What insulation for 2x4 exterior walls

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Nobody

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I am remodeling a section of my house and am looking for some advice on what type of insulation would be the best bang for my buck to insulate the 2x4 walls.

The house is in Seattle, WA was built in 1967 and has cedar bevel siding over 15 lb tar paper and 5/8 plywood sheathing and 2x4 stud walls.

When I went to apply for my permit they said I needed an R-15 for my exterior 2x4 walls. I was wondering if the foam board (R-Max with 2 foil faced sides) sold at Home Depot would be a good alternative rather than the typical fiberglass batts. I was thinking a 2"thick foam board (R-Max says an R-value of 12.9) adhered to the stud bay with can spray foam and then maybe even adding a 1" thick piece of foam board on top of that. Not sure if this would trap moisture in my climate/house situation though. Would this be a better option (Foam board option will be pretty expensive) than a Roxul R-15 batt or an owens corning compressed R-21 batt (compressed R-21 gives R-15 in 2x4 wall I saw a chart somewhere that said this).

Ive gone to the trouble to caulk all my stud bays to try to achieve an air tight situation.

Thanks
 

JohnfrWhipple

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How does your homes rain screen and vapour barrier work?

In Vancouver you need both.

A 2lb spray foam can be installed in a 3.5" stud bay at roughly 3" and meet code. Batt insulation does not cut it for 2"x4" walls anymore and the wall needs to be furred out so 2"x6" Batts can be used.

You better check with your local city hall - these requirements are very struck and closely inspected in the Pacific North Wet
 

Dana

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Cutting up foam board for cavity fill is a waste of good foam-board due to the thermal bridging of the framing.

Putting in 3" of 2lb foam would get you that performance but it's a waste of good foam too.

With a 3" thermal short through the wood instead of 3.5" , the losses from the ~25% framing fraction (typical of 16" o.c. framing) is higher than with a full cavity fill. Worse yet, most 2x4 studs back in 1967 were doug-fir which is only R1.0/inch rather than hemlock at R1.2/inch. The result is that while it may be R18-ish at center cavity, it's "whole wall" thermal performance is less than R0.5 better than R13 fiber with a full cavity fill, and slightly worse than R15 cavity fill. Allowing R1 for the gypsum + sheathing you are looking at whole-wall R (after calculating the thermal bridging of doug fir studs) of:

R13 fiberglass: R8.75

R18 closed cell foam (or 3" of polyiso): R9.0

R15 rockwool/fiberglass : R9.25

If they're hemlock instead of doug fir the whole-wall numbers rise by about R0.5-0.75, but the relationship remains the same- the R15 batts outperform the R18 foam.

What's more the vapor permeance of 3" is about 0.35 perms, which virtually blocks the sheathing from being able to dry toward the interior. (Foil faced board foam would be even worse.) And the HFC245fa blowing agent used with 2lb foam has a global warming potential about 1000x that of CO2, and in your climate the carbon savings over the lifetime of the house may never get the climate damage back to zero if you're using 3" of the stuff.

Open cell 0.7 lb polyurethane spray is blown with water, and hits R15 in 3.5", as does high density fiberglass and rock wool.

If you can spare a half-inch of interior space, putting a half inch of rigid foam (any type) between the studs and sheet rock, if you used damp-sprayed or dry-blown cellulose in the cavity you'd be at ~R13 for cavity fill but the additional R2 of continuous EPS insulation as a thermal break of the stud edges it raises the whole-wall R to about R10.75. That's 16% better than a code-min solution, and 19% better than a 3" closed cell foam solution.

And by using unfaced EPS, you would not prevent drying toward the interior, since it runs 5 perms + @ 1/2", which is about as vapor retardent as standard latex paint. If you then put sheet rock with latex paint on it the drying capacity would run in the 1.5-2 perm range, which is just about perfect for your climate.

In a Seattle climate with cellulose insulation, with your stackup you do not need an interior side vapor retarder- the cellulose itself buffers and redistributes any moisture that finds its way to the sheathing (either from the exterior or the wintertime interior.) With fiberglass or rock wool you would need something but sheet poly is a two-edged sword. The best solution would be to use a "smart" vapor retarder such as Intello Plus or Certainteed MemBrain, but worst-case you can do just fine with "vapor barrier" latex primer on the wall board, which runs about 0.5-0.6 perms (1.5x the drying rate you'd get with 3" of 2lb foam).

If code inspectors insist on a vapor retarder even with cellulose cavity fill, either a smart vapor retarder would still work great, but v.b. latex would be tighter than ideal. The house is really better off without it if using cellulose (especially if you installed half-inch unfaced EPS under the gypsum) the vapor retardency of standard latex paint (3-5 perms) is better than 0.5 -0.6 perms when you have the hygric buffering of the cellulose to work with, since the drying rate is an order of magnitude higher. Smart vapor retarders become fairly vapor-open when the relative humidity of the air on either side of the membrane hit's 50% or higher, so as the cavity gets more humid while releasing moisture from the sheathing, it allows it to dry toward the interior at a maximal rate.

If unfaced half-inch EPS is too awkward to work with, 3/8" fan-fold XPS siding underlayment works. It usually has thin polyolefin facers that bring it down to 0.75 perms or so, which is fine. XPS isn't as benign as EPS, since it's blown with HFC134a (about 1400 x CO2) instead of pentane (7x CO2), but when you're only talking 3/8" the lifecycle environmental cost/benefit is still clearly in your favor, unlike 3" of 2lb foam.

Any interior side foam should be glued to the studs with foam board construction adhesive for air tigthness, and the seams should be taped with housewrap tape, then overlaid with a thin layer of duct mastic (which sticks better than the tape adhesive.)

Prior to insulating the cavities, seal the sheathing to the studs with a bead of caulk- acoustic sealants are the best, but a good exterior rated acrylic is way better than nothing. If you have horizontal seams the sheathing in there, tape & duct-mastic those too. Lay a bead of caulk between doubled-up top plates of the studwall, and where the bottom plate meets the floor/subfloor. As much fuss is made about vapor permeance, typical levels of AIR LEAKAGE carries an order of magnitude more interior moisture than vapor diffusion through latex paint. Air sealing all of the electrical & plumbing penetratations (even lateral wiring runs through studs or vertically through the top or bottom plates is also important.

If going with R15 batts rather than blown cellulose, take care to install them to near-perfection. Split the batts where they go around wiring or plumbing, and sculpt them to fit around electrical boxes, etc. When installing it, tuck/compress the edges & corners in to ensure that you don't leave voids at the sheathing/framing junctions, then tug them out lightly until the batt is just proud of the stud edge, so that it's a compression fit for the foam or wallboard layer.

It's OK to go with half-inch unfaced EPS or 3/8" fan-fold XPS with a batt solution too which would deliver about a 15-20% improvement over code min. With rock wool or fiberglass cavity fill the lower permeance of 3/8" fan-fold XPS means you could skip the vapor barrier latex, but you'd still have at least some drying capacity toward the interior.

NEVER use foil faced goods in an assembly without doing the full moisture migration analysis on the stackup, since like 6-mil polyethylene, the foil facers are true vapor barriers, which blocks drying paths.
 

Reach4

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Since closed cell spray foam is expensive, would filling the center of an area with equivalent foam board and follow up with the closed cell foam to fill the gaps and seal the edges be effective and cost-effective?
 

Dana

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Cut'n'cobbled foam board can work, but it's also prone to failure. At more than 3" even Type-II EPS drops to less than 1-perm, limiting the drying capacity of the sheathing.

The additional thermal performance you get by going with even a full 3.5" cavity fill of R6/inch foam is minscule- about R1, since it's thermally bridged by the 25% framing fraction. With douglas fir framing a full 3.5" fill of R6/inch foam results in a whole wall R of 10.33, compared to R9.25 for the same wall with R15 rock wool, and it's a riskier build, since the moisture is trapped in the sheathing rather than hanging out as water vapor in the air between the fibers. Foil faced polyiso runs about0.05 perms per facer- all moisture in the sheathing MUST dry toward the exterior, which can be painfully slow in foggy-dew PNW climates. Even unfaced polyiso (it exists, but you'd have to special order it through roofing supply distributors ) runs about 2.5 perms @ 1", which becomes about 0.7 perms @ 3.5", about the same as the facers on any thickness of fan-fold XPS.

If you put in even 1/4" fan fold XPS (about 1.25 under the interior gypsum wall with R15 fiber in the cavity it would have the same thermal performace as the full cavity fill of unfaced polyiso. The vapor permeance toward the interior is comparable to a full cavity fill of unfaced polyiso but the moisture load is shared by the air in the cavity. Polyiso is also hygroscopic- it would wick moisture out of the sheathing, but it's performance falls when that happens, unlike cellulose, which has a hollow fiber characteristic, taking the bulk of the adsorbed moisture inside the fiber without changing it's R-value by much. Freeze/thaw damage of the outer part of unfaced damp polyiso would also degrade performance over time.

The best foam solution is 0.7 lb open cell spray polyurethane foam. It would need an interior side vapor retarder to protect from wintertime moisture drives, but it would air-seal the cavity better than closed cell foam without needing caulking details.

Caulked seams & R15 batts with R1.25-R1.75 fan-fold XPS under the gypsum is going to deliver good bang/buck. Skipping the fan-fold XPS and using vapor barrier latex under the finish paint is a good value as well, but lower performance.

The IRC 2012 code-min for Seattle's climate zone 4C location would be 2x6/ R20, or 2x4/R13 + R5 continuous insulation, which works out to a whole-wall R of about R13-R15, depending on the actual framing fraction & framing species. The R15 + R1.75 (3/8" fan-fold) solution comes in at about R12-R13, which isn't bad, fairly close to code-min for new builds, and if you do a"near-perfect" job of it, the real-world performance will be better than a code-compliant new build at "typical" air sealing & insulation installation.

Washington state has adopted IRC 2012 for the basis of their residential building code, but excluded chapter ll , which is where code min got bumped to R13 + 5 c.i. for all zone-4 locations. In fact, that chapter is excluded in all recent versions adopted into code in WA, where instead a home-grown set of climate zone prescriptive R-values have been in effect since 1 July 2013. See Table 402.1.1 on page 20 of this document.

Note that they specify R21 rather than R20 for framed walls, probably to avoid arguments that low-density R19s are close enough, which they really aren't, since they only perform at R18 when compressed to 5.5", and are more of an air filter than an air-retarder. R21 fiberglass batts are easy to find- easier than R23 rock wool in most US locations. Both are pretty good products, light-years ahead of the ubiquitous R19.

Compressing%20fiberglass.jpg
 

Nobody

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Thank you Dana for all the excellent information......it will take me a while to absorb it all.

Another question: each of my bedroom exterior walls are about 12ft wide by 7ft 8" tall......both walls have a window about 6 ft wide and 3 1/2 feet tall with U factors of 0.33. Since the windows have an R value of about 3-4, (and also I cant really insulate the 6 ft 2x10 header above the windows ) don't the windows/headers severely limit the total wall R value capability anyway?.....making it seem like going to a lot of extra effort/spending a lot of money for a 15% improvement in R value on the 2x4 studded part of the wall kind of not really worth it in the overall walls R value?.

Thanks
 

JohnfrWhipple

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Thank you Dana for all the excellent information......it will take me a while to absorb it all.....

Me too.

I think I need to go back to school.

Not so simple this business about insulation and vapour barriers. That is why we leave th ecall to the inspectors and the building department.
 

Dana

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Thank you Dana for all the excellent information......it will take me a while to absorb it all.

Another question: each of my bedroom exterior walls are about 12ft wide by 7ft 8" tall......both walls have a window about 6 ft wide and 3 1/2 feet tall with U factors of 0.33. Since the windows have an R value of about 3-4, (and also I cant really insulate the 6 ft 2x10 header above the windows ) don't the windows/headers severely limit the total wall R value capability anyway?.....making it seem like going to a lot of extra effort/spending a lot of money for a 15% improvement in R value on the 2x4 studded part of the wall kind of not really worth it in the overall walls R value?.

Thanks

Adding interior foam would insulate over the header framing too. If you can give up an inch of interior space and deal with the window sill/trim issues you can cut the losses of the framing portion by more than half with an inch of EPS, and still not create a moisture trap.

Is it "worth it"? Depends on what your goals are. From a net-present value on future energy savings for the next 3 years, maybe, maybe not, for the next 10 years probably, and over the next 15 years, CERTAINLY. And that's assuming you're heating with something cheap like a condensing gas furnace. If you're heating with propane, electric baseboards or some other more expensive source dial it back in. But there is the long term resilience issue too, which also has value.

The way you framed the question is sort of like asking, "Since the back door gets left open a lot, is it really worth insulating the house?"

Of course it is. It may also be worth retrofitting the back door with an automatic door closer too, but that's a separate project on a separate budget.

Heat loss is on a per-square foot basis, as is insulation cost. If it's worth it for 10 square feet it's worth it for 100 square feet, and conversely. Code minimums are VERY conservative on cost of construction vs. cost of heating/cooling that building. If you plan to live there for more than just another year or three, just do it, and know that the after-tax payback of the "extra" investment is going to beat your 401K (by a lot.) Any time you get into a project, it's worth bringing performance up as close to current code as you can, and it's ALWAYS worth making it as air tight as possible.

While it's rarely worth it to gut a wall simply for the purpose of raising the thermal performance, if it's already opened up, taking it to code min (or as close as possible) is a financial no brainer if you are using cheap high density fiber for the cavity fill, saving the foam-budget for where it is thermally breaking the more conductive framing.

On a lifecycle basis for new construction there is a good argument for hitting "whole-wall" R values in the R20-R25 range (that's whole-wall, not center-cavity R, such as , a 2x6/R20 plus 3" of continuous rigid EPS). In a retrofit that's not really an option unless you're going to re-side the place and re-mount /replace all of the windows & flashing, and put the fat foam on the exterior. See Table 2 p.10 of this document, (but read the whole first chapter for the rationale.) Seattle is climate zone 4, so refer to the row for zone 4, bearing in mind that those numbers include the thermal bridging factored in, and using insulation materials that are lower $/R wherever practical & appropriate.

Closed cell foam runs about 17-18 cents per R per square foot. R15 rock wool even at box store prices runs less than half that: A 12 batt bag is about 60 square feet, at R15 makes (R15 x 60'=) 900 R-feet, and costs $45, making it, ($45/900=) 5 cents per R-ft.

R15 fiberglass may be somewhat cheaper still (though my personal preference is to work with rock wool, for a number of reasons.).

Unfaced 1.5 lbs density EPS runs about 9-10 cents/R-ft^2, open cell spray polyurethane about 12-14 cents, XPS (except fan-fold) about 13-15 cents/R-ft^2

Fan-fold XPS is a bit more expensive per R- running about $45 at box stores for 4' x 50' ( 200 square feet) for the R1-ish 1/4" stuff slightly more for the R2-ish 3/8" stuff, or roughly the cost of closed cell polyurethane, but since it's it's thermally breaking the studs, you get the full-R out of it. Some box-stores carry it, most don't, so you may have to shop around. The most commonly available versions comes in blue (Dow), pink (Owens Corning) and green (Pactiv), as well as a few others, but they are all pretty similar in terms of R and vapor permeance.
 

Nobody

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Dana......Your information is amazing and very helpful ......your perspective on "is it worth it" helped me a lot. Thanks.
 

Marty Gordon

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If you are insulating your basement I'd definitely go with spray foam. It will eliminate the chances of moisture or condensation accumulating behind wall. It may seem like overkill but it definitely happens. I've experienced it first hand. Check this video to see what I mean.
 
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