, can be found here in GBA or FineHomebuilding. Stripping 3/4" plywood into batten strips to minimize splitting is one suggestion I have noted. Be sure to consider what fasteners are required for your siding choice. If you do go with bare metal that becomes rusted ( a popular choice over here) be aware that the rust will also stain below the walls. Sidewalks and light stone will become very tanned.

    As to the WRB, I put the Blueskin directly to sheathing over the entire house. It is vapor open, but with the 6+ inches of foam that is really moot. My back gap is static air due to being only 3/16 wide and as noted, capped at the top. To get air flowing at my wall plane through that gap would take quite a lot of suction. I used the Blueskin to seal against water and air in one bite. No taping. If bulk water ever makes it that deep, I have real problem.

    If you did choose to tape all the plywood and then mount the foam directly without a gap, then your frontline defense against water intrusion will be the rainscreen gap and WRB on the face of the foam. This approach seems widely accepted, with the understanding that flashing details, window placement and overhangs will make this arrangement secure against water ever getting near the sheathing plane. Outie windows and roof profiles with overhangs all the way around should perform well with this approach. If you were doing a mid-century modern Mies box or zero overhang roof, I would suggest WRB at the sheathing. And being the way I am, another Tyvek like wrap on the foam face held by the battens. You might be able to deal with the winds by starting with one batten pinning the roll goods and chasing along with every other batten to keep control. If you get lucky and find foil faced iso for the second layer, just tape and go. And remember the sample you have has been through far more than what a wall should experience, so judge accordingly.

    I have commented before on the amount of air exchange that actually occurs in a rain screen gap, which I would maintain is less than expected. If you detail in a way that the gap vents at the top just short of the eaves, then you will need to provide Corrovent or equivalent protection of the gap there just as you do at the bottom. Bees and hornets will be searching for gaps there. This is assuming traditional siding types which overlap, not the currently trendy nickel gap choice. Nickel gap will most definitely have free air exchange. It will also ship water pretty well, too.

    There are as you have found many ways to put a house together. GBA is a good resource, so keep clicking on pages and read as many comments as you can. Last thing about the soil comment. I did pay for a soil profile, which showed 3-5% expansive clay in a layer 3' down. The bedrock here was only 4-5' down so the cheap choice was to keep going til bedrock. If you are in deep clay, it might be just as solid as you perceive or just waiting to get wet. If your lot is flat, the bigger issue will be keeping water from accumulating in the newly disturbed soil around the foundation. Even with 11" of rain total, the ground flow each spring shows up in my footing drain peek hole. And I have 5' of elevation drop across my footprint with drain to daylight working for me. Check around with the neighbors. Cold beer can elicit illuminating stories.

    1. don_christensen | | #6

      Lots more to think about here. Akos - I hear you about finding contractors who are comfortable with the plan. There was a recent post here, from someone in Denver area as I recall, who was unable to find a contractor willing to do exterior foam. He ended up using Zip-R. Comfortboard may not be an option either. I haven't really pursued it yet; so far just research and early planning. I had R30 whole-wall in mind as a rough target. Following Harold Orr's 'superinsulation' formula (HDD/180) would call for R34, but R30 or close represents a big step up from normal practice. I downloaded the BEopt tutorial but only viewed the first section so far.
      Roger - My thinking for the gap behind the foam was to minimize hydro-static pressure pushing moisture through the WRB, but as you note, there shouldn't be much water there unless you made some pretty serious mistakes. My present house has wide overhangs, which I see as a valuable feature. Also, I would for sure get some soil analysis done before breaking ground on any project.

      I'm curious, what do you have between the EPS and stucco?

  6. onslow | | #7


    The stucco is essentially the greatly feared EIFS system with a modification of the base coat. 1 1/2" low density foam sheet was glued to my base EPS with 3/8 notch applied cementitous adhesive in vertical only lines. The foam was then surface abraded everywhere and shaped with soft radii around doors, corners, and three sides of window openings. The top side of each window opening was left flat and squared with a plastic trim piece that creates a drip edge to prevent water from tracking down from above and back to the window. As I noted before, the windows are "middies" which makes them about 2 1/2" recessed from the main surface.

    The performance so far has been very good. I did have a few cracks appear first year near two windows which seem stable now after caulking them. I have one new crack that needs attending after four years. Hopefully, I will continue with this very low level of stress cracking. Why I don't have three coat is an offline story.

    In my area the flickers and woodpeckers are a notorious problem with EIFS over foam. Some say the stucco over foam sounds like a nice dead tree to nest in when tapped. Increasing the mesh grade was one way to toughen the coat, but quick tests showed the ready mix brand base coat was still too soft. I chose a Parex product that requires mixing from dry powder, but has twice the impact rating of the standard stuff that comes in buckets ready to apply. ( I am deliberately leaving out the brand name.) The Parex with regular mesh has so far kept them at bay despite a few of them attempting. The textured top coat is essentially just very dirty latex paint, which seals the surface against rapid water absorbtion.

    I am clueless to whether there is significant moisture drive with this form of stucco. I can only surmise that there is likely not too much. If there was a lot of driven water being pushed into the first layer of foam, I think I would experience the equivalent of frost heaves what with the rapid temperature shifts we get here. In any case, the water management plan is to have the 3/8 notched glue level perform as primary capillary break for driven moisture should it get that far. If a really major blowout occurs somewhere up at a parapet junction (a distinct design flaw on my part) and water does manage to get all the way to sheathing, then the Blueskin and 3/16 gap will send it down to the perforated tray used as a starter strip.

    You are very wise to be doing extensive review of options and methods before building. I am a bit obsessive and consumed GBA for 5 years before committing. Even then finding people willing to work with somewhat unconventional processes can be a major challenge. I got lucky. There were a few bumps along the way, but the results I feel were worth it.

    Here is a link to a Colorado Geological Survey. org page. Might be a good start point to see if you have any worries about expansive clays. A quick look at some of the distribution maps might be encouraging.

    1. don_christensen | | #8

      Thanks for link, and for the info on your stucco design. What I have read--I'm no expert here--is that EIFS failures occurred mostly on face-sealed systems, which lacked adequate drainage/drying, while properly flashed water-managed systems (like yours I would think) have performed well. Even better when it's woodpecker-proof. Thanks - Don

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |