We bought a fixer-upper in Florida. Interior walls have been removed for an open concept. The drywall is bad and wiring as well so I plan on re-sheeting. There is no insulation in the 2x4 cavity so now is the time to do it right. Current wall assembly is ½” drywall, reflective foil, empty cavity, T 1-11 sheeting and cheap vinyl siding. The siding will have to be replaced down the road. Thus the question, what would be a suggested wall assembly for the hot, humid climate of central Florida? I like the idea of strapping the interior wall with 2x2’s going horizontally for wiring/plumbing chase. I understand vapor retarders are supposed to go on the outside; can I apply a liquid vapor retarder from the inside on the T 1-11 sheeting? Or should I wait until I reside?

You can start here. In general, make it somewhat (but not too much) vapor permeable in both directions, much more so on the cold side, and air seal it well (preferably both sides).

Other hot climate advice - maintain a slightly positive building pressure and use dehumidifiers.

Thanks for the rapid reply. I read that as well as some other information specific to hot humid climates. One specific to Florida highly recommended a vapor barrier as opposed to a vapor retarder and foam insulation on the outside. Therefore I'm thinking some fluid applied vapor retarder/air barrier over the T 1-11 sheeting then EPS foam over that strapped with 1 by 4s to hold the siding. On the interior side I'm thinking something like SmartBatt or unfaced batt insulation with MemBrain in the 2x4 cavity. I'm also thinking about using 1 1/2" ESP on the inside wall as well. However, does that make for the dreaded foam sandwich?

Any recommendations for fluid applied vapor barrier or smart vapor retarders that have efficacy and reasonable cost?

Again thanks for the information.

Having foil on the interior side of fiber insulation would be a mistake in FL, since the temp of the foil during peak AC season would be below the outdoor air's dew point, and you'd get condensation on the foil & rot conditions inside the stud bays.

True vapor barriers (like foil or polyethylene) are dual-edges swords, and would cause as many problems as they solve in your climate unless managed correctly in the stackup. When dry, unpainted T1-11 would have a vapor permeance of about 1-perm, the edge of Class-II vapor retardency, but on the humid high-RH exterior side it's vapor permeance would be higher, maybe 3-5 perms. If it has been painted it'll be a bit less, depending on the paint.

An air-sealed layer of foam (any type) over the exterior T1-11 would isolate it from the exterior humidity, but a layer of foil-faced polyiso would isolate it completely. The assembly would then be able to dry toward the air-conditioned interior, as long as you get rid of the interior foil, and don't use foil or vinyl wallpapers, only standard latex paint.

A layer of unfaced EPS would be more vapor-open, but if air tight the T1-11 would drop to 1 perm or less. As long as the foam & T1-11 are detailed to be air tight, a ~1-perm exterior is NOT a significant latent cooling load even in sticky central FL. The assembly would be able to dry in both directions. AIR LEAKS would be far bigger latent cooling loads to contend with, so caulking the framing to the T1-11 with the appropriate sealants while you have it open is key to making the assembly less susceptible to moisture problems. This is an important step even if using a spray applied WRB on the exterior, but would be of lesser importance if installing open cell spray foam.

MemBrain on the interior wouldn't hurt, but it wouldn't help as much as even 1/2" of exterior foam and an air-tight sheathing approach. In your climate the more vapor retardent layers want to be on the exterior.

From a financial sanity point of view in US climate zone 2 anything over a whole-wall R of about R15 could be hitting the limits. A T1-11 sheathed wall with R15 fiberglass or rock wool and half-inch gypsum comes in pretty close to R10 whole-wall. Adding 1.5" of EPS or 1" of foil-faced polyiso on the exterior would already put you in that range. If you went with 1.5" exterior polyiso you'd be literally double the performance of a 2x4/ R13 wall with no foam, which is the IRC 2012 code-min for US climate zone 2. Literally TWICE code min!

At that point there are probably better places to be applying the money. Download BA-1005 from this page, and review the zone-2 row of Table 2, p10 to compare the performance points of other aspects of the house:

http://buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones/view

Installing a additional 1.5" EPS without facers on the interior is still about 2 perms. It's a modest vapor retarder on the "wrong" side of the assembly for your climate but it's not a serious impediment to drying. With that R6 or more on the interior it's unlikely to end up being cold enough on the studwall side of the foam to create condensing conditions, but it would still be better to put it all on the exterior side of the assembly, bringing all of the structural wood a little bit closer to the conditioned space conditions which is almost always drier than the outdoors in your climate.

Dana1 you are the wizard!! I also read Habitat Congress Building America Hot-Humid Climate Case Study. Thus if I'm hearing you correctly I would be close to optimizing my wall assembly by installing 1" of foil faced polyiso on the exterior, fiberglass or rock wool in the cavity but making sure everything is air tight. I was thinking when I have the cavity open to use acoustical caulk along the intersection of the T 1-11 sheeting and 2x4 studs. Or are there other products that could serve to air seal the sheeting?

The Habitat Congress Building America Hot-Humid Climate Case Study also recommended installing foam on the underside of the floor joist. Any thoughts on that recommendation?

Again, thanks to all!

I wouldn't put a foil vapor barrier anywhere and I don't buy into using acoustic caulk in place of a caulk made and sold for such things. Use tape instead. You can air seal the interior drywall to get a beneficial double air barrier. I agree with no liquid applied barrier, no interior foam and no MemBrain.

Acoustic caulk will work, but sealants like ProPink et al are purpose-made and less messy to work with.

Tapes may work on the horizontal seams of the T1-11 or not, depending on just how clean the T1-11 is. You may have to both tape it and goop over it with duct mastic or something. With T1-11 there's no way to seal it with tapes on the exterior.

The foil facers are not a hazard in this climate as long as they are on the exterior of all structural wood. In colder climates that would be much riskier to do that unless you had enough exterior R for ample dew-point margin at the sheathing. In FL using foil faced polyiso gives you higher R/inch, and with a rainscreen gap between the foil and the siding gives you an additional performance boost.

Calling 1" of exterior polyiso "optimizing" isn't necessarily the case- it depends on what your energy costs are and what you expect them to be in the future. If you want to buy a hedge against future energy inflation bumping that to 1.5" doesn't cost much more in labor costs, and only a modest upcharge in material cost.

You'll note that the prototype in the Habitat Congress Building America Hot-Humid Climate Case Study added R10 foam sheathing to a 2 x 6 wall sans cavity fill. This is lower performance than 1.5" foil faced polyiso sheathing on a 2 x4/ R15 wall, but would be similar to the performance you'd see if you skipped the R15(!).

If yours is a pier foundation or vented crawlspace, yes, installing rigid foam under the floor joists makes sense. If it's a vented crawlspace with ducts or other mechanicals in the crawlspace it may make more sense to convert it into a conditioned unvented crawlspace with insulated walls and a ground vapor retarder, but no insulation on the floor joists or the crawlspace floor, "earth coupling" the house to the thermal mass of the dirt below (which is at a favorable temp in central FL.)

jonr--I was thinking about using the acoustical caulk like EcoSeal or EnergyComplete sealant on the interior cavity in addition to taped building wrap therefore having triple the air sealing when counting the sealed drywall. Or is that overkill, if so which method gives me the best outcome?

Exterior foam--EPS or polyio with foil or polyiso without foil? Which choice is less like to get one into trouble? Or most resilient, if you will?

The home has a crawl space/partial basement containing no mechanicals but has a window and door. So what would be the recommendation?

Thanks so much!

I think you will get slightly better R/$ and moisture performance from unfaced EPS vs foil faced polyiso. Right from the case study - "Ideally, building assemblies should be designed to dry to both the interior and exterior."

There is a lack of long term data regarding different air sealing methods - but even short term, caulks/sealants aren't doing as well as gaskets or the top tapes. Matt Risinger has sealed and tested many homes and writes "I believe the tape is a much better overall strategy."

Posted By jonr on 20 Aug 2015 09:39 PM
I think you will get slightly better R/$ and moisture performance from unfaced EPS vs foil faced polyiso. Right from the case study - "Ideally, building assemblies should be designed to dry to both the interior and exterior."

There is a lack of long term data regarding different air sealing methods - but even short term, caulks/sealants aren't doing as well as gaskets or the top tapes. Matt Risinger has sealed and tested many homes and writes "I believe the tape is a much better overall strategy."

R10 unfaced EPS would have a vapor permeance of about 1 perm, which is about the same as dry.unpainted T1-11.  At 1.5" unfaced Type-II EPS comes in under 2 perms.

Painted T1-11 with a couple coats of exterior grade paint would run about a half-perm.  It really HAS to dry toward the interior.

The case study prototype used 2" XPS, which runs about 0.6 perms and unpainted OSB (about half-perm when dry, 5 perms when wet), which isn't exactly a huge drying capacity.

With painted T1-11 the difference in drying rates between 1" foil faced polyiso and EPS of equal R is pretty much in the noise- it's primary drying path is toward the interior, at about an order of magnitude higher vapor than the combination of 1.5" EPS & exterior paint.

To quote Dana elsewhere: "...you'd have more than 0.5 perms of drying capacity toward the exterior which is WAY better than the 0.05 perms (best case) you'd have with foil-faced foam". Different wall and climate, but 10x to ~.5 still makes for significantly more drying.

When the outside of the T1-11 gets wet due to rain or air movement, ~.5 perms to the interior plus ~2 perms to the exterior significantly outperforms .5 perms plus .05 perms.

That's right- if that's your only drying path due to the interior side foil referenced in that link, and the vapor retardency of the foam is on the "wrong" side of the assembly for the climate. In that case the interior foil faced batts blocks drying toward the interior, and foil faced polyiso on the exterior would create a severe "dries never" moisture trap. But that's not what we have going on here.

In central FL having the foil vapor barrier on the exterior side protects the T1-11 and framing cavity from very significant exterior moisture drives. And with a 3-5 perm interior vapor retardency through the wallboard paint into a known-dry mechanically air-conditioned space, even minor bulk leakage at the windows during hurricanes can still dry toward the interior at a reasonable rate.

Yes, 0.5 perms is still WAY better than 0.05 perms...

... but it's still an order of magnitude lower than 3-5 perms.

In FL nothing (much) really dries toward the exterior- it's a SWAMP, a swamp in a climate where the temperature promote mold & fungus growth! Unpainted wood left outdoors in the shade will grow mold and rot fairly quickly. T1-11 siding is a pretty crummy product for that climate- it'll grow moss if you let the paint go. Vented attics in that climate drag more moisture in than the venting purges, raising rather than lowering the moisture content of the rafters and roof deck.

If for example you install two layers of polyiso 1" / 1" for a total of 2" each layer is taped/sealed at joints and terminations.

Would it be best that the first layer of polyiso installed be faced with foil on the front side and a fiberglass face on back side which would rest against the wood sheathing.....The second layer would be foil faced on both sides and taped also?

The reason I say fiberglass for the face which would be in contact with wood sheathing is moisture could collect on the metal foil against the sheathing if the dew-point resides at that depth.

Great thoughts. People tell me that one has to power wash vinyl siding annually because of the mold which by the way is growing on my fixer-upper. However, there was no mold growth on the painted T1-11.

I'm glad Dana1 mentioned attics. The fixer-upper attic has a row of vents on each side about half way up. When I looked in the attic I noted a fairly strong level of air movement. However, there are no vent chutes bringing air from the soffit. The soffits in fact are solid, which made me think oh, oh. Therefore the question, what is the proper method to deal with attic ventilation? The roof is only 8 years old so I'm reluctant to do anything there. When I go to re-side i plan on doing a proper fix, whatever that entails.

Thanks again.

what is your wall "stackup" if you use some type of foam? Air sealing techniques vary.

My thought is with a wall stack up (from interior to exterior) drywall, unfaced fiberglass, T 1-11 sheeting, Tyvek, 1 1/2" EPS foam, 1 by furring strips, steel siding or fiber board siding. My air sealing process would be to caulk the joints where the studs contact the sheeting on the interior. Caulk and tape the T 1-11 joints, tape all Tyvek joint and tape EPS joints. I may swap out the EPS foam for polyiso.

What makes the most building science sense?

I'm unclear why T-1-11, an exterior siding product, is in the middle of your wall. You could use fir plywood instead; this allows you can tape and air seal the exterior joints with SIGA Wigluv or 3M 8067 tape. Caulk the joists & studs at the the edges of the wall.

Bob I The T 1-11 is the existing exterior sheeting/siding which the previous owner applied vinyl siding over. I plan to take the vinyl off which will give me the opportunity to seal the T 1-11, install Tyvek, foam insulation, furring strips and re-side. Is there a better alternative? I'm retired and finding my planning horizon is shortening.

Again, thanks for all the good thoughts.

Any thoughts on venting the attic?

Since T-1-11 is a rough products and difficult to tape, you can caulk the joints with a good urethane or similar caulk. The tape the foam that you install over it.

I agree - tape the EPS with SIGA Wigluv. But with primer and a little sealant to fill perpendicular grooves under the tape, external tape on T1-11 might be fine too.

Has your attic been working well so far?