I have looked at the data by DOE and decided that some type of radiant barrier in the attic and possibly as a house wrap would be a good idea. I do, however, have some concerns. Tyvek or similar is permeable so that moisture does not become an issue. A radiant barrier would seem less permeable? Could this cause a problem? According to the DOE numbers, the benefits outweigh the costs, but do they outweigh the concerns?


Second part, for the house wrap, would you go with foil faced sheathing or a wrap over the OSB. The siding will be brick or stone. About 1” gap between OSB and exterior covering. ANy recommended distributors?

A little more about the house: Full daylight finished basement, 2x6 exterior walls. Tennessee.

Add perforated foil and Tyvek® AtticWrap to the options.

Have you used these products? Are you saying any perforated foil or Tyvek Therma Wrap (I think this stuff is really expensive).

Tyvek AtticWrap in place of radiant barrier?

In bang per buck you'll usually get more performance out of adding more insulation than with RB, the exception being if you have ducts & air-handler in the attic.

As a house wrap the thermal benefit of RB is almost nil unless you have at least 3/4" of ventilated air-gap between the brick/masonry siding and the sheathing. Better yet is foil-faced INSULATING sheathing (seams taped with FSK tape, and edges sealed with spray foam) over the structural OSB, which gives you both the RB effect and a conductive thermal break over the studs. With an inch of foil-faced polyisocynanurate or unfaced XPS (which is cheaper) over the studs you'll improve the "whole wall" (thermal bridging of the studs included") R value up from about R14 to about R20, a 30%+ improvement in performance for that extra inch of wall thickness, at an installed cost of about a buck a square foot. (sometimes less.) In TN with an inch of exterior foam you can then (and should) use UN-faced batts or spray/blown fiber in the stud bays to let it dry toward the interior (through latex painted wallboard) year-round for optimal moisture control. See: http://www.buildingscience.com/documents/guides-and-manuals/irc-faqs/irc-faq-insulating-sheathing-vapor-retarder-requirements

Without the exterior foam you might need kraft facers on the interior side to mitigate winter moisture accumulation in cooler parts of TN, which could then run into summertime moisture problems where sun hits rain & dew moistened brick. Putting something more vapor retardent than #15 felt between the brick and the OSB is a GOOD thing, especially if you can avoid using interior vapor barriers.

If this is new construction you're best off bringing the ducts & air handler completely inside the thermal boundary and out of the attic, even if it means adding a foot of height to the wall framing to accomodate it. It's not dramatically more expensive if planned in from the get-go, and make air-sealing the place much easier for a HUGE low-cost boost in thermal performance for the whole building.

And CRRC rated cool-roof materials are more effective than an interior radiant barrier, usually for less overall cost, on new construction. See: http://www.coolroofs.org/

I haven't used them or looked at them closely - just mentioning that they are out there and could help with concerns about moisture. I generally (as in most of the US, including TN) disagree with Dana about his "make the exterior less permeable and make the interior as permeable as allowed to make up for it". Code may give you a lot of leeway, but I wouldn't push it to either extreme.

> an inch of foil-faced polyisocynanurate or unfaced XPS (which is cheaper) over the studs you'll improve

As Building Science writes (as a principle that they sometimes violate), "Avoidance of using vapor barriers where vapor retarders will provide satisfactory performance". Ie, not foil-faced iso.

Posted By jonr on 06 Jun 2011 01:05 PM
Add perforated foil and Tyvek® AtticWrap to the options.

Yes going in for a perforated foil would be suiting your needs quite well, do go for this.....

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Posted By jonr on 06 Jun 2011 04:03 PM
I haven't used them or looked at them closely - just mentioning that they are out there and could help with concerns about moisture. I generally (as in most of the US, including TN) disagree with Dana about his "make the exterior less permeable and make the interior as permeable as allowed to make up for it". Code may give you a lot of leeway, but I wouldn't push it to either extreme.

> an inch of foil-faced polyisocynanurate or unfaced XPS (which is cheaper) over the studs you'll improve

As Building Science writes (as a principle that they sometimes violate), "Avoidance of using vapor barriers where vapor retarders will provide satisfactory performance". Ie, not foil-faced iso.

Which is why XPS rather than iso is a preferable solution for the studwall/masonry-cavity interface.  At about 1 perm foam the studwall can dry into the exterior but won't let much in during the peak vapor drives of sun on saturated brick.  Keeping the interior 2-10perms is completely safe in a TN climate from a winter moisture accumulation point of view, and maximizes the drying capacity. 

But foil-faced iso at R6 or higher is also more than sufficient for protecting the sheathing from winter moisture in that climate. The 2+ perms of interior drying capacity is far higher than a standard construction of 2-5 perm felt on the exterior, ~0.5 perm (kraft facers) on the interior, since the exterior moisture drives are quite high in summer, while interior drives in the winter are quite modest in TN.  Foil faced R6 on the exterior would not be an extreme in this climate, since it would have more than 50% more exterior R than the minimum required to be protective for Zone 4 per the IRC prescription.  Half inch iso would be cutting it too close, and perhaps extreme.

But the benefits of the radiant barrier aspect of foil facers is also pretty small in walls compared to attics.  Without the exterior foam foil-faced wood sheathing might still make sense in TN, but leads to drying capacity issues.  If the cavity space just isn't available for 1" of foam, 1/4" fan-fold XPS with ~0.6-0.8 perm facers are available, and would allow you do do pretty well with a 0.5 perm interior.  It would dry in both directions and be protected from winter moisture accumulation by the lower interior permeance, but would be less resiliant to air leaks from either side as compared to the "right" solutions:  1" XPS or 1" iso solutions with 2+ perms on the interior.

I don't take any credit for this approach- it's been well-studied, has a powerful theoretical basis, and is enshrined in the IRC at this point.  I'm just pointing out what's been shown to work.  Run a WUFI simulation, if you're skeptical.

Seems like some debate on what to use. Hopefully responses keep coming.

I have a unit in the attic, in the house I live in now - Never again. The unit works way too hard to cool the upstairs. I have foam and batt now and the down stairs feels good, but I can feel heat from the roof. That is why I am interested in the thermal barrier. I will add extra insulation in the attic this time, and the 2x6 walls are for better insulation. But I want to do the best I can to keep the house cool and energy cost low.

More than 2x6 construction, next time, consider using advanced framing technique (24" o.c. studs, rafters, windows & door framing aligned with studs, tuck corners & tabs for hanging the corner wallboard to allow for maximal insulation minimal framing fraction) and use enough exterior foam to bring the "whole wall" R (thermal bridging of studs factored in) above R20, maybe even R25 which would be a cost-effective (long term) high-R solution for Zone 3/4 (TN has both- what's your zip code?)  That's economically achievable with 1-2" of XPS over any 24" o.c. 2x6 construction with low or mid-density wet-sprayed cellulose. (Sprayed/blown always works better than batts, since batts are never installed perfectly, and low density batts don't perform well a the temperature extremes, performing a double digit percentages below their rating at 75F.)

See table 0.2, p10: 

http://www.buildingscience.com/docu...mate-zones

For verification of the exterior foam approach and other possible R20 wall layups, see:

 http://www.buildingscience.com/docu...gh-r-walls

Case 2a + 1 more inch would give you ~R25 for a whole-wall number, and the interior of the sheathing would NEVER reach the dew point of the wintertime interior air (OK, maybe 5% of the winter hours, but it would have to be 10x more hours than that to accumulate any moisture in the sheathing.)   A couple inches of XPS comes in at ~0.6 perms too, so there is still some drying capacity to the exterior, but going with 2-10 perm paint on the interior would be more than sufficient drying capacity, at essentially zero risk of winter moisture accumulation issues in your climate.

To hit the numbers in the table...

In the attic, design the trusses  &  framing for ~18-20" of cellulose for R55-ish (after settling) and count on supporting it with 1/4" OSB rather than just gypsum on 24" o.c. framing or it'll bow like crazy.  With R50+ cellulose upstairs the presence or absence of radiant barrier wouldn't be felt the way it does with R19 or R30 fiberglass attics, but it's still worth using cool-roof materials on the exterior, since the cost uptick is usually minimal "in the noise", but the heat-rejection effect is as-good or better than shiny stuff mounted inside.

Any sub-grade walls can be done with R16 insulated concrete forms (about the slimmest made), and think carefully how to thermally break the band joist & foundation sill.  Running the studwall exterior foam all the way down to the concrete on the outside of the band joist & sill, then using 2" of spray foam on the interior of the band joist down to the interior foam of the ICF might work, but you may need a bit thicker wall to accommodate the brick too.

Any slab on grade would take 1-1.5" of XPS at the slab edge, 1" XPS center slab.  In a basement, float the slab, making the sub-slab insulation foam completely in contact with the wall-foam to avoid thermal and moisture bridging the slab to the wall/footing.

With those types of whole-wall R values, the heat gain/loss numbers are then dominated by the glazing size & type.  Make the case carefully for every square foot of window, because it's an R2-R3 hole in your R20+ wall, and the smaller they are the lower the heat gain/loss.  Use fixed-windows wherever possible- they're the tightest, but where windows must openable, casements & awning leak less air (they're inherently easier to weather strip and seal positively) provide more air flow and egress area per square foot than slicers or double-hungs.  Push-out vs. crank out types are usually tighter too.  Avoid glass sliding doors too- swinging patio doors are easier to weatherstrip, whereas sliders always warp over time and leak air.  Reducing the size of the glazed and going with simpler windows is usually a cost savings that provides a performance boost.

Define the primary air-barrier in the assembly, and inspect regularly.  With acoustic sealer or other high quality caulk under the studwall plates, between the subfloor & band joist, and the bandjoist & foundtion sill, as well as the interior corner of each stud bay, and between the top plate and the OSB you hang for supporting the cellulose, painting & taping the seams of the ceiling OSB, staggering the seams with those of the gypsum etc you can define a continuous air-barrier that is somewhat protected from interior penetrations by picture-hangers, etc.  You then have to get religion about spray-foam sealing every plumbing & electrical penetration in the primary air barrier too,  but hang the doors & windows prior to insulating, and do a blower door test, find & fix all of the leaks you can, and it's usually possible to get your total leakage under 2 air exchanges @ 50 pascals pressure.  (The standard blower-door measurement.)  This is by far the cheapest most cost effective energy efficiency enhancement you can make to the place.  A well-insulated wind-tunnel is still expensive to heat & cool, but even a modestly insulated building envelope is reasonable if air-tight.  The Canadian R-2000 standard specs out < 1.5 ACH/50, and is pretty easy to meet most of the time, but not if you're asleep at the wheel when framing & sheathing the place.

A house built with 25% less glazed area and true R20-25 (whole wall) R values at <2 ACH 50 and all other thermal bridging insulated by R10+ foam will use ~30% of the heating & cooling energy of a standard R19 2x6 R30 attic house with no foundation insulation and standard double-panes.  Your current flash & batt wall is probably a bit tighter, and might be as high as R16 for a whole wall R.

On the existing house, the cost of adding 6" of cellulose over the existing batts/blown/whattever is pretty low, probably lower than a retrofit raditiant barrier, but will add R20+ to whatever actual R value it's running.  If you have low density fiberglass up there, it's underperforming it's 75F rating by quite a bit under a 120F roof deck, but an overblow of denser goods will restore it's rating while adding more.  The net effect is more than simply R20 at the temperature extremes, since it reduces the convective R-value loss of the fiberglass it's covering.

Dana,

Sorry, I have been out of pocket. The zip code is 37876. I have looked at advanced framing techniques, but I do not want to lose the stability of 16" on center. I have seen too many homes with wavy walls. That is why the 1/2" OSB wraps the entire house. Could I use 1/2" XPS over the OSB? Are you saying cellulose over foam and batt?

Gaitlinburg is definitely well into Zone 4, which has a slight hazard on the exterior OSB if you went with latex-only as the interior vapor retarder, but with literally just about any amount of exterior foam it would make it on 2x4 construction, but it takes at least R4 (1" EPS) with 2x6. See:

http://www.buildingscience.com/docu...quirements

But 1" XPS (R5)  or 1" iso (R6) would give you a bit more margin.  The best bang/buck in this situation is to go with 1" XPS, but you might want to double that and have HUGE margin over the winter moisture issues and have a pretty high performance building for the climate zone.

Yes, I'm staying use spray cellulose rather that flash'n'batt, and put use the savings to beef up the exterior foam sheathing, since that's where it does the most good.  If you went with 2" of spray foam on the exterior it would give you a whole-wall R of about R25 (in a 2x6, 16" o.c. wall) and be a more reliable air-seal than if the same 2" were in the wall cavity.  Alternatively 2" of exterior XPS in 2 1" layers with the seams taped with housewrap tape and seams staggered on the layers, with the edges sealed with low expansion 1-part foam gives you ~ R24 , usually for less money than with closed cell spray foam.

R25 done that way is still cost-effective long-term in Zone 4, per the table on p10 of the link in my prior post, so it's not all that outlandish to suggest:

http://www.buildingscience.com/docu...mate-zones

Wet sprayed cellulose on open stud bays is pretty cheap- similar to a batt-installation in cost (sometimes at a slight premium, but often at-parity. ) But unlike batts it's more likely to be near-perfectly installed. Since blown & sprayed goods have no voids or compressions, and conform to every anomaly, knothole and bump convection around the fiber layer is impossible, and the-air retardency of cellulose is vastly better than low density batts, which keeps it from suffering R-value losses from internal convection when the temperature differences across the wall are high. To get simlar air retardency with fiberglass you have to go with a "dense pack"  (1.8lb density) new-school ultrafine blown fiberglass like Optima or Spider (both great products), but bang for buck it might still be better to spend the difference between the better f.g. & cellulose on fatter exterior foam. 

The difference of R4/inch (Spider/Optima @ 1.8lbs) vs. R3.5 inch (low density wet sprayed cellulose) may sound like a lot- it's a difference of R2.5, center cavity), but when the thermal bridging of the16" o.c. studs is factored in the difference in center-cavity R makes in the whole-wall number is about R1. That's also why spending the money on the cavity foam is wasted- you only reap about half the benefit of the extra R if it's not thermally breaking the studs, whereas foam on the exterior adds R to those thermal bridges, not just the center-cavity.

Even with double-headers and 16" o.c. spacing the 2x6 wall will perform well, taking only about R1-R1.25 off the whole-wall R compared to "Advanced Framing" or "Optimum Value Framing", but it's at a price premium on the framing.  There are many reasons for going with the heavier framing- ease of wall flatness is one, higher structural capacity is another, and those are very reasonable design goals.  To regain that whole-wall R-difference takes only ~1/4" more exterior foam.

2" layer probably would require extra brick ledge, I would have to check, but that would seem to have detrimental effects to cost, not to mention brick ties.
 
I am assuming that with all this sealing and making the house air tight, I would need to start looking into air exchangers?

Heat recovery or energy recovery ventilation is always a good idea. Having an air-leaky house is no guarantee that the leaking air is ventilating the most-appropriate place.

Having a properly ventilated air-tight house has a bigger effect on energy use than a high-R but leaky house. The combination is "money in the comfort-bank"- a more comfortable & healthy house at a lower operating cost (NPV on energy savings be damned!) But at R20-25 will payback within your lifetime if done judiciously.

In TN going with ERV (which does both a humidity and temperature exchange) is advisable, since you'd otherwise have to remove substantially more humidity from the air via air-conditioning or dehumidifiers.

If adding thickenss to the wall would be a primary cost adder issue, consider some alternatives:

For the same thickness as a 2x6 wall with half-inch foam (~R17) a 2x4 16" o.c. cellulose wall with 2.5" of XPS comes in at ~R22.5.

For the same wall thickness as 2x6 with no exterior foam (~R14) a 2x4 wall with 2" of XPS would hit ~R20.

For the same wall thickness as 2x6 with no exterior foam (~R14) a 2x4 wall with 2" of exterior closed cell spray foam would hit ~R22+ (and make air-sealing dead-easy).

If you went with the 2x4 studwall and 2" of foil-faced iso instead of XPS you'd hit ~R22.

Any of them is good- the 2" XPS R20 wall would be the cheapest, the R22 spray foam wall would probably be the tightest (or could be, with some testing & remediation prior to doing the cavity insulation.)

To deal with the masonry tie issue with thicker foam it may be easier/cheaper to use metal Z-furring in the cavity through-screwed to the studs to establish the cavity depth, then screw cheap L-strap type ties to the furring.

Variations on the theme are common in commercial construction, where CMU or metal-stud walls are often insulated with 2" XPS, with a brick veneer on the exterior, eg: http://www.homeconstructionimprovement.com/brick-facade-metal-stud-framing/

http://hci.frontstepsmedial.netdna-cdn.com/wp-content/uploads/2009/06/brick-anchors-and-foam-insulation-300x225.jpg

This sort of system developed for metal studs are easily adapted for wood frame construction- it's primarily a difference in fastener type.

See also:

http://www.foam-tech.com/case_studies/exterior_wall_masonry.htm

http://www.buildipedia.com/on-site/construction-materials-and-methods/item/1019-extruded-polystyrene-as-a-wall-insulating-option?tmpl=component&print=1

If you need less thickness at a good price, I would look at a 2x4 16" o.c. cellulose wall with 2" of taped XPS. Or you might be able to stay with 2x6 but add the XPS to the inside. You want to have the house tested to make sure you have achieved your air tightness goals.

Putting the foam on the interior leaves significantly more thermal bridging, and parks the sheathing next to the masonry- don't go there without vetting it with a WUFI simulation first (and without actually performing that exercise it may sometimes fail in that climate, especially on a sun-baked southern exposure.) Putting at least SOME substantial vapor retardency between the cavity and the wood is advisable, even with a vented cavity, and the venting of the cavity becomes more critical.

See: http://www.greenbuildingadvisor.com/blogs/dept/musings/when-sunshine-drives-moisture-walls

I don't think masonry was specified, but if that is used, yes, vent it and put building wrap next to the OSB.

That 2x4 with 2" with cellulose sounds like the way to go. If I did my math correctly, I need a little over 150 4x8 boards. $25 is the cheapest I found it. Thats ~$4,000!! Is there a cheaper place to buy it than the Big Box stores? Also blue or pink?

And then I need to start thinking about the attic.

Box stores are usually competitive on commodity products like that, but sometimes commercial distributors can beat that. Mind you, that's not an installed-price and there's wastage on the edges and window/door openings, whereas 2lb foam @ ~$1/inch/square foot is the installed price, no overage necessary. In big flat installations it's usually cheaper per unit R to go with rigid than sprayed, but in complicated cut''n'cobble it's not a no-brainer- depends on your labor rates and scrap fraction. Pink, blue, green, gray, faced, unfaced doesn't much matter. (Some 1" goods come with facers to lower the perm rating, but in this app it doesn't much matter. Price is king- with XPS it's all good.) T&G goods have some advantage for gaining good flatness, which helps when air-sealing the layer. IIRC the big blue box store has store-branded XPS ship-lap or T & G sheathing (manufactured by the same manufacturers of the green stuff) available at a slight discount below blue or pink goods in some markets.

If doing it as 2 layers of 1" XPS rather than sheets of 2" good is easier to air-seal reliably. Hold the first layer in place with blobs foam-board construction adhesive tacked with only a very few nails to the structural sheathing to keep it there as the adhesive cures. Tape the seams with housewrap tape, and glue the second layer in place over that with a foot or so of seam overlap, also with as few fasteners as possible, then lay down the furring through-screwed to the studs as the primary method of keeping it in place. Foam seal all exposed edges. If doing 1-layer of 2" T& G or ship-lap, caulking the seams with construction adhesive as you go is at least as reliable as housewrap tape on square-edged goods.

Detailing the structural sheathing as the primary air barrier makes the air sealing more robust overall. Using acoustic sealants or other high-quality caulking on the framing at the OSB/ply edges as it goes up works, as does painting the seams with 2" stripes of acrylic latex primer as a bonding surface for housewrap tape. A bead of caulk under the studwall plates etc also counts.