ratio of closed cell foam to other insulation for ceilings and roofs
Hi – we are building an energy efficient house (radiant heat in slab) in southwestern Montana in CZ6 and trying to figure out our insulation options. We’ve read an article by Dr Lsitburek from Oct. 2017, Insight – Hybrid Attics and Hybrid Walls. He shows two tables – Table R 806.5 and Table R 702.7.1 from the International Residential Code – that show the Ratio of Rigid Board Insulation or Air Impermeable R-Value to Total Insulation R-value. For our zone 6, the recommended minimum “Ratio” is 35% for walls and 50% for ceilings.
We have a double stud wall (12″ total width, 2 x 6 outer, 3 inch space, 2 x 4 inner). Sheathing is CDX plywood and then Corten siding. No rain-screen gap as we are making this house very fire resistant and are worried that the gap could be an entry way for fire.
For this 12″ wall, we are thinking we want to do 3″ closed cell then 9″ BIB fiberglass. This gives a “Ratio” of 39.7%. Presently, we are not planning on using any other vapor barrier given the fact we are using 3″ closed cell. Is this a feasible option? Are we missing anything? Should we be using a vapor barrier between the inner stud wall and the sheetrock and if so, what type? Should we be using a vapor barrier paint if not adding the vapor barrier behind the sheetrock?
For our cathedral type ceiling, we will have an unvented space above a ceiling that is 1/2″ CDX then T&G hemlock. We are planning on 6″ of closed cell, then 5″ of open cell. This has a “Ratio” of 72%. Again, we are not planning on using any other vapor barrier given the fact we are using 6″ closed cell SPF. Is this a feasible option? Are we missing anything?
We really want to make sure we don’t get condensation problems when we have some of our really cold days at 30+ below zero.
Would greatly appreciate any advice, feedback, warnings, recommendations, studies, articles, etc.
Thanks so much!
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I'm far from the leading expert here so hopefully, if I'm wrong, others will chime in. I have, however, been looking at these issues for a few years, so I believe I've got some useful knowledge to share.
1. A few years back I asked here, and searched online, to find out whether rainscreens posed additional dangers to houses in wildfire zones. The answer was that there was no evidence to indicate an increased danger. A gap of 1/4" to 3/4" that is 8' plus high, and which is partially blocked by screens at bottom and top, simply can't develop a flue effect. As added assurance, having it open only at the bottom still results in a useful rainscreen. Also important is to keep vegetation away from the base of the exterior wall. I specify on my drawings a 2' zone of impervious clay cap all around the wall base, with no vegetation or irrigation within that zone. I also insist on foundation french drains in all locales.
2. The insulation ratio is more than adequate. If you had a rainscreen and a class II vapor retarder at the inner face you wouldn't need any vapor barrier at all.
3. I definitely wouldn't use a second vapor barrier it is not only unnecessary, it is also risky. No need for a vapor barrier paint.
I'll leave the ceiling for others to comment.
On those coldest days you would do well to make sure to minimize indoor sources of humidity. For example, you could put the bath fan on the same switch as the light, with a timer that keeps it going for 15 minutes after the light is off. This would assure that people run the fan when showering. And of course, use the stove hood! That's what it's there for.
I've been interested enough to follow the literature on energy
efficient single-family housing for 40 some years now. It's
my impression that there has never been a year when some
article hasn't appeared about someone installing spray foam
which ended in a financial disaster. Why? Because the foam was
installed at the wrong temperature, the wrong humidity, the mix
of ingredients was incorrect, etc., etc., etc. Having said this, I also
note that thousands of homes have had spray foam installed
with no problems. Personally, I have been unwilling to take the
risk even though I'm very aware of the advantages of spray foam.
Having designed, built, and lived in several extremely energy efficient
homes over decades I find that there are other ways (also cheaper
ways) to achieve the same R-Values without spray foam. Your choice -
keep looking - have fun.
"No rain-screen gap as we are making this house very fire resistant and are worried that the gap could be an entry way for fire."
I'm not going to say it's an unreasonable concern but it would be nice to see a solution because rain screens are great but fire concerns are also becoming increasingly important in many areas.
Some thoughts...
1) Don't vent the top. Just have the drainage gap at the bottom. I think this is the most common configuration anyway but should reduce the chance of a draft that would pull hot air and embers through the rain gap.
2) The rain screen gap may actually improve fire performance. Assume your metal siding will get extremely hot. It'd be nice to keep that from coming into direct contact with your WRB.
3) It's possible to put a very tight mesh at the bottom of the screen. You should also be keeping vegetation away from the edge of your building. Combined with a lack of a vent at the top, it seems much less likely that embers would find a way into the gap.
4) You might also consider a thin layer of external Rockwool insulation.
5) If you don't want a full rainscreen gap, you could at least use a drain wrap product (basically just a WRB with some texture/channels.) For example: https://www.dupont.com/products/tyvek-drainwrap.html
6) The furring strips are a weak point for fire. I wonder if there are any practical non-combustible alternatives. With metal siding, non-combustible furring strips (potentially connected directly to the sheathing if the metal siding is heavy) and a layer of Rockwool, you'd have a very fireproof exterior.
+1 to exterior Rockwool as a protective measure against fire.
Rockwool's relative fire resistance has been a selling point with clients the last few years, even given its higher price point locally than other exterior insulation materials (also southwest Montana, Bozeman / Big Sky).
For your roof:
What is the point of the cdx at the ceiling level? I read that as not being at the roof deck but on the underside of your rafters? If so, I would omit this and instead use a variable perm membrane (though as others have stated not really necessary and a type 1 vapor retarder is not even allowed by code in this assembly).
I would not use open cell foam. I see very little reason for anyone to use this and this seems to be 90-99% of the curing issues people see, especially with odors making the house unlivable.
Here is an excerpt from CA WUI code:
"Where the roofing profile has an airspace under the roof covering, installed over a combustible deck, a 72 lb. (32.7kg) cap sheet complying with ASTM D3909 Standard Specification for "Asphalt Rolled Roofing (Glass Felt) Surfaced with Mineral Granules," shall be installed over the roof deck. Bird stops shall be used at the eaves when the profile fits, to prevent debris at the eave. Hip and ridge caps shall be mudded in to prevent intrusion of fire or embers.
Exception: Cap sheet is not required when no less than 1 inch of mineral wool board or other noncombustible material is located between the roofing material and wood framing or deck.
Alternately, a Class A fire rated roof underlayment, tested in accordance with ASTM E108, shall be permitted to be used. If the sheathing consists of exterior fire retardant-treated wood, the underlayment shall not be required to comply with a Class A classification. Bird stops shall be used at the eaves when the profile fits, to prevent debris at the eave. Hip and ridge caps shall be mudded in to prevent intrusion of fire or embers."
If you are serious about fireproofing your structure, I would consider mineral wool insulation on both your roof and walls exterior at min 1" thickness.
(edit)-Exterior rockwool would actually be very similar in price to your closed cell insulation, consider not using ccSPF and just do thick exterior rockwool,
For your wall:
Again, consider whether a double stud wall really makes sense if you are attempting to make this a very fire resistant building and whether a layer of exterior mineral wool, perhaps even 3" thick, would make more sense with a 2x6 wall, or even 2x8 if you need more insulation.
for a rainscreen, what I have done in CA is use a fire approved vent for the bottom, vulcan vent makes a low profile vent that fits this bill nicely, though if you do exterior mineral wool I would worry much less and simply block off the top vent channel so that ember storms cannot get to the bottom of your eaves.
Spray foam is something you design out of a new build. Expensive, can sometimes have bad install and in a fire zone, it is pretty much liquid gasoline in your walls.
For thick walls, I would stick with dense pack with borate treated cellulose or layers mineral wool batts. Thick walls with all fluffy do need a rain screen though, not that hard to detail to keep ambers.
Your roof is a bit harder. You could go for exterior insulation but that is a lot of rigid. Spray foam there might make the most sense, unless you have trusses, I would skip the open cell and go for regular batts. With trusses, all spray foam is easier to install is it is hard to hold any batts in place.