Got a friend I am going to help residing his old house.  He will be tearing off old wood siding, insulating, adding sheathing, and siding with hardi plank.  The stud bays are 4"

He is probably not going to use a spray product, as he will be doing this one wall at a time over several weeks.

My concern is that we will not be able to properly install a vapor barrier using kraft faced batts. 

He is considering strips of 1/2 foam board, foil faced to the drywall or plaster, and then a 3 1/2" rock wool batt over that.

Seems like that would get him a vapor barrier and fill the cavity completely fairly cheap and easy considering the circumstances.

Anyone have issues with that or other ideas?

where are you?

is there a reason he's taking off the sheathing ?
if he's not rewiring , then i'd leave the sheathing and blow cellulose

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In Richmond Va.  No sheathing to take off, it is an old home with siding only.  So we will add sheathing as we go.

Since it will be a wall at a time ,it will not be economically feasible to bring the spray insulation people out several times.

Don't put the foam board on the interior side of the stud bays, instead, put it on the exterior of the sheathing. In Richmond VA, with even an half-inch of exterior foam OR a rainscreen gap between the siding and the felt/housewrap and make the interior side reasonably air tight you don't NEED an interior vapor barrier/retarder. The rigid foam on the exterior keeps the average temp in January at the sheathing is above 37-40F (the dew point of 30-35% relative humidity 68-70F interior air). The average January outdoor temp in Richmond is already 37-38F, so it doesn't take much. If you use unfaced XPS (extruded polystyrene) it allows the sheathing to dry toward the exterior seasonally, but limits the rate of summertime exterior drives, allowing the air-conditioning to handle the very modest amounts of water vapor coming though the fiberglass and finish-wall. Even though a half inch would do, there's a good economic argument for putting up at least an inch of XPS (or 1.5" of EPS).

More than vapor retardency, you need to make both the interior and exterior faces as air-tight as possible, since air-transported moisture can be orders of magnitude higher than vapor diffusion through permeable latex paint on an interior wall. Air sealing is the single most cost effective thing you can do from an energy point of view, and it protects the structural wood far more than any interior vapor barrier ever could.

While you have the stud bays open, air seal the every corner, edge electrical box & plumbing penetration, or crack on interior side of the stud bay with caulks and low expansion gun-foam . The 1-part foam that comes in the scew-on cans is more cost effective on a larger projects than Great Stuff or similar, and the application guns adjust for bead size nicely, valving at the tip. They can be left on the cans for days/weeks without cleaning, but clean the gun before storing with a can of foam-gun cleaner, or if the time between use will extend for several weeks/months. You have a lot to do if you're doing the whole house- decent guns can be had for under $100 online, and a case of 20-24oz cans will be worth it.

If you must use batts, use UNFACED R15 high density "cathedral ceiling" batts, and take care to sculpt them to a good fit around plumbing, and split them rather than compress them around any later electrical & plumbing. When you put up the sheathing, caulk every stud & plate with acoustic sealant or high quality caulking as you go to make it as air-tight as possible. Paint the exterior of the seams in the OSB or plywood 2" either side of the seam with an acrylic-latex primer, and use housewrap tape to further seal the seam.

Stagger the seams of the XPS with that of the sheathing for even better air-tightness, tape the XPS seams, and gun-foam seal the top/bottom edges. Use blobs of "foab am board construction adhesive" (available at box stores) to put the foam up, press in place and put as few nails in it as possible to keep it in place. Then apply vertical furring at each stud long-screwed to the studs, and hang the siding on the furring rather than attempting to long-nail through the foam (which always sags, and makes 1000 holes in the foam. You'll need to screen the top/bottom of rainscreen cavity to keep it from becoming a critter-haven, but keep it vented both top & bottom where possible. A 3/4" rainscreen gap gives a HUGE boost to the ability of both the structural wall and the siding to dry to the outdoors, and any wind driven rain penetration gets out promptly rather than stored up in the Hardi board to be driven into the house when the sun hits it.

As for how to flash the windows and where to place the felt or housewrap, it depends. see:

http://www.greenbuildingadvisor.com/blogs/dept/musings/using-rigid-foam-water-resistive-barrier

It occurs to me that if you went with 1" of foil-faced iso + rainscreen gap you can get a good seal on the seams with FSK tape and gain some benefit from radiant-barrier aspect of the foil facers. Foil faced iso would be an exterior vapor barrier though, so drying to the exterior goes down to nothing, so you're relying on the ratio of the foam R to total R to keep the sheathing above the wintertime interior air dew point, so an inch (R5.6 @ 25F) is better than a half inch to give you more margin. It would block moisture drives during the hot humid summer better than XPS or EPS though.

An R13 batt insulated 2x4 studwll with the batts installed perfectly yields about R10 whole-wall average (after factoring in the thermal bridging of the framing- the studs, plates, headers, band joists, etc). An inch of XPS or iso over the sheathing is a 50% improvement in R value, which cuts the heat gain/loss through the walls by over 30%.

By contrast cut'n'cobbled into the stud bay as you had suggested yields at best a 10% improvement in whole wall R value using about the same amount of material, with more labor involved too.

Also, putting a highly retardent vapor barrier like foil on the interior side of the wall with cement-fiber siding and no rainscreen gap puts the sunny-side wall at risk, since the siding retains rain & dew moisture that gets driven out at high rate when the sun hits it, leading to higher moisture content in the sheathing and framing wood. As long as the exterior foam is sufficient to meet the wintertime dew-point criterion (which is pretty easy in Richmond VA), it's by far the better solution, since it protects the structural wood both summer & winter.

As always, thanks Dana for the detailed and insightful replies.  I sent a foreward to my friend as you have much better arguments than I can make to do it the right way.

One question I still have is how important it is given an exterior insulation scenario to fill the 4" stud bays in their entirety?  If I am correct, the R15 batts are still 3 1/2".  Is it worth the trouble to get additional insulation and split batts to fill completely.  Or is R19 cut down or compressed a better solution?  I am seeing recent research that suggests compression is not the bad idea is was once thought.  Or do we use 3 1/2 and let there be a gap?

Thanks again all for info and suggestions!

I would leave all the old siding in place and spray 1.5 inches of 3LB closed cell foam over the entire house then spray with a silicone base sealer choice of colors and now you not only have a waterproof dwelling your R factor eliminating air leaks will be R50 Your energy bill reduces by 30% see more at SafeDomes.COM

Posted By s.kelly on 17 May 2011 08:44 PM
As always, thanks Dana for the detailed and insightful replies.  I sent a foreward to my friend as you have much better arguments than I can make to do it the right way.

One question I still have is how important it is given an exterior insulation scenario to fill the 4" stud bays in their entirety?  If I am correct, the R15 batts are still 3 1/2".  Is it worth the trouble to get additional insulation and split batts to fill completely.  Or is R19 cut down or compressed a better solution?  I am seeing recent research that suggests compression is not the bad idea is was once thought.  Or do we use 3 1/2 and let there be a gap?

Thanks again all for info and suggestions!

 It's VERY important in batt applications that the batt fills the full depth of the cavity (as is the compressed/installed density.)  Gaps allow high rates of convection within the cavity, sometimes even circumventing the insulation.

If  this a full-dimension 2x4 (old school) application yes, using R19 batts with a compression fit is far preferable to R15 batts with a gap, but R21 higher-density batts would be even better (and R22 low-low density batts would be worse.)  An R21 batt would yield about R17 compressed into a full-dimension-four-by. See:

http://numsum.com/spreadsheet/show/21111

And the higher density of the R21 batt means that it loses less R to internal convection at the temperature extremes than a lower-density R19 batt does.  An R19 batt is the same amount of material as an R13 batt, but "fluffed", leading to degraded performance at high delta-Ts. 

R22 batts seem almost deceptively labeled, since it only runs R19 when installed in the cavity it was designed for!

If he can handle the extra depth involved and the cosmetics of that type of finish (or an EIFS type faux-stucco, etc) buck's suggestion isn't a bad one.  Even a 1" shot of 2lb foam would work in a wall application (3lb foam is more typically use on roof apps- it's a lot tougher, doesn't need as much protection from the elements.)  If going that route, dense-packing cellulose into the cavities from the exterior prior to foaming over the entire exterior makes the interior nearly air-tight.  At full-dimension 2x4s 16" o.c. a dense pack cellulose fill would yield~R15 center-cavity, and the whole-wall R (bridging factored in) would come in at ~R12.  Adding an inch of 2lb foam over the exterior would then bring it up to R18 (whole wall), or with 1.5" of 3lb foam you'd be at ~ R22.  He'd save the cost of new sheathing, and most of the cost of the siding.  Dense-packing dry-blown cellulose would be somewhat more expensive than oversized compressed batts, until you factor in the air-sealing labor & materials, etc.

Come to think of it, if the existing siding hasn't succumbed to rot/ants/termites, just keeping it, dense packing the cavities and doing either spray foam or housewrap + rigid foam solution on the exterior might be quicker & cheaper, even if he still went with rainscreen gap furring & cement-fiber siding for the outermost stackup.

Thanks again all for replies.

The siding does have some issues, so it is frankly just as easy to pull it off and start from scratch, which also allows the cheaper DIY insulating methods.

Dana, thanks again for all the info, been a help to me several times, and I learn a lot just reading your posts.  If you ever get into the Richmond area dinner is on me!

Sent the info on so my buddy will read it and get on board with better if not best practices.

Posted By kermit on 16 May 2011 08:53 PM
is there a reason he's taking off the sheathing ?
if he's not rewiring , then i'd leave the sheathing and blow cellulose

The same question i would like to know as well, is their any specific reason as to why your friend is taking off the sheating, it can still act as the best insulating material for his building.....

I agree with the other comments, leave the old sheathing on and spray in foam. That will be THE most cost effective solution and will save you both some time!

Greetings.

Since you are going to have open cavities the most eff and lest costly is RB, 2 layer mfg by fifoil,com

You also will have a VB, no condensation or loss of insulation values, of cancer causing fibers of toxic fumes.

The RB will perform better than the FG or foam.

You can also insulate the stud losses by installing a single RB over 1/2" furring strips out side or inside and then siding or drywall.

I've done several of this type of retrofit over the years and it is VERY eff.

The RB should be perforated.

Perforated RB alone without additional materials in the stud cavity will result in summertime moisture accumulation on the wallboard/plaster layer in an air-conditioned building in a Richmond VA climate- it's too vapor & air permeable.

A cellulose fill and even modest vapor retardency on the exterior would eliminate the summertime condensation potential, and even R5 rigid foam on the exterior would limit the wintertime condensing temp hours of the sheathing to inconsequential levels. (Even R2.5 would be sufficient anywhere in US climate zone 4.)

Fifoil doesn't make nearly as grandiose an estimate of their products' performance in wall stackups, even in a FL climate. According to their numbers a 3 reflective layer stackup is about 10% more effective than 3/4" (R3.75) XPS mounted on furring with no interspersed framing, or 1" iso (R6, center-cavity) cut up by the thermal bridging of 1x furring, and no other insulation.

Compare the "savings" numbers of wall 4 (the 3 reflective layers) to wall 5 (R3.75 XPS) and wall 6 (iso cut in between 12" o.c. 1x furring, with a 9% framing-factor.)

http://www.fifoil.com/pdfs/productDocs/MicrosoftEvaluationSix.pdf

OK, so it's 10-15% more savings over an uninsulated 8" CMU wall than the savings of ~R3.75 of uninterrupted XPS with an air-film (big whup!). That's the standard of comparison?

It's not a stretch to think that they didn't bump out to 1" XPS (R5) on the furring mounted foam for the analysis of wall #5 is that it outperforms the 3 reflective layer stackup.

And that's a FL climate. In colder places with longer & deeper heating seasons (such as Richmond) that 1" of type-II EPS (R4) foam or cheap foil-clad 1" type-I EPS would likely beat the annual performance of 3-layer RB.

None of the stack-ups in that analysis even meet code-min performance for wall assemblies in any of those FL cities, and certainly doesn't meet code in Richmond VA, so it's something of a moot point.

Mind you, a 16" o.c. studwall (~20% framing fraction typical) with blown cellulose fill comes in at about R10-R11, add an inch of iso on the exterior and you're at R16. That's 1/4 the U value of R4, a 75% reduction in heat transfer over the R4 case. If you leave a rainscreen gap between the exterior foil of the iso and the siding buys a modest something due to the RB effects of that single low-E, high IR-reflectance surface w/air-film.

Greetings,

QUOTE>Perforated RB alone without additional materials in the stud cavity will result in summertime moisture accumulation on the wallboard/plaster layer in an air-conditioned building in a Richmond VA climate- it's too vapor & air permeable.
ANS OK, then use non perforated. OOPS, I forgot, vapor passes thru wood and forms on the other side. Hmmm, and bulk insulation the condensation forms in the mass or at the coldest surface. Gee, the home owners in Richmond are cursed. It's strange that all the conversations I had with the owner of ALFOIL that manufactured on the east coast never mentioned that sort of thing. I suppose you can give me names and phone #s of the the poor souls who suffered with this. You're pathetic. READ further.

By the way a foil in the wall IS NOT a RB, it is "reflective insulation". I keep emphasizing this difference by using the term multi foil, BUT, you keep referring to RB tests. I've come to the conclusion you are doing this deliberately. I also have challenged you to publish the btu/sf/hr of drywall temps for cell and foam, in situ, for summer (95-100 deg day) with drywall facing 75 deg surface. Apparently non of the materials you promote can come to the 2 btu/sf/hr performance of the RI other wise you would have published the figures.

I have also asked you to explain why the attic temp of a RI house with 3 foils in the joist space only gets to 11 degs over ambient on a 95-100 deg day. No response except to push expensive alternative roof shingles, etc.

I have also noticed the use of the term "opaque to RE" in referring to cellulose, not necessarily by you.
To me this term means that the cellulose does not absorb RE. Now we know this isn't true, so can some one explain the use of this term. By the way wood is in the middle 90's on emissivity, so it's absorbing that amount of RE.

QUOTE>A cellulose fill and even modest vapor retardency on the exterior would eliminate the summertime condensation potential, and even R5 rigid foam on the exterior would limit the wintertime condensing temp hours of the sheathing to inconsequential levels
ANS> If it doesn't get the vapor form the out side it'll get it from the inside. The NBS test house , near DC, showed about 15% moisture by weight. That's about 75% increase in heat flow. That doesn't sound inconsequential to me.

QUOTE>Fifoil doesn't make nearly as grandiose an estimate of their products' performance in wall stackups, even in a FL climate. According to their numbers a 3 reflective layer stackup is about 10% more effective than 3/4" (R3.75) XPS mounted on furring with no interspersed framing, or 1" iso (R6, center-cavity) cut up by the thermal bridging of 1x furring, and no other insulation.

You need to reread the Fifoil data. IT'S 3 REFLECTIVE SURFACES, NOT THREE LAYERS.
As I have mentioned before, THERE IS NO definitive in situ test for foil insulation. The foil industry is forced to use dysfunctional tests that show "R" factors. Foil DOES NOT HAVE "R" FACTORS. THE only fair test is the one that I promote and there are foil manufactures that agree. In fact the tests for bulk insulation are NOT in situ test either and do not reflect the conditions the consumer encounters,
and are skewed to give a result that sell product, not performance.