Rainscreen differences when using mineral wool rather than foam?
Need to replace siding and roofing on my 1942-vintage house. Climate 4C. 8-in-12 gable roof. 2×4 walls.
Because I’ll be removing all the contact-clad stucco and replacing it with rainscreened Hardi-Board, I thought it would be a good opportunity to add insulation to the outside of the house (right down to the footings). Ditto with the roof. Thought it’d be a good opportunity to add on over/under-roof with an air gap on top of the new insulation.
I’ve found lots of resources on using foam board (EPS, ISO) but little on using mineral wool in this application. Lstiburek mentions mineral wool as an acceptable sheathing material but doesn’t say if/how it affects the choice/placement of the air/water membranes.
I really would prefer to use mineral wool rather than foam because:
– it’s cheaper here,
– it’s easy to work with, and
– it’s fire-resistant.
I know that mineral wool boards lets moisture and air through, so how do I account for that in my wall/roof design?? Are there any resources you can point me to that specifically deal with using mineral wool in this application?
Thanks!
…/j
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
John,
A few builders have begun using mineral wool on the exterior side of walls. Here are some links to relevant articles on GBA:
Installing Mineral Wool Insulation Over Exterior Wall Sheathing
Installing Roxul Mineral Wool on Exterior Walls
Wrapping an Older House with Rock Wool Insulation
Mineral Wool Boardstock Insulation Gains Ground
The minimum R-value rules for rigid foam insulation in this location don't apply to mineral wool insulation, because mineral wool is vapor-permeable. That means that your mineral wool can be as thin or as thick as you want.
Semi-rigid mineral wool panels are often used on the exterior side of roof sheathing, especially in Europe, but only on low-slope (flat) roofs, as far as I know. If you want to experiment with using mineral wool on the exterior side of roof sheathing on a steep roof, you will be a pioneer, forging new ground, and making new mistakes for the first time ever.
Thanks for the articles Martin. Good to see that mineral wool slabs are gaining in popularity.
Here's what I'm thinking (inside-to-outside) for the wall assembly retrofit:
- plaster-and-lathe
- 2x4 with dense-pack cellulose (already installed)
- remove 1x6 T&G sheathing and replace with 5/8" exterior plywood (much of the T&G is rotten, looking from the inside of the unfinished basement walls)
- peel&stick membrane; should this be vapour permeable (like BlueSkin VP)? or not (like BlueSkin WP)?
- 2 layers of 1-1/2" Roxul ComfortBoard IS =>R12 tacked to studs.
- vertical 1x4 furring strips, screwed to studs through mineral wool
- Hardi-Plank sidings, attached to furring strips.
Does this look right? If so, where is the drainage plane? Back at the peel&stick? or the outside surface of the mineral wool?
i.e. should my flashings around doors/windows/vents extend all the way back and under the peel&stick?
In an foil-faced ISO-insulated wall, many of the articles use the outside face of the ISO as the place to attach flashings.
Am I correct in assuming that the flashing detail would be different for mineral wool?
John,
You get to decide how to create an air barrier, and you get to decide what type of water-resistive barrier (WRB) you want to install. You also get to decide where the WRB goes.
It makes no sense to install an expensive peel-and-stick product if you don't know what it's for. Is it an air barrier? (You decide.) If so, there are cheaper ways to seal your wall -- for example, by taping your plywood seams.
Is it a WRB? If so, there are cheaper options -- like Typar or Tyvek.
You might want to read these articles:
All About Water-Resistive Barriers
Where Does the Housewrap Go?
The peel-and-stick was intended to form an Air Barrier like in the PERSIST system. The thinking was to have it fully cover the roof, walls, and foundation to tighten up the envelope.
Based on what I see when I tear off the stucco, I may only replace the rotten parts of the existing t&g sheathing with plywood so there will still be lots of air gaps in the original sheathing.
I'm leaning towards innie windows so it sounds like flashing would be easier with the WRB against the sheathing. I'd then have to extend the jambs out from the windows to cover the exposed edges of the mineral wool.
In such a case, would you rely on a peel&stick flashing for the top side of the upper jamb extension? Or would you cap it with a metal flashing? My concern is that it's a place for water to collect (and therefore needs protection), but a metal flashing could result in cold sheathing/condensation at that spot.
John,
The PERSIST system assumes that all (or almost all) of the wall insulation will be installed on the exterior side of the wall sheathing. Is that your plan? Will your stud bays have no insulation?
There is dense-pack cellulose in the main walls and poorly-fitted fibreglas batts in the second-story walls (from previous owner's reno).
I would be adding R12 on the exterior which, in this zone 4-Coastal region, would result in sheathing temperature above the dew point year-round. All-time low temperature here is -8C (18F); design temp low is -5C (23F), for a sheathing temp of around 10C (50F) during the winter with 50% of the insulation on the exterior.
Getting back to the treatment of wall penetrations, how would you flash the top of innie window jamb extensions if there is vapour-permeable peel & stick on the sheathing?
- Peel & stick from the jamb extension up onto the sheathing? or
- Slope the top of the jamb extension towards the face of the wall and then peel & stick? or
- Slope the top of the jamb like a "frog flashing" on a chimney (sloping towards each side of the window & then peel & stick? or
- Cap with sheet metal (with end dams) and peel & stick to sheathing? or
- Something else?
Thanks!
.../j
John,
With innie windows, the peel-and-stick you are describing is usually the WRB. So the exterior jamb extensions are on the exterior side of the WRB, and don't really need flashing. If water gets behind the jamb extensions, the water is already on the exterior side of the WRB.
Of course, it's usually a good idea to make the exterior sill (which the bottom of the exterior jamb extensions butt into) waterproof, since the exterior sill sees a lot more water than the jamb extensions.
John,
If you have 2x4 walls filled with dense-packed cellulose, and you plan to install R-12 insulation on the exterior side of the wall sheathing, you aren't building a PERSIST wall.
In general, you don't want to install a vapor barrier in the middle of an assembly unless you are sure of what you are doing. The peel-and-stick membrane works on a PERSIST wall because it is on the interior side of the insulation. You're proposing something different -- so if you really want peel-and-stick in the middle of the insulation, I advise you to choose a vapor-permeable peel-and-stick.
Thanks. BlueSkin says its BlueSkin VP is vapour-permeable although I haven't seen it for sale up here in BC yet.
What are the other leading contenders for vapour-permeable Peel & stick?
John,
Another is VaproShield WrapShield SA.
Martin writes: "The minimum R-value rules for rigid foam insulation in this location don't apply to mineral wool insulation, because mineral wool is vapor-permeable. That means that your mineral wool can be as thin or as thick as you want."
That is (sorta) true for climate zone 4C (or warmer), but not true for colder climate zones! In zone 4C it only takes R3.75 of exterior R for dew point control at the sheathing on a 2x6 assembly and only R2.5 for 2x4 assemblies, with only latex paint as the interior side vapor retarder. That's less than the the R-value of the thinnest rigid rock wool products- you can't even BUY rigid rock wool that would be below that threshold.
It's also true that in zone 4C ANY rainscreen would be sufficient to allow latex-paint as the vapor retarder. Even R1 1/4" XPS siding underlayment with 0.8-perm facers does not impede drying sufficiently to cause a problem if rainscreened, despite having a lower-R value than the IRC prescriptive minimums.
Plywood/OSB/plank sheathing itself is not (very) vapor permeable- it's a class II vapor retarder. An inch of XPS is more vapor permeable than dry OSB sheathing. Even with rock wool on the exterior OSB sheathing not going to be passing much moisture through to the exterior until it has accumulated the very moisture load that you're trying to avoid. The average winter temperature of the sheathing is still the most critical parameter for limiting the amount of interior-side moisture that ends up in the OSB when no interior vapor retarders are used. The enhanced drying rate through the rock wool to the exterior compared to foam is but a secondary factor in climates colder than zone 5. With a rainscreen you can get away with thinnest-available ~1" rock wool on an OSB sheathed 2x6 assembly in zone 5, but not zone 6.
In climate zone 6 it takes a more vapor-permeable sheathing than OSB or plywood (such as gypsum board, fiberboard), to be safe with less than the requisite minimum R-value.
In zones 7 & higher even gypsum sheathing isn't sufficiently vapor permeable to cheat the prescriptive minimums for exterior R.
Thanks Martin -- I'll check VaproShield out.
Dana, given that I'm in climate zone 4C and am planning on installing R12 mineral wool on the outside, I think I will be well within your specs for preventing condensation. In the basement (where fibreglas batts and incomplete poly have been installed for 15+ years) there is significant sheathing rot -- demonstrating the problems of having no external insulation and face-sealed (but cracked) stucco.
The finished attic has a poly vapour barrier on the inside but, from what I've seen in the other renovated walls I've opened in the house, workmanship is mediocre at best so it's definitely not acting as an air barrier: big voids in the insulation, poly that doesn't reach the top plate, insulation-bag plastic used instead of 6-mil, etc.
I used fine lines of spray-foam to seal the t&g joints and the edges of every stud cavity from the inside of the basement wall. Made a big difference in its air sealing (eliminated the significant cold drafts that were coming through) although I realize it is not a permanent solution. I'll need to replace the sheathing from the outside to do that.
Bit by bit...
Dana,
You wrote, "In zones 7 & higher even gypsum sheathing isn't sufficiently vapor permeable to cheat the prescriptive minimums for exterior R."
Yet builders do it all the time -- in conventional 2x6 walls as well as double-stud walls. The reason these walls work is that the sheathing which gets damp every February is able to dry out by April.
How much foam &/or rock wool are they putting on the exterior of the double studwalls, and with what cavity fill?
You can buy a lot of buffering with cellulose (even in 2x6 walls) enough to make out OK with an inch of ~1-1.5 perm XPS on the exterior (but zero exterior foam would be better from a drying rate point of view.
Dana, I see how that double studwall would dry outwards more quickly with no XPS/rockwool on the exterior, but wouldn't it also be more prone to wetting (due to condensation) then?
Dana,
My point is that the mineral wool doesn't slow drying. You're worried that the sheathing will get too wet with mineral wool -- but these walls work even with no mineral wool or any insulation of any kind on the exterior side -- so the mineral wool, however thin, makes things better, not worse.
Martin- For exterior rock wool as insulating sheathing to work at an R-value lower than the prescriptive minimums, it has to be sufficient R to make the average winter temp at the sheathing comparable to a zone 5 climate, where a rainscreen alone is sufficient.
In zone 6 even with a rainscreen it requires highly permeable sheathing types at R0 exterior R to be able to use the interior paint as the only interior vapor retarder, and in zone 7 & higher even with highly permeable sheathing types would not cut it.
If there is sufficient exterior rock wool to make the sheathing's average winter temp mirror that of a zone 5 climate it would be fine with OSB or plywood sheathing, but less than that runs some risk. That's not quite as much exterior-R as the IRC chapter 8 prescriptives, but it's still a significant fraction thereof.
Of course the exterior rock wool doesn't make things worse, but it has to make it sufficiently better. The R-value still matters in cold climates, even if it doesn't matter in zone 4C.
Dana,
I respectfully disagree with your analysis.
Most walls in North America don't include exterior insulation. Most aren't rotting, for the reason I gave. (Although wall sheathing gets damp in February, it dries out in April.)
Are these American walls the most robust walls you can build? Of course not, as GBA has been reporting for years. Walls with an adequate thickness of exterior insulation perform much better.
The problem with too-thin foam is that the foam inhibits drying to the exterior without keeping the sheathing warm enough to be out of the danger zone.
Mineral wool doesn't inhibit drying to the exterior. If you take a run-of-the-mill North American wall and add exterior mineral wool, you are making the wall more robust, not less robust, regardless of the R-value of the mineral wool.
Most run of the mill buildings in North America with OSB or plywood sheathing have interior-side vapor retarders more effective than latex paint, even if it's only asphalted kraft facers on batts.
We don't know if or what type of vapor retarders are in this 1942 vintage house, but unless the walls are have been gutted and re-insulated it's probably just the interior side paint. But in zone 4C it's "who cares?" and rock wool is the best way to go if for no other reason it can't create a moisture trap even if there is interior side foil faced batts (common in that area the 1950s & 1960s) or poly sheeting, whereas low-permeance foam could. The sheathing in that region in the 1940s was typically 1x ship-lap planking too, which isn't quite the same cooked up mold-candy that OSB is. I'm not at all concerned about this installation- the exterior R is many times the prescriptive R, and the wall has withstood the moisture drives of stucco siding for 73 years.
I'm more concerned about prescribing thinner than-code exterior rock wool on an OSB sheathed house in zones 6 or higher without the use of class-II or class-I interior side vapor retarders, or that ".. your mineral wool can be as thin or as thick as you want", on the theory that the OSB will always be dry by April. With R0 exterior insulation in a rainscreened wall assembly in zone 6, without interior side vapor retarders the sheathing (particularly on the north side of the house), can become nearly saturated, and the insulation can even become wet.
That's why the IRC requires either Class-II or Class-I interior side vapor retarders for wood sheathed buildings in US climate zone 6, even IF it has vented siding.
But since the IRC is fine with Class-III interior vapor retardency on wood sheathed buildings with vented siding for zone 5, that's the climate where it will dry sufficiently quickly come April, with no appreciable frost/moisture accumulation in the insulation. So, if you can keep the wintertime temperature profile at the sheathing to a zone 5 type winter temps, the drying performance to the exterior through 25-30 perm rock wool and 2-5 perm felt will be about the same as in the zone 5 case. But with less rock wool that that the sheathing will see progressively higher moisture accumulations, and higher risk.
Not every building that doesn't meet code will fall apart in 5 years or even 50, but from a relative risk point of view, if you're going with less exterior rock wool than the code prescriptives, it has to be at least enough rock wool to make the sheathing experience a zone 5 climate, or it starts climbing up that risk path.
The north side of the building in N.America is the wall at highest risk, since it gets no direct sun warming, and dries much more slowly. If you're going to cheat the code prescriptives cheat only the sunnier sides of the house, don't cheat the north side.
Dana,
You are wildly exaggerating the effect of vapor diffusion (as opposed to air leakage) in conventional walls. Please provide a reference to a study that supports your view that interior vapor barriers are necessary to prevent sheathing on conventional walls from becoming "nearly saturated, and the insulation can even become wet" -- so wet that the sheathing doesn't dry out by the end of April.
Note: we're talking about vapor diffusion here, not air leakage.
There have been many, many field studies and test-hut studies looking at this issue, and none of them have produced the results you postulate.
Lots of good info here so far.... :-)
Getting back to the question of using mineral wool on an over/under roof... The Revised BC Building Code (Jan 2015) calls for RSI 6.91(=R40) on ceilings below attics and RSI 4.67 (=R27)on cathedral ceilings and flat roofs. http://bccodes.ca/BCBC_9%2036%20EnergyEfficiency.pdf
The existing roof covers a 1-1/2 story house which has 7" of fibreglass batts (R24?) between doubled-up 2x4s and an inside 6mil poly layer over the vaulted living area. The side attics have 10" of loose rock wool.
So I'm not sure whether an over/under roof would need to meet R40 or R27.
From the bottom up, I'm considering the following assembly for my over/under roof retrofit:
- existing 2x4 rafters (this is a 1942 house, region 4Coastal, 8-in-12 pitch gable roof). Cut off overhangs.
- existing 1x6 skip lath (every other board, installed when original cedar shingles were installed)
- existing 1/2" OSB
- add peel-and-stick membrane as an air barrier (vapour permeable?)
- add multiple offset layers of mineral wool (1-1/2" sheets are R6 each). I'm thinking 4 layers of R6 mineral wool would meet the 50/50% interior/exterior ratio to prevent condensation on the sheathing.
- add 2x4's on their side, screwed through mineral wool to rafters
- add 5/8" exterior t&g plywood as a nailing surface and extend over edge of walls to create 18" overhang
- add peel&stick ice shield to entire roof and connect to peel&stick layer on walls.
- add a ridge vent to ventilate the 1-1/2" air gap
- add fibreglas shingles over top.
How's this sound? It's like a rain-screen wall, but with an extra waterproofing layer on the outside.
Thoughts?
Real houses (particularly, but not exclusively old houses) have minor air leakage, they are not test huts.
Wall #7 was an unvented stucco test wall with only latex paint as the interior side vapor retarder in the is 4C climate, which regularly experienced outright condensation on the OSB:
http://www.energy.wsu.edu/documents/AHT_ComparingTheMoisturePerformance%20Of%20Wood%20Framed%20Wall%20Systems.pdf
The descriptions of the wall assemblies tested are on page 12, the graphic moisture profiles at different sensors for wall #7 are found on p.21. MCc3 is the moisture content sensor at the OSB, which shows moisture content north of 25% for nearly four months out of the year, and over 40% for three months (peaking at 50%), highly correlated with the period of time when the RH in the insulation near the sheathing hit 100% (sensor RHc_3), tracking as the inverse of the seasonal outdoor temperature averages.
The RH sensor on the cold side of the insulation shows both episodic and sustained wetting of the insulation in that test, with the wettest insulation occurring between the beginning of January and the end of February. (100% RH for two months straight- tell me it wasn't wet!)
Drying of the OSB to sub-15% levels did not happen until May, and that was on the sunny SOUTH side of the test assembly!
While this is exaggerated due to the high 55% RH interior-side conditions of the test. That's worst that worst-case, but super-different from the vapor pressure differences across the latex paint that would be seen at 35% RH interior in a zone 7 climate, where the average temp of the sheathing in winter is more than 25F colder than in zone 4C.
I'd be curious to see test-hut data that shows OSB sheathing is always fine on the north side of a rainscreened assembly (or unvented) in a zone 7 climate (or even zone 6) with only class-III vapor retarders on the interior, with the interior kept at 20C, 30-35% RH.
Regarding the roof assembly, I may need a picture, but maybe not. It sounds like you're venting the nailer deck with 2x4 furring, for a 1.5" ventilation gap?
You don't need the peel'n'stick to be vapor permeable unless the assembly has zero venting to the exterior If you retain 6-mil poly on the interior you absolutely need the assembly to be able to dry toward the exterior, and the 1.5" of soffit-to ridge venting under your 5/8" nailer deck would be plenty of exterior drying on an 8:12 roof.
If the peel'n'stick is impermeable, the above/below R-ratio doesn't need to be anywhere near 50% exterior/ 50% interior to avoid excess moisture adsorption/condensation at the OSB layer next to the skip sheathing. Even 20% exterior / 80% interior would meet the IRC prescriptive for zone 4C to be able to use latex paint as the interior vapor retarder. A ratio of 40% / 60%would be fine in zone 5.
Dana,
Stucco is a special case, and I've written about it extensively, including here: To Install Stucco Right, Include an Air Gap. There is lots of evidence that stucco-clad walls without a ventilated rainscreen gap are very risky.
It sounds like you are admitting that these are air leakage issues, not vapor diffusion issues -- in which case the best remedy is not an interior vapor barrier, but a better wall design (for example, a wall without stucco, or a wall with a ventilated rainscreen gap, or a wall with better interior air sealing).
Clearly better wall designs help, and part of that is better interior side air sealing, but zone 7 climates don't offer much in the way of early-spring climates. In fact, drying in zone 7 doesn't really begin in earnest until late April.
In say, Winnepeg, the average outdoor temperature is below the dew point of 20C/35% RH air from about the third week in October until half-past April. The amount of drying that takes place between 1-March & April 15 is effectively zero, since there is still significant amounts of moisture coming through the paint TOWARD rather than away from the sheathing.
https://weatherspark.com/#!dashboard;ws=28187
It's unfortunate that the WSU test did not include a vented stucco clad or a vented lap siding assembly with only latex as the interior side vapor retarder. I'm confident that the vented lap siding assembly would make it on the south side, but I'm not so sure about the north side given the high interior RH. The dew point of 55%RH/70F air is about 53F, and the outdoor temps in Puyallup WA average below 53F from early October through the first week of May, dates that correlate highly with the RH of the air at the cold side of the unvented assemblies being over 70%.
https://weatherspark.com/#!dashboard;a=USA/WA/Puyallup
But NONE of the unvented stucco clad assemblies in the WSU tests with interior side vapor retarders (either polyethylene or MemBrain) had excessive moisture in the OSB. It's not so much moisture drives from the stucco that made it or broke it on those south-facing assemblies, the interior vapor retardency on the interior side was the primary factor.
Wall #1 (p.15) was unvented stucco with a poly vapor retarder, and the OSB never exceeded 15% moisture content, despite the reservoir cladding in a rainy climate. While that was worse than the unvented lap-siding assembly's peak OSB content of ~10% (Wall #12, p.26 also with an interior poly vapor barrier) it isn't the driving factor here.
Wall 2 was the same assembly as Wall 1, except for MemBrain instead of poly, and had a similar peak moisture content to the OSB, and a slightly higher winter average.
Wall 3 was the same as Wall 1, except vented, which dropped the peak moisture content of the OSB to about 10%, and the ONLY vented stucco clad assembly tested, but with a poly vapor barrier it's not a great comparison to a vented assembly with latex paint as the vapor retarder. The RH of the cavity insulation air was higher than the others on average (over 70% for most of the year!) with comparable peaks to the unvented poly or unvented MemBrain walls. Venting is good, but it's not everything. (Which is sort of what the IRC prescriptives imply.)
Unfortunately they did not publish the graphic data for the only north-facing assembly, Wall 15, which was unvented stucco-clad OSB wall with a poly vapor barrier.
I don't want to steal John's thread here, but it seems as though it's already been stolen. Dana - are you suggesting that every double- stud walled house in Maine is going to get in trouble if they have only drywall tightly sealed on the interior. I know these homes are considered "risky", but I thought they have been tested, and, as Martin says, they dry out by April. So - In your opinion, are rain screened,taped plywood sheathed, 12" dense packed walls, with carefully sealed drywall and paint( no vapor barrier) going to fail? We value your opinion and a lot of builders in the northeast build these homes.
Kevin: Dense packed cellulose can buffer VERY significant quantities of moisture, harmlessly redistributing it within the insulation layer of a double studwall.
I'd be more concerned if it was dense packed fiberglass, which has effectively zero moisture buffering capacity. I think the jury it out on those. It might make it if it has deep roof overhangs to limit direct rain-wetting, but you're asking a lot of ~1-perm OSB to pass the same amount of moisture as the 3-5 perm interior paint into the rainscreen gap to keep from loading up. It's conceivable that the entrained cavity air in the fiberglass layer of very thick walls is sufficient buffer, but I'm not comfortable with it, especially the north walls. YMMV.
On a wall like that a layer of smart vapor retarder is cheap insurance, since it'll be under 1 perm during the winter. If it's sufficient to protect the OSB in the unvented fiberglass insulated stucco clad wall in rainy-foggy-dew Puyallup, WA it should be more than sufficient to protect the sheathing of a double-studwall fiberglass filled assembly in the north woods of New England.)
Hi John - if you are replacing all your siding and adding insulation, you might qualify for Deep Energy Retrofit funding. Maybe you knew that already. If not, it's worth looking into. I know National Grid and NYSERDA have programs in the northeast, not sure about the rest of the country.
Thanks for the heads-up. I'm in British Columbia, Canada. Haven't checked recently to see what programs are available here. Probably won't start work on it until next Spring.
To original poster John: there is a relatively new publication at the Building Science Corp website that has a bunch of explicit building suggestions for using exterior insulation on walls, including details for innies, outies, and what you might consider if you're using mineral wool. Look for BSI-085.
Here is the link to the article that Andrew just mentioned: Windows Can Be a Pain.
Another note to the original poster.
There are several YouTube videos from Hammer and Hand showing exterior mineral wool in action on a house under construction in the same climate zone (rainy Portland)
I find these really helpful to understand how all of these materials can get installed together.
Before mineral wool goes on discussing air barrier and WRB:
https://www.youtube.com/watch?v=QICamiVkKNA
After mineral wool is on discussing the rainscreen details etc:
https://www.youtube.com/watch?v=XEiFCW9Tbbw