Dana, more specific to the oc vs cc for a retrofit. The house I purchased recently is a 1929 structure with lath and plaster on 2x4. The first floor has a stone facade and the 2nd floor has plaster and exposed wood (Tudor revival style). After a freezing winter I checked the walls - there is nothing in that wall cavity. Now I am considering filling that cavity with some kind of polyurethane foam cc or oc ? I hear all this talk about the moisture barrier effect of the cc, but is that necessary on an above grade application ? Can I safely use the oc and halve the budget ? What am I missing?

Posted By Lica on 23 Aug 2011 01:51 AM
Dana, more specific to the oc vs cc for a retrofit. The house I purchased recently is a 1929 structure with lath and plaster on 2x4. The first floor has a stone facade and the 2nd floor has plaster and exposed wood (Tudor revival style). After a freezing winter I checked the walls - there is nothing in that wall cavity. Now I am considering filling that cavity with some kind of polyurethane foam cc or oc ? I hear all this talk about the moisture barrier effect of the cc, but is that necessary on an above grade application ? Can I safely use the oc and halve the budget ? What am I missing?

There are a few types of moisture situations of concern in above-grade applications.  In cool/cold climates in winter the outdoor temps are below the dew point of conditioned space  indoor air, and some of the structural wood (typically sheathing or the cold edge of studs/timbers) are too, and if the wall structure is permeable to water vapor (or worse, leaks air)  moisture originating in the living space will condense and be absorbed into the wood, raising the risk of rot.  While air leaks are the primary moisture transport mechanism creating this type of problem, in cold or very cold climates (US climate zone 5 or colder) vapor permeation even through air-tight walls can be of concern.

In masonry/stone/stucco clad buildings, there can be severe exterior-moisture drives creating problems as well.  These types of materials are porous, and will wick up and store significant quantities of moisture from dew or rain, and releasing it as water vapoer it to the susceptible interior side (particularly) when heated rapidly.   The classic cases are summer sunrise heating up dew wetted east-facing walls, or sun breaking out on E/S/W walls after a summer shower, causing chronic repeated condensation on the interior surface that runs down the inside of the wall creating mold conditions in the wall cavity as a whole, and rotting out the wood near the floor.

If the walls have exterior wooden sheathing and a substantial gap (1" or more) between the stone or stucco, and that cavity is vented both top & bottom,  in most of the US you'd be fine filling the stud-bays (but not the cavity) with open cell foam or cellulose (dense packed is cellulose preferred, due to the high moisture cycling, which would cause low density celluose to settle over time.)  The cavity between the facing and the sheathing is sometimes referred to as a "rainscreen", but it has multiple functions, one of which is to purge moisture from both the masonry and the sheath via convection driven outdoor air.  The temperature of the sheathing can't drop sufficiently below the dew point of the outdoor air to cause moisture to accumulate from the outdoor air, and since most of the time it's well above the dew point of outdoor air, it relieves the moisture of the summertime vapor drives of sun-baked masonry, and allows any rain seepage to dry quickly as well.  In winter the cavity enhances drying as well, but it becomes a balancing act between the rate of moisture permeating through the wall against how quickly it's removed by the outdoor air convection.  With a closed cell cavity fill that water vapor permeation rate is extremely small, and the risk is low, both winter & summer.  With open cell the vapor permeability is a lot higher, and it kinda depends...

So...

Is there any type of wood sheathing exterior to the studs, with a gap between it an the masonry?  (If no vented masonry cavity exists there's risk to filling the stud bays fully with ANY type of foam, closed cell or open.)

If there is a cavity, there some sort of asphalt paper, rosin paper or some other sort of air-barrier or  vapor retarder on the cavity side of the sheathing?  If yes, what is it?

Are there deep overhangs from the roof that mostly keeps rain off the upper-story walls?

Where is this house located? (For weather data.)

Cincinnati is in SW Ohio, and ASHRAE says this is at the northern edge of zone 4, but this past winter was definitely a 5. I have lived in upstate NY for many years and it felt no different, maybe next winter will be better but I am not willing to bet on it. Summers are hot, regularly in the 90s and humid mostly 60-80 RH.
Roof overhangs are short, no more than 1 ft.
There is no vented masonry cavity "rain screen" !
The 2nd floor wall has lath on both sides of the 2x4, and covered with about 3/4" plaster on inside and about 1-1/2" stucco on the outer wall. I have drilled a couple of inspection holes from inside and as far as I can see there is no other air-barrier or vapor-retarder material. Maybe the outer wall has that material but I did not drill any inspection holes in it. The house was completed in 1929 and the walls appear to be untouched - is anybody familiar with construction techniques in fashion for that era?
On the 1st floor, as far as I can tell from my inspection holes, the only difference is for the outside wall. This is a combination of stone and brick, resting directly on the foundation, with the 2x4 occasionally touching the stone.

My main concern is that by filling the stud bays with insulation, moisture may be trapped in that space. Either moisture running of the closed cell foam or trapped into the open cell foam. Is this a real concern?

You paid $5.20 for 7 inches and he gave you 10 inches in some spots? I think more than a $40 tip was justified. I just have a few questions :) Is this company still in business? Did he do a good a job? Is he willing to travel :) $$$ PS... 10" at $5.20 and I'm paying YOU to have your home insulated by trained profesional with high dollar equipment.......How long ago was this? (Soy based formulas have taken quite hit in the last few years)...

Dana, I missed this earlier... "Either can be used as an air-barrier if applied with care, but neither is guaranteed to be an air-barrier in a less-than-meticulous application." Less than meticulous? In the construction business, nothing is really guaranteed. With that being said, it's pretty hard to screw up a foam job. You should always ask around and get more than one, prefferably three, quotes if you can. You won't find us in the yellow pages and I'm sure other areas are the same. Ask your hardware store for references. Call around. There are plenty of people out there that willing to give there $.02 for next to nothing

Pretty hard to screw up a foam job?

Really?

There's a vast supply of internet images of screwed up foam jobs. Whenever a process becomes more idiot-proof, more talented idiots always seem up to the challenge. ;-) It's doesn't take the talents of Michelangelo or von Braun, but there's both an art and a science to it, and the details count if a primary goal is air-tightness.

Even applications that look pretty good visually can have significant leakage, and shrinkage gaps pulling away from the framing (sometimes days/weeks later) aren't rare. Spraying the exterior has far fewer issues with installation defects leaking air.

Dana,
As far as outside foam is concerned is I have been looking at products like Hardy Board and my concern would be going beyond an inch. Most nailing guns only go 2 1/2 inches and 1/2 inch is most likely not enough as far as nailing. My question is if I go 1 inch on the outside for thermal bridging what would you recommend on the inside? I have heard some negative issues with wet blown cellulose. I have had a energy rater tell me that he would recommend an inch or so of closed cell with fiberglass. Also, with the application on Hardy board and foam would I need some other type of rain screen between the siding and foam. I am getting a little confused because there are so many ways to skin a cat here. Thanks

Posted By peteinny on 24 Aug 2011 06:39 PM
Dana,
As far as outside foam is concerned is I have been looking at products like Hardy Board and my concern would be going beyond an inch. Most nailing guns only go 2 1/2 inches and 1/2 inch is most likely not enough as far as nailing. My question is if I go 1 inch on the outside for thermal bridging what would you recommend on the inside? I have heard some negative issues with wet blown cellulose. I have had a energy rater tell me that he would recommend an inch or so of closed cell with fiberglass. Also, with the application on Hardy board and foam would I need some other type of rain screen between the siding and foam. I am getting a little confused because there are so many ways to skin a cat here. Thanks

You don't nail through the foam, you nail to the 1x (some prefer 2x) furring, and the furring is through-screwed to the studs.  For fiber-cement siding you can get away with 24" on center screws on the furring. See:

http://www.greenbuildingadvisor.com...athed-wall

The only difference between using spray foam vs. rigid foam is that with the spray foam you install the furring first, using chunks of cut up XPS as the depth spacers for the furring.  The furring itself forms a rainscreen gap- you don't need a full mesh-type system behind there, but it's a good idea to use some sort of screen material at the bottom & top vents to limit access to insects.

Flash & batt in the stud cavities can work fine, but it doesn't fill in as well as sprayed/blown, and as a rule there WILL be performance robbing gaps & compressions.  It also doesn't seal the whole wall, since there's still gaps between the stud plates and band joists, etc, and flash'n'batt does NOTHING for breaking the low-R thermal bridging of the studs. Flash 'n' batt protects the sheathing from wintertime moisture drives by providing a liquid-water-impermeable condensing surface (the foam), and the number of condensing hours are reduced substantially, since  there's R6+ between the fiber and the exterior, and a 4.5" deep compressed R19 batt is ~ R16-17 center-cavity.  With more than 25% of the total R between the condensing surface and the exterior, and with a condensing surface that does not degrade with moisture, you're good to go almost anywhere in the lower 48- it can't condense enough to cause a problem.

But 2x6 flash'n'batt without exterior foam is at most an R15 wall, when the bridging of the studs are factored in.  Putting the same 1" foam on the exterior with a spray cellulose fill protects the sheathing even BETTER, and gives you and R20 wall. If  you're ADDING that foam to the exterior bringing the 1" depth up to 2" , the sheating is protected by keeping it even warmer, since now you have R12 on the outside, and R18-21(center cavity) on the inside, with more than 35%  of the exterior of the wood. The sheathing is now the condensing surface, but with that R ratio the number of condensing hours per season on the interior surface of the sheathing is negligible.  Putting an inch of foam on the OUTSIDE rather than in the stud bays raises it to R20, and but making 2" makes it ~R26. That first inch is a huge performance boost over R15, and the second inch is still a significant boost over R20.

If you don't like wet-sprayed cellulose as cavity fill (it's usually the best value), you can use dry-blown-in-mesh dense packed cellulose at 3.2 lbs/ft or more, or blown in mesh new-school superfine fiber glass such as Optima or Spider blown to 1.8lb density, but that's a lot more expensive than mid-density wet-spray, and it barely affects the whole wall R or air-infiltration when you have a solid exterior SPF air barrier.  Before adding the exterior foam, with 2x6 framing the difference in whole-wall R between any of those cavity fill solutions (including flash & batt) is less than R1, due to the R5-ish thermal bridging of the studs- they're all about R14-15. 

Depending on where you are in NY (US zone-5 or zone-6) you may need to put either 1.25" (zone 5) or 2" (zone 6) to fully protect the sheathing from winter drives without an interior vapor retarder, but 2" is good anywhere in the state.  See: 

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

Building to anything less than R20 whole-wall values anywhere in NY is crime, even if it's code-legal.  Spray cellulose + 2" of foam will cost a buck, maybe a buck and half more per square foot than flash'n'batt 2x6, but seriously, it's nearly twice the wall- it's both tighter, and 67-70% higher-R, (and in zone-6 it's a difference you can FEEL every morning in January.)

Greetings,

ANY fiber type insulation for the wallls is going to cause problems, particularly FG. Your only path is foam. Not only will it seal all cracks but it will be less prone to mold and condensation since the foam will adhere to the sheathing components of the wall.

Being a radiant barrier supplier I would normally recommend RB for maximum eff., but, you would have to tear down your inner wall to install it.

Be sure to air blow the seam where the sill plate contacts the floor, that is if you have that type of construction and silicon caulk the crack. If you plan to replace windows use foam to seal around the windows, NOT fg.

If you have no insul in the attic go to fifoil.com and check the two layer RB. It will you the highest insulation value for your money. You can also install a perforated foil over existing insulation and get a notable increase in eff.

You can also increase the eff by adding ceramic beads to the exterior and interior paint. See koolcoat.com for info.

Nice set of misleading & unsupported assertions there, rbi! ;-)

Dana,
Thanks for all the info.

Can you use open cell SF on top of closed cell? So if you wanted to do 2" to 3" of closed cell, and then switch to open cell to save money? Is open cell just as good as blow in cellulose or similar? Would 3" of CC and 3" of OC outperform 3" of CC and 3" of blow in?

I've not seen open cell foam applied directly to closed cell, but I can't think of any reasons why it can't or shouldn't be done.

Open cell foam has a similar R-value to blown cellulose but it's often more than 2x the cost. It's a better air-barrier than cellulose, and isn't affected by moisture, but it also won't buffer any moisture, so it doesn't protect the structural wood as well from vapor-permeation issues.

In the 3" closed cell + 3"-other, situation the thermal performance difference between open cell vs. cellulose will be negligible, and which has the slight performance edge will vary by density, and to some extent climate. The 3" of cellulose at any standard density adds a modest but real amount of thermal mass, whereas the thermal mass of half-pound spray polyturethane is very low. At dense-packed densities the cellulose will have slightly higher R too, but with the thermal bridging of a studwall that aspect is really in the statistical noise, not enough to be a driving factor.

In new-construction (or even re-siding), putting the closed cell on the outside of the studwall is always the better value compared to closed cell as cavity fill, since you get the full-R out of it that way. Between the studs, the thermal bridging of the studs dominates the heat transfer across the wall, and there's remarkably little whole-wall performance difference between an R20 center-cavity R and an R30 center-cavity. When you have ~ R5 framing (2x6) bridging the sheathing to the wallboard for 15-20% of the wall area the framing begins to dominate the surface to surface heat transfer. An inch of closed cell over that framing more than doubles it's R value, cutting the heat transfer of the framing by half.

Greetings,

QUOTE: In new-construction (or even re-siding), putting the closed cell on the outside of the studwall is always the better value compared to closed cell as cavity fill, since you get the full-R out of it that way

A better way would be to furr out the wall and attach a perforated RB. That will give you far better results. By the way what is the full "R" value? Since bulk insulation are tested under conditions that do not match the installed conditions, that's a bold statement. OH yes, how about the BTU's used to phase change the moisture in the air causing condensation against the wall sheathing. Doesn't that count against the "R" value?

QUOTE: Between the studs, the thermal bridging of the studs dominates the heat transfer across the wall, and

If I'm reading this right your saying there is more energy exchange between surfaces thru the stud conduction than between two surfaces of the wall cavity. If that's right then you had better go back to science 101.