Rock wool in old farm house
DaveG1234
| Posted in General Questions on
We have an old farm house. The walls from inside to out are: Sheetrock, fiberglass batt, wood lap siding (no sheathing), 1″ extruded foam, vinyl siding. Is it safe to just remove the sheetrock and fiberglass batts and replace with rockwool and then re-sheetrock? Do I need some sort of vapor barrier? Will the insulation work without sheathing? I doubt the old siding is airtight.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Whether or not this is safe depends on your climate zone. If you have enough exterior rigid foam for your climate zone to eliminate moisture issues in the wall, then you can just replace the existing fiberglass with mineral wool. If you don't have sufficient exterior rigid foam for your climate zone, then you need a vapor retarder when you replace the fiberglass with mineral wool.
Since you suspect the exterior is leaky, I'd do two things while you're doing this project:
1- Air seal the exterior as best you can while you have the wall open. I'd use a combination of canned foam (using a gun is much easier for this than the one-time use cans with straws, BTW) and urethane caulk.
2- Install a smart vapor retarder on the interior and detail it as an air barrier too.
Bill
Thanks - we're in upstate NY, just north of the capital region. So you suggest we seal each board of the old lap-siding from the inside? Should we spray a thin layer of foam over the wood? Then the wood would be sandwiched between foam board and spray foam - wouldn't this result in possible water issues?
We only have one inch of rigid foam on the exterior.
Hi Dave,
This article (Insulating Walls with no Sheathing) presents an approach to insulating walls with no sheathing that makes for a durable assembly and can create an air barrier on the exterior side of the insulation, but limits the amount of insulation you can fit in the stud bays. Because of that, it may be best paired with this approach: Breaking the Thermal Bridge. If you were to use rigid foam in the stud bays, then leave the interior vapor open. But an interior air barrier is important.
You won't get much R value or air sealing benefit by replacing the fiberglass batts with rockwool. Unless you need to remove the drywall for other reasons, it is probably not worth it.
What might be simpler and cheaper if you can find somebody to dense pack over the fiberglass. You can do this without removing the batts or drywall. Dense packing also does a good job of retarding air flow. This would work well provided there is a WRB in your wall somewhere, either above or bellow the foam.
1" un-faced foam (either EPS or XPS) is still somewhat vapor permeable, you should not need more than a coat of latex paint over the drywall for vapor control.
Thanks for the help everyone! We plan on eventually renewing the vinyl siding - if we use house wrap at that time we would then have from inside to out: Sheetrock, rock wool, wood lap siding (no sheathing), 1″ extruded foam, house wrap, new vinyl siding. Would this work to air seal the house and not lead to any water issues? Does that sound like a good plan?
We are doing this project because there are very likely large voids without any fiberglass and we also have lots of things living in the walls - so also plan to try to barricade them out where we can. Also, we opened up a wall to install a slider door and those fiberglass batts were fully tunneled by mice and smelled like mouse urine. Would make us happy to get that out of the house.
>"Sheetrock, rock wool, wood lap siding (no sheathing), 1″ extruded foam, house wrap, new vinyl siding. Would this work to air seal the house and not lead to any water issues? Does that sound like a good plan?"
If the "...1" extruded foam..." is XPS it's not a good plan. XPS is the LEAST green insulation material in common use today due the extreme global warming potential of it's HFC blowing agents, and is only warranteed to R4.5 @ 2o years (90% of labeled R), as those HFCs escape the foam to do it's damage:
https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png
Worse still, the XPS runs about 1-1.5 perms @ 1" close to being a Class-II vapor retarder, inhibiting drying toward the exterior.
Unfaced 1" Type-II (1.5lbs per cubic foot nominal density) EPS is the same polymer, same density but has an order of magnitude less CO2e footprint. The vapor permeance is about 2.5 - 3 perms, still a Class-III vapor retarder, but not a whole lot tighter than standard interior latex paint. It performs at R4.2 now, and will perform at R4.2 in 50 years, with no intermediate term performance boost from it's isoopentane blowing agent, most of which leaves the foam while at the factory, where it is recaptured, not vented to the atmosphere.
Neither R5, R4.5, or R4.2 is sufficient for dew point control at the sheathing layer for full-dimension 2x4 framing in your zone 5/6 climate, but with a Class-II or tighter interior side vapor retarder ("vapor barrier" latex primer, or a "smart" vapor retarder such as Intello Plus or Certainteed MemBrain, or even 4 mil polyethylene) the sheathing can dry to the exterior air behind the vinyl siding just fine through an inch of unfaced EPS,.
The housewrap is another issue. To make plank sheathed houses air tight to the exterior it's "worth it" to use a FULLY ADHERED vapor permeable housewrap (eg Vycor Env-S, Henry Blueskin VP 100, etc) directly on the plank sheathing, between the exterior foam & sheathing.
Dense-packing cellulose over the existing batts working from the exterior (preferably before installing the fully adhered housewrap) before putting up the foam also tightens up the airleakage quite a bit, and offers a bit of hygric buffering to better protect the sheathing.
Dealing with the window flashing details is the most critical aspect though- even high perm assemblies can fail if there is chronic bulk-moisture re-wetting of insulated walls.
Hi Dana,
The foam board is already in place - it's old Styrofoam TG, which I assume is XPS. I also assume it's more green to leave the XPS as opposed to removing it and replacing it with EPS. We are currently planning on opening up the sheetrock to remove the old broken-down fiberglass batts, replacing with rock wool and new sheetrock. Sometime in the future we plan to renew the siding and at that time I assume tape the old foam that's in place already. I am not a pro and don't want to mess anything up. Is there anything we should do on the inside now? Vapor barrier?
Yes, also in the plans to upgrade any electric that needs it.
For rodents, etc., plan to put boric acid powder on everything and hardware cloth + spray foam any openings.
Thanks everyone!
Your best bet to air seal from the outside is to tape the seams of the foam with a quality tape. Detailing housewrap as an air barrier is possible, fussy and easy to damage.
If you have mice in your walls, you need to do critter proofing before any remediation. Mineral wool will not keep critters out, they love to make nests in it. Boric acid treated cellulose does deter certain pests, might be the better choice if you can't find where they are getting into your walls.
DIY non-pro perspective:
Are you planning to redo your electrical? You don't say what the age is but I'd guess you don't have a modern standard and if the mice have been running rampant inside the wall, the electrical might be damaged. Is redoing the electrical a benefit or an unaffordable expense?
Have you considered simply over framing the wall with a new 2x4 wall with new electrical? Abandon the contents of structural stud wall to provide whatever benefit they do or don't provide? Avoids demolition of mouse-mold-fb wall, allows modernization of electrical (if you have access to refeeding source into the new 2x4 wall). Costs you ~4.5" of interior space. Just something to consider. If you go this route, there are quite a few technical details to consider (electrical, whether to put a low perm paint on the existing drywall, whether to space the new wall off the old wall and if you do how/if you'll fill that spacing with insulation).
I suspect that while you won't gain much paper R-value by reinsulating and air sealing the wall with mineral wool or batt or blown sheep wool (an option you didn't mention), you might gain a lot of quality of installation thermal improvement and noise control that lightweight fiberglass doesn't provide.
I do want to disagree with one pro opinion. If mice are getting in the wall, I can't see how dense pack cellulose filling it would solve anything. You'll still have bulk moisture and air intrusion and the rodents will likely ruin the insulation.
Just a DYI perspective.
Hi All -
So I opened up one wall of our living room in our upstate NY 1850 farmhouse. Outside is an uninsulated attached shed with an earth floor. The wall, from outside to in, is constructed of 1" plywood, thin (about 1 mm) foil faced styrofoam (perforated on one side, solid foil on other), tar-paper (about 1 mm thick), 1" board, paper, 1" board with gaps between each board. The stud bays are 8" deep.
I'm thinking of putting R30 rock wool batts in, but I'm concerned about condensation. Would a vapor barrier on the inside of the rockwool prevent condensation?
The adjacent exterior wall does not have an attached shed outside of it. This wall from outside to in contains vinyl siding, 1" extruded foam panel, wood lap siding, then the 6" deep stud bay. Here I was planing on adding R23 rockwool. Do I also need a vapor barrier here?
Any suggestions?
Also, these are the only walls we're opening up at this time.
Thanks for any help!
Dave
Are those bays actually 6" and 8"? If so, then 2x6 and 2x8 Roxul won't fill them as those products are ~5.25"/7.25" thick. Depending on your desire for maximum effectiveness, leaving 0.75" unfilled may cause convective looping, reducing the effectiveness of your install.
Yes and no. Some are 5.25"/7.25" thick. Nothing in this house can be measured to the 0.5" - for example the bays are between 13 and 18" on center. Would it be better to have the air gap on exterior or interior of the wall?
Would I benefit by sealing in the exterior part off the stud bay with rigid foam? or would that make matters worse because there seems to be a vapor barrier on the outside of the wall?
You have a kind of crazy wall. Have you considered having it WUFI modeled? I know the 475 used to offer this service but I don't know if they still do.... googling... here you go. https://foursevenfive.com/475-does-wufi/
I wonder if you have considered a more hygroscopically buffering insulation such as wool or cellulose? Something to discuss if the WUFI modeling makes roxul look like a poor choice. I've personally been considering havelock wool for a reinsulation project but am just not (yet) convinced my project is right for it. Loose fill wool looks to be about the same R per inch as Roxul but appears to be even more expensive than Roxul. Of course, your project sounds like a labor of love not an avalanche of square footage.
Another option would be to limit/prevent penetration into the insulation from the interior via air tighting your electrical boxes and using a proper smart membrane such as Intello Plus and the appropriate tapes. Again, WUFI would model you whether those elements limited your peak moisture content. I'm sure the folks at 475 would be more than happy to sell you their very awesome (and very expensive) Proclima products. (Thumbs up for Intello Plus and Vana).
Reminder, I'm just a DIY guy so definitely listen to what the pros say. But I have done some uhhhh how do I insulate this projects with custom cut Roxul and Intello and loved the results, if not the effort.
Going back to your Roxul question, if you didn't have vapor retarders/barriers outside your inner condensation plane, you could think about cut and cobbling styrofoam to fill the 6" (my local box stores sell 0.75" EPS). Similarly, you could think about having a foam flash fill to the depth you need and then installing roxul (I have a hard time believing you'd get a super flat flash fill). But it's doesn't seem like a good idea either with that exterior vapor barrier.
If you are sticking with Roxul, you can change the direction of its installation and custom cut each piece to fit the depth of the stud bay. It's not as awful as it sounds for a wall (it's awful in a ceiling) but requires super high attention to detail to insure every piece of very tight to the next piece without being squished either. I'd only trust myself to do that type of work. You can also rip down the thickness of batts to fit custom bays. Again, kind of awful to do for a whole house but doable for one wall. Also, the waste factor, unless you plan out something perfect, goes through the roof. I've used all of these techniques to deal with oddities like headers or blocks but I can't recommend them for 100s of sf.
Another choice, (slightly more dodgy) would be to use 2x6 R23 Comfortbatt and then a piece of Roxul Safe N Sound, which is ~2.75" thick. Technically, the safe n sound is not rated for an R value or exterior insulation but various people and places suggest it is worth R-10 or R-11. That would give you a R-33/34 assembly in your 8" true bays. Of course, if you need a documented R value above R-23, that won't work. Similarly, for 6" true you could use a 2x4 batt (3.25") and a safe n sound batt (2.75") to get to something like R-25. I don't know what the pros would say about this option.
So Roxul and a vapor membrane (CertainTeed MemBrain) then taped sheetrock would be ok? Would this prevent condensation?
Dave,
No idea. That's a question for the computer model. Understanding peak moisture content isn't something our brains can do very intuitively, especially given your wall being unusual.
I think the WUFI modeling fee from 475 is a bargain to prevent mold and water damage. I'd pay them to figure that out. I am a past and future 475 customer but have no other affiliation with them.
I think you need to consider paying 475 or another professional modeler to make a WUFI model. Then you'll know what science says will work for your wall and climate.
Thanks - I'm thinking that with a vapor retarder inside the calculation you mention won't be necessary. I've done my own calculations that show the end of the bay will be below freezing in he winter- but I'm thinking the smart vapor retarder (CertainTeed MemBrain) will stop the moisture from getting there so it won't matter. That's what I'd like a pro to weigh in on.
Also - would we benefit from fitting extruded foam to the back of the bays and air sealing with canned foam? This might cause problems considering what's outside the wall... Or just do the Roxul and vapor retarder?
Thanks!