New construction - need some advice on in floor heating....
Last Post 02 Nov 2011 10:38 AM by ilgeo. 33 Replies.
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Dana1User is Offline
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12 Apr 2011 11:38 AM
Yup- the primary benefit of the foam cavity fill is the air-sealing, but that's fairly easily achieved by detailing the Thermax as an air barrier. At K values of ~R4/ inch the low-performance R1/inch framing begins to dominate the heat transfer of the whole-wall (sub)assembly of a typical residential wall assembly, robbing the performance of the center-cavity R considerably.

The R18-R20 center-cavity value of a full cavity fill of 2lb foam is pretty much wasted by the R3-R4 thermal bridging of the framing. With 3-3.5" of 2lb foam and no exterior Thermax you're looking at a typical whole-wall R (that includes the thermal bridging framing factors of headers/plates/windows doors etc) of only ~R12-R13 in a 2x4 16" on center cavity,compared to R10.5 whole-wall for the same structure with cellulose, a less than R2.5 improvement, for a whole lot more money.

With 1.5" of exterior Thermax it rises to ~R23 for the 2lb foam cavity fill, or ~R20 for cellulose. Increasing it 2" of Thermax instead of 1.5" with cellulose fill makes them equivalent-performers, as long as the seams of the Thermax are taped, and the edges foam-sealed.

"Flash and fill" using 1" of foam to seal the cavity and filling the rest with spray/blown fiber is another approach that could be taken for air-sealing. The difference whole-wall R with 1.5" of exterior Thermax with flash'n'fill 2lb foam + spray cellulose and an all-foam show works out to be ~R1, about a 5% performance delta. Increasing the exterior foam to 2" in a flash'n'fill is cheaper & higher performance than 1.5" of Thermax and a full cavity fill of 2lb foam. But it adds another step in the schedule, which has to be played off on the "time is money" equation. In general you get more bang per foam-buck putting the foam outside the framing, where you reap the full performance of the foam.

FWIW: From a design point of view, in climates where it stays below 25F for weeks/months at a time, the K value of exterior Thermax needs to be derated to ~ R5.6/inch, whereas polystyrene K-values can be up-rated about 10%. At 0F they're more equivalent than the ASTM C 518 ratings at 75F might indicate. Given that foil-faced Thermax is an exterior vapor barrier and Styrofoam (tm) XPS is not at thicknesses of 2" or less (just like 2lb foam), if using 2lb foam at 2"+ as cavity fill, consider switching to XPS on the exterior, as it is more protective of the structural wood, since it provides a drying path toward the exterior. An inch of 2lb foam is vapor-retardent enough to protect the wood from inteior vapor drives, and with well over half the total R at center-cavity exterior to the cellulose in a 1.5-2" XPS + 1" SPF + 2.5" cellulose stackup there won't be wintertime moisture accumulation in either the cellulose or the sheathing/framing, and it can dry in both directions.
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12 Apr 2011 12:37 PM
Naturally your numbers are sound but in residential renovation, infiltration is THE factor and nearly impossible to address without foam. The skin is often off limits e.g. Queen Ann downtown St.Paul and the stucture is often nearly impossible to access with other types of insulation. The difference in comfort and performance is often quite dramatic inspite of the often overstated "bridging" affect. From a creature comfort standpoint, even here in Minnesota, thermal bridging is a non-issue. For the purist foam is not GREEN enough, but for a pratical matter what we save in fuel is green in itself.

I have a hot roof in my own 1921 balloon frame and don't have to worry, given the weighted load for roof insulation, but would not trade the performance of foam in terms of overall performance and labor savings in the wall either. When I think of "super" insulated in a cold climate foam always sticks out if labor is considered.

When you speak of double-walls, hay bails etc. I think DIY retired, laided off etc. Maybe good for a few, but will not help the vast majority living in pre-1978 homes. The same is true of passive-solar designs in cold or cloudy climates. If you have enough money to invest and/or will suffer large ambient temperature swings, you,re all set.

There is much talk about performance, but as you point out, the performance is based upon the quality of installation. It is hard to cheat on 2# foam.
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12 Apr 2011 02:46 PM
If you're not allowed to touch the siding as in historical districts, etc, yes, 2lb foam is the way to max out the R-value of what is the possible, but even in those cases the performance difference between a flash'n'fill and a full 2lb foam fill just isn't very much- far less than the center-cavity R value differences might imply.

It's the applications where you were talking about using exterior Thermax where a full cavity fill of 2lb foam is likely to be mis-spent. At a buck a board foot or more it's hard to rationalize just for the air sealing, over a flash'n'fill approach or detailing the air barrier at the rigid-foam layer. And any more than 2" of standard 2lb foam with exterior foil-faced insulation creates a moisture trap at the sheathing layer, and thus not best-practice.

It was the only way to retrofit the 6" raftered cathedral-ceiling attic rooms of my house up to any reasonable R without re-roofing with exterior foam, which I intend to do when the time comes. I used 2lb Icynene taking a ~10% hit in the whole-assembly R value under other 2lb polyurethane foam, but it allows the roof deck to dry toward the interior due to it's higher perm rating. And when I put foil-faced iso above the roof deck in short years it won't create a moisture trap the way most 2lb foams would. I considered doing it flash'n'fill, but it was a pretty small job and the insulation cost was subsidized by the state & local utility, so the cost difference was less of an issue than the time-factor of 2-steps vs. 1.

I've never been in the "no foam" camp- it's more a matter of how MUCH foam to achieve both the air-tightness and vapor retardency that makes sense for that layer & climate. Spray cellulose is cheap stuff, with a very reasonable K-factor for use as cavity fill. Dense-packed cellulose in wall cavities also air-retardent enough for retrofit air-sealing, from a comfort point of view, but requires the interior vapor retardency to be adjusted with paints (and or ventilated siding) to avoid excessive moisture cycling of the exterior sheathing in places as cool as MN.

I never talk about hay bales, and rarely about double-walls/larsen trusses etc. except in the context of PassiveHouse type super-insulation, never as retrofit. I DO talk about air-sealing though- and advocate air-sealing the sheathing and verifying/remediating with blower doors PRIOR to the cavity insulation.
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12 Apr 2011 08:48 PM
I appreciate your grasp of vapor migration, something still lost in many building codes. I don't believe plastic vapor barriers are effective enough for most application except in basements where they shouldn't be used at all. I am glad to flesh out the complexities of insulation with an real expert on the subject.

Were it that everyone did a blower door test before during or after. This fall we will provide a blower door test with every new high efficiency boiler installation (with foam if applicable).

I would not dispute the value of celluose but am concerned with settling in paper products, a factor of no concern with foam.
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13 Apr 2011 02:51 PM
Morgan, I applaud your blower-door testing service!- It's not a usual service for a heating contractor. But air-sealing is often/usually the lowest of the low-hanging fruit on building efficiency, and knowing the infiltration rates up front the heat-loss calc is more precise.

Wet sprayed (aka "stabilized") cellulose is dimensionally stable over decades due to the water-activated adhesives used.

The stability of dense packed cellulose is also very good, if installed at the density appropriate for the climate. Annual humidity cycling in dense-pack introduces some creepage/shrinkage over time if it's density is too low. This has been studied fairly carefully for calculating minimum cellulose densities for Scandanavian climates:

http://www.nordicinnovation.net/nordtestfiler/rep565.pdf

Basically at 3.5lbs density or higher even the coldest parts of the lower 48 will experience no shrinkage over time. Since the density increases with settling the process is inherently self-limiting, so even significant under-density cavity blows will stabilize to within 99% of it's 200 year number in 20-25 years, and it's typically in the low single-digit percentage in 2-2.5lb density "two hole method" low density blows. At only ~3.2lbs density dry blown in US climate zone-5 is good forever, and you'd have to look at a century-long time-frame to find settling worth remediating in the colder parts of zone 6. Dense-packing stabilized cellulose at even 3lbs is also good pretty much forever- it eventually glues itself in place, even when dry-blown.

Foam (particularly closed cell foam, which is less flexible than open cell) also has shrinkage & bonding issues over long time frames (or even in the short term in the hands of an incompetent installer. There is no free lunch, and all things-construction are transitory, grasshopper.

Plastic vapor barriers cause as many problems as they solve, IMHO. Almost all moisture problems in wall structures are ascribable to either bulk-leakage (poor flashing), or air leakage from the interior side in cool climates. A plastic vapor barrier on the interior in a cold climate is typically full of holes from plumbing & electrical penetrations, or even hanging pictures on the walls with nails, and relies on vapor diffusion & air leakage from the exterior side to remove air-transported moisture from the cavity. In a foam-sandwich that's perfectly air-tight it's an all vapor-diffusion show, but nothing in the real world is actually perfect. It's possible to build assemblies to be resilient to both interior air leakage and vapor diffusion by adjusting R-values and the vapor permeance of both the interior and exterior sides so that A: the average winter temperature of the wood stays above the average dew point of the interior air (37-40F for a 30-35% relative humidiy 68-70F conditioned interior) and B: is permeable enough to dry seasonally should some moisture find it's way in (by whatever path).

Using somewhat permeable exterior sheathing foam and providing a 3/8" (10mm) vented cavity between the foam and the siding (aka "rainscreen") to dry into improves the year round exterior drying capacity to the point where you can get best results using only standard latex paints (2-3 perms) as the interior vapor retarder with relatively modest sheathing to cavity R ratios. Without the cavity there will be many annual hours where the micro-space
between the foam & siding is at or near saturation, with zero drying going on. With a 2-3perm interior the vapor diffusion into the wall is slow enough to limit moisture accumulation by diffusion alone, and has substantial spring/summer/fall drying capacity toward the interior.

Cellulose cavity fill can buffer a LOT of moisture (up to something like 20% by weight) without losing R-value, and actively wicks condensation away from the sheathing during the coldest hours when condensing might occur. A flash coat of 1" of closed cell foam on the wood makes the foam (and not the wood) the condensing surface, but a condensing surface that can't wick moisture toward the wood. Yet at 1" it remains vapor permeable enough for the wood to dry toward the interior. The inch of foam only adds ~R2 to the center-cavity R (and ~R1 to the whole-wall R) over an all cellulose fill, but it puts a modest vapor-retarder at a near-optimal point within the stackup, while providing at least half the detailing necessary to make the sheathing an air-barrier. This makes flash'n'fill using 1" of 2lb foam + wet-spray cellulose a real value-proposition, using the properties of both the foam & cellulose to best price/performance advantage.
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13 Apr 2011 04:20 PM

Your conclusion makes sense and I have seen it done, though selling it to the layman is a bit of a chore. I have lost many a job to a monolithic salesman touting open, closed, cellulose or even glass batts. And then you have to argue with the local code official about what constitutes a vapor barrier and where it should be. In the end it is as much about perception as reality and why I have to get involved with fields outside my immediate expertise.

I appreciate the technical nature and astute analysis, as it will help with the more technical of my client base.

There is still a gray area for me when sorting out new construction - an area to which I am increasingly called- and renovation of older houses - which I was born in.

In renovation - as in your ceiling - space and structural limitations seem to dictate 2# foam (knee walls, finished attics and sill plates full holes, cracks and mechanicals come to mind). Open foam may suffice with lower R-value per inch but is not waterproof and the same can be said of blown cellulose.

The blower before - to help with confirming the heat load - and after to prove our work.

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jonrUser is Offline
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14 Apr 2011 09:47 AM
Vapor barriers do involve a lot of debate. For example, here is an actual test in Minnesota where it looks to me like every test wall with plastic on the interior worked well, even the one with plastic on the interior and the exterior. Most of the other walls have mold.

http://www.forestprod.org/woodprotection06huelman.pdf


I would pay more attention to infiltration, either non-absorbent sidings or a rain screen, overhangs (keeps siding dryer), interior humidity and pressure differentials (which will push moisture into a wall).
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14 Apr 2011 10:10 AM
I looked at the slides and didn't get that message. I got bulk water problems and a common synthetic stucco failure with poor workmanship thrown in. I work on them all the time and get calls to fix plumbing leaks that are really poor site prep and rain run-off problems. This was an uncharacteristically poor presentation raising more questions than answers.

My first rule is to do more work than you make, I think they broke the rule.
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Dana1User is Offline
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14 Apr 2011 10:17 AM
Posted By BadgerBoilerMN on 13 Apr 2011 04:20 PM
Your conclusion makes sense and I have seen it done, though selling it to the leyman is a bit of a chore. I have lost many a job to a monolithic salesman taughting open, closed, cellulose or even glass batts. And then you have to argue with the local code official about what constitutes a vapor barrier and where it should be. In the end it is as much about perception as reality and why I have to get involved with fields outside my immediate expertise.

I appreciate the technical nature and astute analysis, as it will help with the more technical of my client base.

There is still a gray area for me when sorting out new construction - an area to which I am increasingly called- and renovation of older houses - which I was born in.

In renovation - as in your ceiling - space and structural limitations seem to dictate 2# foam (knee walls, finished attics and sill plates full holes, cracks and mechanicals come to mind). Open foam may suffice with lower R value per inch but is not water-proof and the same can be said of blown cellulose.

The blower before - to help with confirming the heat load - and after to prove our work.


Open cell foam is too vapor permeable to use in unvented cathedralized ceilings in MN-  moisture diffused in and accumulated in the roof deck too quickly in winter.  (0.7lb Demilec could work if you go thick enough, but not half pound polyurethane or half-pound Icynene.)  The 2lb Icycnene  MD-R-200 (and not MD-C-200) is something in-between- it's K value is ~R5/inch (compared to ~R6+/inch for polyurethane), but is 1.3 perms @ 3" thickness,- a thickness at which 2lb polyurethane is under 0.5 perms.  Even though I object to some of that company's marketing strategy (selling people on insulating to sub-code R values, in particular) their 2lb product is useful in applications where you want to keep the vapor permeance higher at R values that would be more vapor-tight than best-practices using polyurethane (or Icynene MD-C-200) 

At 5-5.5" in a 2x6 cavity it'll be about 0.7-0.8 perms, which is about ideal for Zones 5-7 in most stackups.  It's only R26 @ 5" as opposed to R30+ for most 2lb polyurethane, but with the fraction of R5 thermal bridging at the rafters/studs the difference it makes in whole-assembly R is quite small.   At under 4" it would probably be useful to use vapor retardent latex on the interior to reduce the interior vapor permeance for zone-7 unless there is sufficient exterior foam to keep boost the average winter temp of the sheathing above the interior air dew point.

But as I stated before, despite all the discussion about vapor barriers/retarders, the primary causes of moisture related rot or mold in buildings is still bulk water incursions and air leakage, both of which are mitigated considerably with even an inch of closed cell foam. In air-tight vapor-tight assemblies (even double-vapor barrier situations with extremely slow drying rates) the risks are low. It's the non-so-air-tight assemblies with vapor barriers where the vapor barrier becomes a "solution-problem", causing as many issues as it solves.
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14 Apr 2011 11:09 AM
Note, regarding the Minnesota study, I wasn't clear that I was referring to the test walls they built, not the houses (which had a wide variety of problems). Fixed.
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14 Apr 2011 12:31 PM
That study pretty clearly debunked the notion pushed by some cellulose installers/manufacturers that you shouldn't use interior vapor retarders. (See bays 5 & 6) But it's pretty clear that cellulose would do OK with only vapor retardent latex, about the same ~0.5 perm rating as kraft facers (see bays 4 & 9 with the kraft faced fiberglass).

The vinyl siding used in the test bays are a rough approximation of a rainscreen gap, but not an exact equivalent.

Apparently no exterior foam was done in the test bays, which would have increased the temp of the OSB, reducing the number of condensing hours at that layer in any stackup.
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14 Apr 2011 05:11 PM
Sorry jonr I overlooked it...should be focused on my current drawing anyway.
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19 Oct 2011 02:43 PM
What does this have to do with radiant heating.
ilgeoUser is Offline
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02 Nov 2011 10:38 AM
I generally say go with hydronic heating but it seems that all you want is floor warming so the advantage of electric would be low installed costs and low maintenance may out weigh the lower operating cost and flexibility of a hydronic system. If you wanted to heat a garage or basement slab to go along with the rest of it then i would use a gas hydronic system. You could use some type of boiler water heater combo.
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