Vapor-open rigid foam insulation on the exterior?
I’m curious is anyone has used Roxul (or similar, mineral wool) rigid insulation on the exterior of a structure. In a previous thread we discussed that this would provide a vapor-open insulation on the exterior, thus allowing drying to the exterior and eliminate condensing on the sheathing. Ideally I think this would be placed over plywood sheathing (for it’s hygro-thermal properties). I am wondering how/if it works as a WRB in a rain screen detail, or if it would need to be covered in bldg paper or a permeable house wrap (permeable). Can the seams be taped/sealed like foam boards? Will the same rain screen- furring etc. details work with this material? Is it too vapor-open to be placed directly on studs over let-in bracing? Do you see any issues with this concept?
My climate of interest is Mixed-Humid.
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rock wool can and is frequently used on rainscreen assemblies (commericial in US, and is done for residential and commercial pretty regularly in EU)
seams typically aren't taped... it's sandwiched between rails. the WRB, if i recall correctly, is placed inboard of the rock wool.
i don't think it can be placed directly over studs... if you are looking for vapor diffuse to exterior, wouldn't you use OSB instead of plywood?
Roxul is not a foam, it is a basalt/slag mineral fiber board (in its semi-rigid form) of 3.5 pcf density and R-4.1/inch (Comfort Board) and 8 pcf/R-4.3 (Drainboard). ThermaFiber also markets a mineral fiber drainboard as a "rain barrier", though probably requiring the foil facing option.
I would think any vapor-open mineral wool board would require a WRB overlay if used as exterior insulation under the cladding.
It's probably a good alternative to petrochemical foam boards, but mineral wool requires an energy-intensive manufacturing process and likely has a high embodied energy and likely a high CO2 contribution (though it does contain some recycled slag).
OSB is less vapor permeable than CDX (far more resin and less contiguity of wood fiber) and does not increase its permeance nearly as much as CDX when the RH rises. OSB also is much more vulnerable to moisture than CDX.
Hunter,
You don't need Roxul to "eliminate condensation on the sheathing." Any foam will work -- XPS,EPS, or polyiso -- as long as it is thick enough.
"it's sandwiched between rails"
Wouldn't that defeat the purpose of exterior sheet/board insulation of minimizing thermal bridging by being continuos, or am I misunderstanding what you mean by that? Placing the WRB behind the roxul makes sense.
Thanks Martin. I'm interested in a detail allowing walls to dry to both sides and not creating a vapor barrier on the outside. I've read that EPS is somewhat permeable, but I'm just not a fan of the "styrofoam cup" board, much like I refuse to accept containers from restaurants of a similar material. I just think they should stop producing that stuff. Though I guess one could argue that at least in a building it's not intended to be a one use then trash deal.
Hunter,
If you don't want to create a vapor barrier to the outside, and you want to eliminate thermal bridging, why don't you use a double wall, or Larsen Truss?
Eliminate condensation from occurring on exterior sheathings by creating an airtight wall assembly. This can be accomplished by air sealing either at the exterior, or interior, or both.
You can't eliminate condensation, since it occurs all the time. You can only reduce its rate within a certain range conditions.
Condensation is not the problem. Moisture accumulation beyond a material's or an assembly's safe storage capacity and ability to dry is the problem.
So there are two approaches:
1) attempt to reduce the condensation rate as low as possible even if it requires an assembly that has very limited storage and drying potential
2) accept that condensation is a natural occurrence and allow limited condensation to be safely stored and easily dried (this will also have the advantage of buffering indoor RH, like thermal mass reduces indoor temperature fluctuations)
Ironically, Dr. Joe refers to the "good ole days" in which vapor open assemblies built of durable natural materials had sufficient heat flux to allow drying in all directions. He explains how increasingly tight structures with increasingly more vulnerable manufactured materials and highly reduced heat flux makes drying problematic and moisture problems a near certainty. And then he recommends "solutions" which use the very same problematic materials and further reduce heat flux by adding exterior insulation and convective capillary breaks (rain screens) which limits solar radiant inward drying as well as outward drying, all in the hope that the system will sufficiently limit condensation and accumulation (assuming that there's never a leak of bulk moisture) such that the probability of moisture problems will be reduced to a safe level.
Sounds to me like the same form of hubris or madness which has created every technogenic problem the world is now facing.
"You can't solve a problem with the same mindset that created it." - Albert Einstein
hunter,
placing rails/battens on the outside of an insulated wall may be a small thermal brige... i guess if you had a 2x6 or 2x8 wall and ran the 2x rails inline but outboard of the plywood or OSB. but you would still have an R-10 at this point...
robert,
the cradle to grave embodied energy of mineral wool is significantly lower than that of petroleum-based boards. more info: http://www.buildinggreen.com/auth/article.cfm/2009/9/25/Mineral-Wool-Residential-and-Commercial-Insulation/
Mike, so the rails/battens you are describing are placed on top of the mineral wool, or directly on the plywood and mineral wool fit between them? I haven't worked with the material before and not sure if it's rigid enough to place them on top and nail through to the sheathing/studs and be able to hold the siding, and not compress the wool. Maybe I need to call the mfgr for some samples and do some test assemblies.
Robert, I agree with you. My thinking is that the mineral wool will raise the temp of the plywood some and reduce the condensation, at least to a point where it should have no problem storing it until it can dry. The mineral wool would hopefully meet the thermal requirements without stopping all of the vapor flow. Maybe making the best of both worlds (that which you've described and what Joe and Martin suggest). It looks like the detail would have nontaped/sealed seams of the insul board and the WRB directly on the plywood (building felt).
Mike,
The only statement about the EE of mineral wool in the article you linked to is this: "hough the life-cycle impacts of mineral wool production—primarily energy consumption—are significant, some of these are mitigated through the use of pre-consumer recycled slag from iron manufacturing."
From the numbers I've found, dense mineral fiber drainboard has about the same EE per CF as EPS foam board (about 50,000 Btu/CF or about 1000 Btu/R-SF). However, I'd feel better about putting it in a land fill at the end of its lifecycle.
And, according the the chart in Alex Wilson's article on the global warming potential (GWP) of various insulation materials, rigid mineral wool board has the same SF-R lifetime GWP as water-blown SPF (more than any other insulation except HFC-blown SPF), and the highest embodied GWP of all common insulations.
http://www.buildinggreen.com/live/images//embodied_GWP_with_title.gif
That's not to discount its overall comparative value (it has many important attributes, such as fire/insect/rodent-resistance, sound attenuation, water draining ability and vapor permeance), but it's also important to fully understand the spectrum of impacts that each of our consumer choices entail.
hunter, this is the assembly similar to what we've spec'd on a few projects:http://guide.rockwool.co.uk/products/building-insulation/rainscreen-duo-slab.aspx
the rails are fastened to plywood and the battens are compressed between them and fastened.
according to uni bath:
dense pack cellulose EE=.94-3.3 MJ/kg
fiberglass EE=28 MJ/kg kgCO2/kg=1.35
mineral wool EE=16.6 MJ/kg kgCO2/kg=1.20
rock wool EE=16.8 MJ/kg kgCO2/kg=1.05
EPS EE=109.2 MJ/kg kgCO2/kg=3.4
XPS EE=88.6 MJ/kg kgCO2/kg=2.5
Thanks for the link Mike. I wonder how the thermal bridging of the thin metal rails compare to wood batten strips. In either case I would line them up with wall studs for structural integrity.
O.K., so we're designing a new home and I've been reading a lot; books, magazines and forums such as this hoping to find the answer to the question... what is the most cost effective system to build an energy efficient home. SIPs, double stud walls, 2x6 wall with rigid foam insulation on the exterior, spray foam, cellulose and so on. So many options, so little time.
I'd also like to avoid as much as I can petrochemical based products and use as many natural and/or recycled products as possible. I understand the argument that using foam insulation in the end saves more oil than is used to produce it but if I can get equal or better results using natural/recycled, all the better.
The concept being discussed here, a vapor open wall system seems a sensible approach allowing the wall to dry to the interior and exterior.
I know that it's difficult to say what's best, but any thoughts on what works and what doesn't would be greatly appreciated.
We're in the cold climate of New England (RI).
John,
You should have started another thread, as this is a different discussion and likely to elicit many different answers.
I have, however, been building in the way you propose for more than 30 years and have developed what I consider to be the optimum approach using conventional materials (though straw bale with earthen plasters beats them all).
I've modified the Larsen Truss system to optimize it for new construction, and use locally-sourced rough-sawn (often green) lumber for all framing and sometimes for siding and exterior trim as well. Other than subslab or foundation insulation (for which there are no good alternatives to rigid foam board), but which I minimize by using shallow frost-protected grade beams or rubble-trench foundations, I use only cellulose with borates to create a thermally-broken R-45 wall and R-65 ceiling with fully vented roof. I do use some butyl caulk (Tremco) for the air-tight drywall approach and some gun foam around doors and windows and mechanical penetrations, but my use of artificial materials is minimal compared to most approaches.
I'm a firm believer in vapor-open walls, particularly exteriors in a northern clime, and in low-tech ventilation systems (exhaust-only with passive make-up inlets). All my homes are designed for passive solar and typically have woodstoves in addition to possible central radiant heat.
My last such house is described and pictured at http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/LarsenTruss.htm
I also have designed several modifications of the double-wall system for others to build with KD lumber and plywood sheathing. The only plywood I typically use is for the deep door and window boxes. All other sheathing or decking is rough-sawn boards (makes the most breatheable exterior skin) or simply thick siding over housewrap without redundant sheathing. With board sheathing, I prefer felt to plastic housewrap and I use felt roofing underlayment rather than the new plastic varieties.
Thanks Robert. I started a new thread in this category as you suggested.
https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/18127/most-cost-effective-system-energy-efficient-h
I also looked at the house at the Builditsolar site and it looks great but for me raises some questions.
The design is a cape style using the "attic" space for bedrooms and workspace so there will be a cathedral ceiling. What's the best option to insulate the roof?
Because of the existing topography and a high water table we're going to have a basement under part of the house and a slab floor for the rest. I'm looking into using Thermomass insulation system for the foundation walls. Any suggestions here?
Does a 2x4 24" OC load bearing wall meet code or do building officials consider the truss as a whole?
We're aiming for a nearly maintainance free exterior without using vinyl and are planning on using Hardie Plank clapboard style siding. Would we better off sheathing with plywood/boards with felt paper and a rainscreen like Homesliker? Any thoughts on the Hardie products?
Why do you use the dry cellulose and not the damp spray?
I've gotten some prices on radiant systems and it seems they are quite expensive considering the minimal heating requirements of a house like this. A warm floor is certainly quite appealing but is there a less expensive way to get there? Perhaps an electric floor "warming" system?
Thanks very much for sharing your expertise and enthusiasm.