Outsulation question
Rik27
| Posted in Green Building Techniques on
Hello!
im building a house in Wisconsin zone 6.
I’m chemically sensitive and builder is asking me for materials. This is what I’d like to do for wall insulation but want to make sure this is the right order of things and will not lead to moisture issues or if I’m missing a barrier. Siding-vertical strapping-weather barrier-rockwool boards-plywood structural sheathing – 2×6 wall with sheep’s wool Insulation- drywall
thank you!
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How thick are you planning on making the exterior continuous insulation layer? You need to have the correct ratio of external to internal R values for your climate zone for this assembly to be safe. You can go thicker than recommended for the exterior insulation, but not thinner.
I would recommend you use polyiso on the exterior and not "rockwool boards" (probably "comfortboard"). Polyiso will be cheaper, and get you more R value per inch. It's a stable material, and on the exterior, so you won't have any issues with smell.
I'm not personally a fan of sheep's wool insulation. I prefer mineral wool, but Owens Corning makes a high density R15 fiberglass batt they call "Pure Safety" that is intended for people with allergies and things like that. That might be a good fit for you. Mineral wool is another option here. I don't like natural fiber insulating materials out of concern for things growing in them or eating them over time.
BTW, you can use a smart vapor retarder on the interior under the drywall for some extra insurance against moisture problems. CertainTeed's MemBrain is a nylon material, very stable and no smell, as one example.
Bill
Bill,
Unlike those using foam, If the exterior insulation is permeable like rockwool, the thickness doesn't matter. The sheathing may be cold enough to accumulate moisture, but a) would still be warmer than the standard wall we typically build without exterior insulation, and b) have a good drying path to the outside.
Beyond price I don't see any problems with what Rik27 is proposing.
I’m not that familiar with the IRC. I think it requires a class 2 vapour-retarder in climate zone 6.
Yes, you're correct about that the exterior insulation thickness/R value doesn't matter in terms of ratio with the interior stuff when the exterior stuff is vapor open in this way. I appear to be programmed to think in terms of impermeable continuous insulating materials, likely because that's what I mostly see.
Comfortboard is fine as a product, just very expensive per unit R value. My recommendation for polyiso (or EPS if cost is paramount) is mainly because you can get more R value for your money that way compared with Comfortboard.
For the OP: I agree that I don't see a performance issue with your proposed assembly, but I do see it being much more costly than it needs to be. I also have concerns using the natural fiber insulating materials that I mentioned earlier, but that's just me -- others have used those materials successfully in the past.
Bill
Bill,
I agree. Perhaps strangely I'm seeing a lot of mineral wool exterior insulation on medium-rise residential buildings here. Maybe driven by fire spread concerns?
Fire resistance is the one area where I'd look at using Comfortboard, since it is basically fire proof and all the various rigid foams will eventually burn. I've recommended Comfortboard on GBA a few times for people in fire zones looking to maximize the fire resistance of their finished structure.
Ultimately it's about getting the right mix of product features and product cost for the application at hand.
Bill
Bill, what don't you like about sheeps' wool insulation?
The higher R value of polyiso is sort of a temporary benefit, or a benefit that diminishes over time (albeit one that lasts for many years) because eventually the blowing agent leaks out and you’re left with an R value that’s comparable to other insulating materials.
I suppose it depends on how long you’re planning to leave that insulation intact, but ideally it would last the lifetime of the house, and maintain a constant R value. Rockwool maintains a constant R value. Polyiso does not.
With that in mind, if you’re going to use polyiso, I’d say plan for its future R value, and make sure that will be adequate. Don’t just count on the starting R value.
It’s well documented that its long term thermal resistance is around R-5.6-5.8 per inch. So you’re correct that it’s not the flashy R-6.5 it is advertised as, but it’s still much higher than any other widely available product and can be cheaper than some, especially if you go the reclaimed route. Not to mention MUCH more environmentally friendly than Rockwool in particular. Heating up slag to its melting point uses a lot of energy as you can imagine.
Put your WRB over the plywood sheathing, not over the exterior insulation. If you end up going with a foil-faced polyiso like Bill suggests (and I agree, since it is the highest R-value per inch option), then you can tape the seams of the polyiso with a foil tape and have a secondary WRB for little added effort/cost. But your primary WRB should be on the sheathing in this setup.
The rockwool board doesn’t need a vapor barrier over it? It will then just
Have the furring directly on it. Is it waterproof?
Rockwool isn't "waterproof" per se, but it's basically just rocks - it doesn't care if it gets wet and stays wet. Rockwool Comfortboard 80 is approved for use below grade, so occasional wetting from wind driven rain that finds its way past the siding won't be an issue.
Bill I hear you, I’m a bit concerned about wool too. But it’s been around for a long time. We had wool insulation in our 1950s home and when we tore it out it was in decent condition! Rockwool has formaldehyde, there is a new version without, but still don’t fully trust it.
Also not very environmentally friendly to make it and wool is renewable. Hoping it’s not a bad move everyone is questioning it.
I’m in healthcare and have no clue about building houses…
Rockwool is supposed to "bake out" their binders in the final stages of manufacturing from what I've read about their products. It's better than most fiberglass in that regard. You should check out that "Pure Safety" product I mentioned too.
Timberbatt is an interesting option too. If you're really concerned about what's in it, I'd check what fire retardants they use, if any. Aside from that, I don't think that particular product adds any new critter risks since it's made of wood, just like the framing, and is less likely to attract anything that wouldn't already be attracted by the framing. I'd be a bit concerned about it getting wet though, since it's likely easier for insects to tunnel in compared to framing, so if it's moist enough for the insects to survive, they may be more prone to nesting in Timberbatt material compared to other things.
Bill
Have you looked at the TimberHP batt product?
https://www.timberhp.com/products/timberbatt
Yes I’ve seen it - it’s also a natural
Product so don’t see how it’s so different from wool but I’m open to it also!
Rik27,
One difference is that being quite rigid it appears to be much more difficult to instal.
The semi-rigidity (not really rigid) actually makes it easier to install, I'd say, assuming that the framing uses standard distances (16 or 24 inch gaps) between studs, so that stretching, squishing, or cutting is not required (aside from cutting around junction boxes, plumbing, and wiring). The friction fit holds it in place permanently, pretty much. The same is true of Rockwool, and sheep's wool batts, or any insulation that is friction-fit.
The Timber Board product, on the other hand, https://www.timberhp.com/products/timberboard is rigid, and intended for installation outside of the wall cavity, not between studs.
Paul,
https://www.greenbuildingadvisor.com/question/timberhp-installation-struggles
I missed commenting on the other thread Paul lists, but I now have direct experience with both HP TImberBatt and TimberFill.
TimberFill is a one-for-one swap for blown cellulose and has similar properties. Locally (VT) the installed cost is the same and since both use primarily recycled or scrap as raw materials, both probably have similar "Green" credentials. Both use borates for fire and insect protection, and borates are typically not a problem for chemically sensitive people, but I would still run your own testing on a sample.
TimberBatt does take some serious getting used to. Installing it is sometimes more like carpentry work than insulation work. We tried multiple methods for cutting the batts, and a very sharp machete is one of my favorites. Also, a 24"-36" bare serrated saw blade (basically a long bread knife) works pretty well. For uninterrupted cavities, it's not too difficult to get one side started, then compress the other side to get is started in the cavity, then "thump" the whole batt in place. Once in, they will certainly never sag or fall out again. For walls that have wiring bundled and running horizontally, it's not too difficult to cut the batt at that height and install above and below the bundle. Running the wiring along the top or bottom plate would minimize this problem, but our electrician balked at that suggestion. Plumbing runs or other big interruptions are much harder: cut&cobble is the only way to go and it can be tedious work. Plan to keep all plumbing out of exterior walls to avoid this issue.
Both products are VERY dusty and while the dust is probably not as offensive as fiberglass, it is a serious nuisance because the fibers are still pretty stiff. Expect to do some significant cleanup after installation. Again, chemical sensitivity is probably not an issue, but since individual reactions can be so variable, some personal testing would be prudent.
One place we went to other insulation products is overhead. The current build has an apartment above the garage, with lots of wiring and plumbing in the chases. It also uses I-joists for floor joists, and getting the TimberBatt in between the flanges is almost impossible. We went with a two-layer approach: Unfaced fiberglass batts above the plumbing and wiring, and rockwool batts below. We liked the flexibility of FG to get up and around the utilities, and the semi-rigidity of Rockwool to keep everything in place.
If environmental sustainability is a factor and continuous exterior insulation is a must I would consider EPS exterior (lower GWP than mineral wool, polyiso, and XPS) and cellulose batt or blown on the interior. EPS and cellulose are both very friendly with respect to indoor air quality.
An alternative that would avoid EPS and exterior insulation would be a Mooney wall with R19 batts between 2x6 studs, and R6 batts inboard of them.
For improved indoor air quality I would opt for batt rather than blown cellulose, though in either case, there should be no problem if there is a thorough cleanup after installation. If blown, consider giving time before putting up gypsum board, so that the insulation can "outgas" whatever trace of chemicals may be in it. For fire retardancy cellulose is treated with borates or a borate/ammonia combination. Opt for the former. Batts avoid most of this problem because of the time between their manufacture offsite and their installation on site.
Can you share a source for that table? Because it seems to be very different from the manufacturer's EPD (environmental product declaration) information. Using A1-C4 categories with the standard reported GWP kg CO2e at 1 m^2 and 1 RSI (which is equivalent to 5.678 R-Value):
Rockwool Comfortboard 80 = 5.38 kg CO2e/m^2-RSI
Polyiso w/ Glass Facer = 4.36 kg CO2e/m^2-RSI
Polyiso w/ Aluminum Facer = 2.32 kg CO2e/m^2-RSI
So based on manufacturer's provided EPD, mineral wool does NOT have 60% less GWP like your table describes and is, in fact, worse from a GWP standpoint than the two most common forms of polyiso. Rockwool is a good product if you have a fire-resistance use case. I just don't like the pervasive marketing they have done that seems to have swept everybody in this country off their feet without considering their mediocre R-value per inch and their high prices.
One additional thought: Rockwool's comfort BATT products are lower GWP, maybe your source is using one of those products in its comparison? But since this thread is about exterior insulation obviously we should use their comfortboard product for an apples-to-apples comparison.
Sources:
- Rockwool (Bottom of Page 27): https://www.rockwool.com/siteassets/o2-rockwool/documentation/epd/rockwool-stone-wool-environmental-product-declaration-epd.pdf
- Glass Polyiso (Top of Page 18): https://www.atlasrwi.com/uploads/resources/PIMA_EPD_Polyiso_Roof_Insulation_Board.pdf
- Foil Polyiso (Bottom of Page 9): https://info.nsf.org/Certified/Sustain/ProdCert/EPD10042.pdf