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Alternative (Non-Foam) Outsulation Strategy Using Rockwool

There’s a lot of talk about the benefits of exterior insulation, but also the downsides of rigid foam materials—that is, the chemicals involved, future disposal issues, wrong-side vapor barriers, susceptibility to fire and pests. It is possible to use rockwool / mineral wool as an alternative, but this is not often discussed or practiced, at least in this country. That may be largely due to the lack of compressive strength of rockwool compared to foam boards, requiring some strategy to support the cladding other than simply driving screws through the insulation.

Here’s a simple solution that seems promising: run 2x2 vertical furring over the rockwool, held in place with long screws, and use squash blocks to avoid compressing the insulation. The squash block is a short segment of wood with an oversize hole at center for the screw. It could be as small as a 2x2 compressing into the end grain, but may be a larger profile (say a 2x4 with the longer axis upright) if greater strength is needed. Use SIP screws (e.g. TruFast SIPTP) long enough to penetrate wall framing by 1-1/2”. Space for squash blocks must be cut from the insulation, but it could be as little as a 1-1/2” square on 24” centers horizontally and vertically.

The vertical 2x2 becomes rainscreen furring. A vent space of 1-1/2” is more than needed but a 2x2 does a good job holding nails for siding. There is limited bearing strength but the assembly should hang off the sheathing and support the siding.

Choose a type of rockwool intended for use in cavity walls, e.g. Roxul CavityRock or Thermafiber RainBarrier. The WRB (say #15 felt) goes under the rockwool against the sheathing. Essentially the outer surface of rockwool is the drainage plane and it works like a superthick housewrap. RainBarrier has a perm rating of 50 but absorbs only 0.03% moisture by volume. Water beads up and sheets off the surface.

With an entry level 2” thick rockwool you get R-8 and need 5-1/2” screws.
3”: R-12, use 6-1/2” screws.
4”: R-16, use 7-1/2” screws. That’s an R-36 wall including R-20 from a 2x6 frame. Total assembly is about 12-3/4".

Adding inches requires minimal extra material, just thicker insulation, longer screws and taller squash blocks. Compared to rigid foam, you get enhanced soundproofing, reduced risk of fire, pest resistance, a vapor open assembly, and probably less long-term environmental impact.

It’s true that rockwool has high embodied energy compared to cellulose, and the system could have one large cellulose-insulated cavity as a truss wall if the sheathing moved to the exterior. But that would eliminate the rainscreen gap and would not provide shear strength. It also adds weight and leverage against the long screws and squash blocks. I have tried to adapt the Riversong truss to my purposes—a snug lot and three stories (topped with a living roof) in seismic zone 3. A rainscreen is also a code requirement. My project requires 2x6 framing and exterior shear ply (both sides in a few places), so a single combined cavity is out. Having exterior sheathing makes air sealing easier. It also seems meaningful that a rockwool/SIP screw/squash block strategy could work for retrofits.

Asked by TJ Elder
Posted Oct 24, 2010 2:54 AM ET


40 Answers

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Lots of superinsulation retrofits were performed during the 1980s using a system that is similar to what you describe. You are proposing something similar to a Larsen truss; in the 1980s, these Larsen trusses were usually insulated with fiberglass, but your proposal to use mineral wool batts is an improvement over fiberglass.

The method was described in an article by John Hughes, "Retrofit Superinsulation," which appeared in the April 1984 issue of Fine Homebuilding. You can see an illustration of the technique in my presentation on the history of superinsulated houses:

One point about your proposal:
You'll probably need a layer of plywood on the exterior side of your vertical 2x2s, for stability. Otherwise you'll end up with a wobbly wall (depending on your choice of siding). This layer of plywood would also prevent wind-washing from reducing the performance of your Roxul insulation. Of course, that means you'll need another layer of vertical strapping if you want a rainscreen.

Answered by Martin Holladay
Posted Oct 24, 2010 3:32 AM ET



Yes, it would be most surprising if what I described had never been done. Other than using rockwool, the other difference (from a Larsen truss) is the special screws, which I think are a relatively recent development. The truss web would only be 3" deep with R-12 rockwool and a 6-1/2" long screw should hold pretty steady. These TruFast screws have a 5/8" diameter pancake head that should allow good torque without splitting the upright 2x2. They're also skinny enough that they shouldn't run too much risk of splitting a 2x6 stud. It's possible that a composite 2x2 could work better for squash blocks, something more resilient than solid wood.

Sheathing over the upright 2x2s would be a problem. The siding I had in mind is tongue & groove 1x6 cedar, which should add some strength. It could even run diagonally if needed for stability.

With the WRB against the shear ply, windows must be innie rather than outie. That's another feature to work out, but it is apparently the preferred approach for thick walls in Europe. (Of course this is yet another subject previously covered in a blog by Martin.)

Answered by TJ Elder
Posted Oct 24, 2010 4:52 AM ET


I had a similar idea a while back to install Roxul Drainboard in the same manner as one would install EPS or XPS in a conventional outsulation approach. I had thought to apply the WRB (Tyvek or similar) over the Drainboard with the thought that the drainboard would act as a sort of secondary drainage plane. Drainboard comes in thicknesses of 1" (R4.3), 1.25" (R5) and 2.38" (R10).

Here's a transcript of a brief email exchange I had with a Roxul rep on the subject:
Me: "Hi there. I have a question about your product Roxul drainboard. I realize that it is designed for use as a below grade basement insulation, but is there any reason this product could not be used as an exterior, above grade sheathing? Have you ever heard of anyone using this product in this way?"

Roxul: "Thank you for your inquiry about Roxul Insulation. Regarding you question about Drainboard, there is no reason why Drainboard can’t be used above grade and have heard of some people using it in that manner but unfortunately we have not performed any testing for that type of application. Sheathing is regulated under local building codes so an approved sheathing would be required but Drainboard could be used as an additional layer over the sheathing.
I hope this information is helpful to you and if you require further assistance, please do not hesitate to contact us here at Roxul."

Me: "Thanks for the information. What is the perm rating for Roxul Drainboard?"

Roxul: "Good Day Lucas. We have not tested DrainBoard for a Perm Rating but we know from some of our other products it would likely be above 30 Perm. It is an open cell product and vapour permeable."

Answered by Lucas Durand
Posted Oct 24, 2010 8:20 AM ET


An additional benefit to having rockwool outsulation is fire resistance. I live in the woods so I worry about forest fires sometimes.

Answered by Lucas Durand
Posted Oct 24, 2010 8:28 AM ET


That may be largely due to the lack of compressive strength of rockwool compared to foam boards...

Wow, I didn't realize there was such a big difference.
Roxul drainboard @ 10% deformation: 2.46 PSI
DOW STYROFOAM residential sheathing @ 10% deformation: 15 PSI

I would have guessed that a product meant for below grade applications would have higher compressive stregth.

... requiring some strategy to support the cladding other than simply driving screws through the insulation.

How thick would the layer of rockwool outsulation have to be for this to be an issue do you think?

Answered by Lucas Durand
Posted Oct 24, 2010 2:02 PM ET


Lucas, I don't think it would work to install vertical strapping over any thickness of rockwool without some type of squash block. For a very short span (like 1"-2" thickness) it might be sufficient to put a sleeve of plastic pipe around the screw, which would be simple to do. Using up to a 2x4 squash block (with the screw running through the end grain) I think the rockwool could be at least 6" thick if desired. SIP screws come up to 18" long so they're not a limitation.

As for installing the WRB on top, it's a challenge to resolve the construction sequence. The easiest way to install felt is over a flat surface with staples everywhere, and it's possible to install over open framing. But as I've described the 2x2 vertical strapping runs over the rockwool with only the ends of the squash blocks coming through the wool. Could you install housewrap over rockwool with long screws and big cap washers? That may work well enough, but you also need to drive screws through the squash blocks, and those would be hard to locate under housewrap, plus they don't necessarily have any way of staying in place before the long screws are installed. Cutting openings after installing housewrap would be easier.

The cavity wall rockwool products are intended for exposure beside a rainscreen cavity, and the only downside might be as Martin suggested, losing some thermal value to air infiltration. That's probably not significant with a brick veneer but could be problematic with open joint siding. A relatively tight skin like the T&G siding would presumably help minimize the issue, but of course the wall should also be designed to allow air flow to make the gap effective at removing moisture.

Answered by TJ Elder
Posted Oct 24, 2010 4:49 PM ET


A couple of comments:

It looks like some Europeans are securing large furring strips to the exterior sheathing, then filling the new "cavity" with rockwool. Then they''re placing housewrap on top of that layer, then siding. This doesn't completely eliminate your thermal bridging, but it does get you 75% - 85% coverage of your sheathing. It also gives you a stronger wall and gives you an "outie" window. Depending on the thickness of your furring strip (compared to the rockwool), you also get to keep your rainscreen.

Or you could replace the squash blocks with long strips of polyiso foam board insulation, and again, fill the space between with rockwool. That takes care of your thermal bridging, sounds one heck of a lot easier than cutting 1 1/2" x 1 1/2" spaces out of your rockwool, and still gives you a very vapor open wall. Polyiso has a 25 psi compressive strength (at 10% deformation). You can get XPS from 15 - 100 psi compressive strength.

Answered by Daniel Ernst
Posted Oct 24, 2010 11:02 PM ET



I suppose strips of foam board would still address most of the original issues. I think it could work with the more rigid blue board, so long as the rockwool fits between framing. Earlier I was planning to run 2x4 overframing against the sheathing, then add 3" rockwool leaving a 1/2" rainscreen gap. Thermal bridging would be a problem because the outer 2x4s parallel the studs. Using 2x2 furring rather than 2x4s economizes framing material and reduces thermal bridging.

With a 2" x 1-1/2" ribbon of blue foam, a 2x2 grows into a 2x4. Still, the rainscreen type rockwool comes fully 24" wide rather than 23" for the interior kind. It could be ordered a custom size or even cut to fit, but I'd like to avoid that. Using the interior rockwool might work but it's less waterproof and may be more vulnerable to wind washing. Another variation is to add housewrap over rockwool (and thermally broken sandwich furring) then 1x2 rainscreen furring on top. More sticks, but the rockwool is enclosed and may give more effective thermal value.

Answered by TJ Elder
Posted Oct 25, 2010 12:14 AM ET


Another thought I had related to this is to use a hole saw to cut the roxul (it's amazing how compliant that stuff is!). Now that you have nice round holes, you can just cut the XPS with a slightly larger size hole saw and use XPS (you can use the high compressive strength XPS, if need be, but if the circle is large enough, it's probably not an issue) as your squash block. Then, you can use whatever size of 2X you want....
Just a thought.

Answered by Bob Manninen
Posted Oct 25, 2010 10:47 AM ET


Have you considered nailing I-joists to your exterior sheathing, and insulating in between joists like a conventional stud wall with the mineral wool? I-Joists are certainly more expensive than 2x2's, but you gain lateral stability, eliminate fussing with squash blocks, and eliminate thermal bridging from the SIP screws (the thin OSB web is thin enough to not be a thermal bridge). SIP screws are up to $0.25 each, so there's a substantial cost in that system, along with a lot of labor trying to fit the squash blocks.

Flashing around doors and windows will be easier as well, since you'll have a nearly conventional frame wall to work with.

Answered by Jesse Thompson
Posted Oct 25, 2010 11:32 AM ET



A couple of big issues with I-joists: First, they start at 9-1/2" and there's already a 2x6 loadbearing frame. Walls that thick would take out a lot of floor area because the urban site won't allow outward expansion. Second, this is for a climate with only ~4500 HDD so that much insulation isn't called for. And third, consistent with the original intent to avoid plastic foam, my preference is low-tech materials and I-joists have already been eliminated from the floor and roof framing.

Answered by TJ Elder
Posted Oct 25, 2010 12:58 PM ET


Here's another variation: rip a 2x4 to make two furring strips, rather than using 2x2s. This will add about 3/16" to the depth. Then place 1.5" rockwool batts in between the furring, leaving a rainscreen gap of just 3/16". The code requirement is a minimum 1/8" gap. Now the total insulation increases by R-6 with almost the same overall wall thickness. With 3" squash blocks and a total of 4.5" rockwool, the wall gets to R-38 at an overall assembly of about 12". Or with 1.5" squash blocks it's R-32 and 10.5" thick.

Answered by TJ Elder
Posted Oct 25, 2010 6:49 PM ET


they've got these really awesome screws in europe for a system like this...
basically you'd have a massivholz/cross laminated timber wall, with mineral wool over it. furring strips are placed over the mineral wool and screwed into place. the screws stop at the depth of the mineral wool without compressing them, and the rear ventilated rainscreen is held in place. no thermal bridging. (the screws are stainless steel)

Answered by mike eliason
Posted Oct 25, 2010 9:28 PM ET


Mike, are you saying the screws are designed to span through the mineral wool and basically cantilever from the backup wall, rather than the screw + squash block that I proposed? Do you have a link?

Answered by TJ Elder
Posted Oct 25, 2010 9:57 PM ET


I've seen these screws also. I think they might be a Swiss product but unfortunately I can't find the website. If I recall correctly the screws are coupled to triangulate forming a quasi truss web and do not squash underlying layers. I think they also have threads in different directions to allow for adjustments after they have been sunk into the structure. I have to imagine they are a little spendy.
Thanks for going into detail thinking this through. I'm looking for alternates for foam walls also but trying to achieve R-values approaching 70 for Passivhaus designs in Minnesota.
I know you are seeking alternatives to synthetic chemical projects but I've read about EIFS systems that use mineral wool boards versus rigid foam. With synthetic stucco systems you can bypass the need to fasten back through your deep insulation layer.

Answered by J Chesnut
Posted Oct 25, 2010 10:20 PM ET


thomas, that's exactly what i'm saying.. j chestnut is correct in that they aren't cheap. if i have time, i can look for links. there are also variants for concrete/masonry.

R-70? wow, we're looking at R-30 walls in the NW...

Answered by mike eliason
Posted Oct 25, 2010 10:41 PM ET


J Chestnut, if you need R-70 walls to keep warm, have you considered moving?

I think with this truss system the furthest you'd want to prop out the furring strips is about 6", for a total of 7.5 inches of rockwool. That would be R-50 at 15" thick overall. To keep adding R's you'd need to start furring out the inside. For R-70 basically you'd add 7" rockwool over a Riversong truss.

Seriously, if R-70 is what PHPP is telling you, I'd suggest changing something fundamental, maybe a rowhouse configuration to improve skin efficiency. Huddle together for warmth.

Answered by TJ Elder
Posted Oct 25, 2010 11:32 PM ET


I've never seen these type of fasteners for wood furring, but they are available domestically for brick / stone veneer systems. Here are two examples:



Mike Eliason - If you come across some links for the European SS fasteners, please share.

Thomas - If you are in a 4,500 HDD climate, do you really need exterior insulation? Where is your location / what is your climate zone? A 12" double stud wall / Riversong truss would get you to the R-40 mark without the complexity this system introduces.

Answered by Daniel Ernst
Posted Oct 26, 2010 8:34 AM ET


Every winter my wife seriously considers moving : )
I've had an opportunity to work on Passivhaus projects in my area. I still have not decided for myself if its prudent to try to meet the standard for single family homes in a cold climate. Of course multi-units make more sense and will likely require less R-value in the assembly to achieve PH but so far only single family home clients are interested in commissioning a PH project. For now I believe the PH projects in our area are valuable exercises to push energy efficiency and bring performance into the thought processes of architects and builders.
Maybe down the road jurisdictions in cold climates will have to think more seriously about their energy allotments. What size population should live in a heating climate and how much energy they should use is a fair question. We often criticize desert cities for their unreasonable water consumption, fuel for heating and cooling is often similarly imported.

Answered by J Chesnut
Posted Oct 26, 2010 10:42 AM ET



This is for the Pacific NW, climate zone 4 (marine). I've made variety of virtual wall mockups and considered double wall systems, and I believe the truss approach described here is actually less complicated and requires less material to build. Think of this as a just-as-typical R-20 2x6 frame wall with exterior shear ply (a structural requirement due to lateral loads). Then add vertical 2x2s, basically a somewhat oversize rainscreen furring. Prop the 2x2s out from the sheathing by some dimension, let's say 4", using long screws and short spacers in the gap (squash blocks). Then add rockwool (24" wide batts) and siding. With a 4" gap between sheathing and furring, the rockwool can be 5.5" thick total for R-22. That's a total of R-42 and 13" assembly thickness. This wall has similar insulation and thermal break to a Riversong truss but adds the shear ply and a rainscreen, with just an extra 1/2" overall thickness.

I've seen Passive House wall assemblies with a 2x4 frame as a curtain wall over a 2x6 frame with shear ply. Cellulose fills the outer frame including a 2" gap ahead of the shear ply. The total wall thickness adds to 1" more than the above truss design and requires an additional layer of sheathing to contain the cellulose, plus additional furring strips for the rainscreen gap. It also uses 2x4s where the truss uses half-2x4s and adds 2" to the thermal break.

Answered by TJ Elder
Posted Oct 26, 2010 12:27 PM ET


I'm sorry I'm coming back to this a little late; and please excuse me if I'm asking something silly; why is there so much reluctance to utilize any foam type of insulation? I completely agree with your premise of using Roxul/Mineral wool because, frankly, it's cheap. For R-23 you're looking at ~$.80 a sq ft... Looking at foam insulation board cladding costs makes me wince. But I also think that Martin is correct in that using "skinny" squash blocks will make the structure unstable and you'd probably have to consider applying sheathing over the structure, then another layer of furring for the siding. How are you going to do corners? Personally, I'm thinking fatter squash blocks (on the order of 4 or even 6 inches) and the use of larger furring strips; 2x6 even. Looking at the specifications for Grk fasteners, just two could hold a single furring strip and the load of fiber cement siding, but for the sake of rigidity, you'd probably need at least four squash blocks for a 10' wall....

Answered by Bob Manninen
Posted Oct 29, 2010 10:05 AM ET


Daniel et al:

I'm entering this conversation (which seems to mention my name several times) a bit late, but here are my thoughts.

With any extended exterior curtain wall, supporting it at the foundation (and, if possible, at the rafters) eliminates any concern about load-bearing or lateral shift and the windows can be outies if that's preferred.

I don't like the idea of large steel connectors as thermal bridges at regular intervals, nor do I like them as vertical shear elements when cantilevered out any distance from the framing. I'm not sure that a rainscreen gap offers any advantage when the material (rock wool) immediately behind the cladding is air and vapor open (basically a thicker version of Cedar Breather), but that might require a discussion with the code official.

While you're not looking for much additional outside wall thickness, the original Larsen Truss was developed specifically for this application, whether new or retrofit. Since it uses small dimension wood with thin plywood spaced web members, it's basically the same as a narrow I-Joist with an intermittent rather than continuous web. The plywood webs are glued into slots in the 2x2s and there is no metal for thermal bridging and lots of vertical shear resistance for load-bearing.

It seems you're trying to reinvent a wheel that's already been proven to be both cost-effective and thermally and structurally effective. But, even with the Larsen Truss, I would recommend bottom support on the foundation, both for structural and aesthetic reasons.

Answered by Riversong
Posted Oct 29, 2010 10:50 AM ET


Robert, I had thought about using rockwool without a clear gap but it could be a battle with the code inspector. One advantage would be holding down the insulation continuously. But there is a point to having a clear gap after all, and the walls are high relative to the overhangs so they are exposed. This is also a rainy climate.

In my design the edge of the foundation is the outer face of 2x6 studs. I don't think it would work structurally to stack three stories of 2x2 or 2x3 curtain wall even if the slab did come out beneath it. Another code requirement is R-15 slab edge insulation, which could be 3" XPS. It could also be 4" of the sub-grade type mineral wool. The thickness of the truss assembly helps here, leaving a slight shadow.

These steel screws have a diameter of 0.19" and they lay out in a 24" grid, mostly embedded in framing. Thermal bridging does not seem problematic. I'll report back about how robust it seems when there's a mockup. The depth I'm going for is 4.5" from sheathing to cladding, really not much of a cantilever. This would be a 2" gap (the squash block thickness) below 2x3 furring. Insulation is two layers of 2" so the inner layer can tuck more easily under the 2x3 rails, then the outer layer squeezes between them. The vent gap is then 1/2". I think the squash blocks could even be replaced with segments of skinny pipe, and the screws would still easily support the 2x3s and cladding. They essentially have moment joints at both sides of a 2" gap.

Answered by TJ Elder
Posted Oct 29, 2010 12:33 PM ET


There is a lot of good discussions on ways to insulate. This is fine for owners where cost is not an issue as they mostly require unique and more expensive construction. I have battled for years to find the best way to go beyond normal construction. I have come to the conclusion that a double stud wall with dense pack cellulose is the best way to achieve elevated performance. The advantages are:
- Any builder can easily build a 2x4 exterior wall with another wall interior of that (2x3's work)
- It does not need any petro chemicals or non-traditional materials or construction methods
- Cellulose is very good at absorbing and dissipating moisture should moisture enter the cavity.
- Double stud wall and cellulose is 'Greener'
- The construction cost is almost always less than other high performance walls.
- There is no thermal bridging or shear concerns thru long fasteners to accommodate the rockwool.
- This system has proven very air-tight (0.5 ACH @ 50 Pa with no special air-sealing procedures)

Answered by Allan Bullis, CEM LEED AP
Posted Oct 29, 2010 2:02 PM ET


And Allan
Your Method probably has an Outie Window
Far less complex than any of the Innie Window details that I have seen.

Answered by John Brooks
Posted Oct 29, 2010 5:04 PM ET



I agree with your overall assessment of the double-wall/cellulose system with one exception. If you're building a conventional platform frame and then adding an interior frame wall, you still have the conventional problem of the floor and ceiling assemblies as both thermal bridges and possible interruptions in the air barrier.

I discarded that system 20 years ago in favor of the much easier and more resource-efficient modified Larsen Truss (Riversong Truss). And, when I design homes for conventional builders to erect, I use a KD 2x4 double wall system with the interior wall bearing the floor assemblies and the exterior, sheathed wall bearing the roof. This "load-sharing" approach eliminates the thermal bridge problem and allows the use of the drywall as interior air barrier (which I prefer in a cold climate).

This latter system satisfies code issues, is relatively simple to build, and has the same advantage of my Riversong Truss wall with an uninterrupted thermal layer wrapping the conditioned space.

Answered by Riversong
Posted Oct 29, 2010 7:21 PM ET


I should add that this 12" thick wall system, which requires bearing for both interior and exterior frame walls, is a perfect complement to a shallow, frost-protected grade beam or a ThermoMass poured concrete foundation with midline XPS (CIC, which puts the insulation where it belongs - completely enclosed and protected from UV, insects, fire and physical damage while providing almost the same dynamic thermal mass benefit of exterior-insulated concrete).

Answered by Riversong
Posted Oct 29, 2010 7:25 PM ET


If I were to start with the Riversong truss and adapt that for my situation, it would still need a 2x6 bearing frame, plywood sheathing for shear and a rainscreen. Shear ply would need to go at the interior behind the GB. The rainscreen means adding exterior sheathing (to contain the insulation) and then furring strips (to create a vent gap). I think this would be more expensive and more difficult to build than cladding rail idea proposed here. It would take more layers and more overall thickness. There are also some walls in my project that need plywood on both sides of the bearing frame due to seismic load, which would interfere with the double frame / single cavity design.

With this sequence (from the exterior): cladding>>gap>>rockwool>>sheathing>>framing>>gyp board, there is no separate rainscreen furring strip and no second sheathing exterior of the plywood.

Answered by Thomas Jefferson
Posted Oct 29, 2010 9:45 PM ET


I wasn't suggesting the Riversong Truss for your situation. Your codes are far too restrictive. I was suggesting the original Larsen Truss, which I believe would be easier, more structurally sound and more energy-efficient that what you're proposing. It can be added to the exterior of any code-required wall assembly and could support the weight of "outie" windows.

Answered by Riversong
Posted Oct 29, 2010 11:39 PM ET


Here's another idea that's a little more elaborate, but it addresses thermal bridging through SIP screws. Rather than a single long screw, connect two screw segments through a fiberglass rod. The rod is 1/2" diameter solid fiberglass, about 10 cents per linear inch from McMaster-Carr. Cut this to the desired length, say 3", drill pilot holes into each end and tap those holes for #10-24 machine thread. At one end of the rod, insert a hanger bolt, which is a short steel rod with machine threads one end and wood screw threads the other (from McFeely's). It's 3" long and will penetrate the fiberglass rod by 1". This gets driven through the plywood into framing. Then attach the cladding rails to the rods with screws that penetrate by 1", and there's still a 1" thermal break between steel segments in the middle of the fiberglass rod.

Now the 1/2" rod replaces the previous squash block, and is easier to fit insulation around. The parts cost for this assembly is similar to those SIP screws but there's labor drilling and tapping all the holes. That could be a job for a machinist with a CNC lathe, and it may be pretty easy then. These rods come larger, up to 1-1/2" diameter but the price climbs considerably. I think the 1/2" rod would be plenty sturdy for this small dimension. A slight variation is to drill a pilot but not tap the hole for the screw that holds the cladding rail, and just use a wood screw there, assuming it doesn't threaten to split the fiberglass.

Answered by Thomas Jefferson
Posted Oct 30, 2010 1:54 PM ET



You seem so determined to make your system workable that you will make it even more complicated before considering that it may not make sense at all or at least compared to tried and true systems like an exterior parallel-chord truss.

Fiberglass has incredible tensile strength but very little shear strength. Not only would it probably shatter if a tapered screw was driven in to it, but I doubt it could resist any amount of displacement.

I recently helped form a ThermoMass foundation wall that uses fiberglass tie rods that have the same tensile strength as ½" rebar (also to prevent thermal bridging). But I could twist them off by hand, shearing right through the fibers.

Answered by Riversong
Posted Oct 30, 2010 5:44 PM ET


Robert, at least one of us is being stubborn. You would make a parallel chord truss (Larsen type) to cross 4-1/2"? Bear in mind that a greater wall thickness is problematic and more than R-36 (20+16) is not really called for. The gap would be only 1-1/2" between 2x2 flanges. It would be a lot of pieces and lots of nails. I think the strategy from the original post is the least complicated.

Answered by Thomas Jefferson
Posted Oct 30, 2010 6:40 PM ET


Yes, I do think the trusses are simpler, quicker, more structurally sound and allow the windows and doors to be flush with the exterior plane.

I've site-fabricated Larsen Trusses in a jig, and it's a mass-production process which is quite efficient (you could even order them from a truss shop, but they would probably be assembled with metal gang nail plates). Then the assembled trusses are installed quickly and easily with a nailgun. Rather than inserting the plywood webs into a dadoed slot in the 2x2 chords, they can be side-lapped, glued and pin nailed.

Answered by Riversong
Posted Oct 30, 2010 9:23 PM ET


I have a fieldstone wall basement which protrudes above ground by 1 to 3 feet (less in the front of the house). From what I understand, rockwool "drainboard" might be useful in adding an insulation layer to the outside of the foundation. I have 3 to 4 inches to work with, where the vinyl siding overlaps the foundation, and the insulation would need to continue below grade.

Can anyone point me to some discussion of how to do this? It is not clear to me whether I need an air gap behind the rockwool, or whether I would need to smooth out the fieldstone wall to minimize any air gap.

Answered by rich cowen
Posted Dec 11, 2010 11:09 AM ET


Rich, I don't have a sample of that exact product, but in general rockwool is easily compressible and would tend to form itself against an irregular surface like a fieldstone wall. Any gap behind the insulation would reduce thermal value, due to exterior air getting in there. If you did smooth out the surface with plaster that would probably be helpful. You would also need a cladding over the rockwool that could go below grade.

Answered by TJ Elder
Posted Dec 11, 2010 4:33 PM ET


If you are adding insulation to the exterior of a fieldstone foundation wall, I really think the best product is closed-cell spray polyurethane foam, extended as far below grade as you are willing to dig, and protected above grade with stucco.

Answered by Martin Holladay
Posted Dec 11, 2010 4:34 PM ET


I will admit that the same idea came to mind--closed cell PU foam would coat any rough surface and waterproof it as well. Still I don't like the idea, partly because it becomes a mess for future generations to deal with somehow (rocks coated with plastic). In the short term the PU has high global warming potential that largely negates any benefit to energy savings, and it's also quite expensive.

Answered by TJ Elder
Posted Dec 11, 2010 7:38 PM ET


Thanks for starting a very informative thread! Not sure where you are right now in the conceptualization stage of this approach. I would be grateful for a final summary of the solution you settled on.

Also, what about the idea of running 2x4's horizontally 25.5" o.c. which would provide 3.5" cavity for 24"x... rockwool boards installed horizontally. This lessens thermal bridging as compared to the scenario of thick vertical furring (unless adjacent to top/bottom plate). The 2x4's would have to be secured with SIP screws increasing overall strength of the assembly with any optional housewrap going on top. Lastly, rainscreen furring of any desired thickness and spacing could be installed vertically and nailed to the horizontal 2x4s. This has many advantages for fibercement panel cladding as furring supporting vertical seams can be relocated as needed w/o having to match up with studs. For lesser insulation thickness requirements, 2x4's could be ripped into 2.5" and 1" strips to be used for horizontal and furring runs respectively....

Answered by Val
Posted Jan 25, 2011 7:33 PM ET


New alternative would be to use Roxul Comfort Board IS http://www.roxul.com/residential/products/roxul+comfortboard+is

Answered by John Sisco
Posted Dec 9, 2011 8:37 PM ET


Thanks for the link...interesting that one of their available downloads is one of Martin's blogs.

Answered by Garth Sproule 7B
Posted Dec 10, 2011 12:22 PM ET

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