Justifying mineral wool vs fiberglass for custom or spec homes?
Hey All,
I am working on presenting a bid for a custom build. This build is in Zone 5B? or 5A in Nebraska.
I have worked with both mineral wool and fiberglass before. I personally used Owens Corning Thermafiber mineral wool + certain teed smart membrane on two of my own builds in the past (around 2015).
Since then I haven’t done a new build, but have this custom build coming up for a client. It’s about a 1600 sqft home with a full basement. I’ve been discussing the benefits of mineral wool insulation with them but they are having a hard time swallowing the additional cost. To be frank, I am too. It must have recently gone up in price? I noticed the Smart Membrain has conveniently doubled in cost since I purchased it.
Here’s the numbers I have come up with. 1128 SqFt of wall cavity after subtracting doors and windows – but not studs (16″ o.c.) The mineral wool must be special ordered, and in pallets of 18 bundles. This means they need 3.2 pallets, but I am going to round down to 3 pallets and assume the amount of studs in the square footage will probably get us really close with the 54 bundles. This total comes out to $1,444. Then the membrain (cheapest I find is $140 for 1 roll of 8 x 100, so two rolls cost $280. The grand total to install the mineral wool comes to $1,724.
Back to Fibgerglass: It’s R21* batts, paper faced, so no smart membrain.
$736 for a total.
That puts the mineral wool materials at $988 more. They are looking at this as more than double the cost for an R-Value increase of only 2, R23 vs R21. Now I know from installing and working with mineral wool that it is a better product, but at the end of the day when the client does not see inside the walls, they can’t really justify that difference. I realize fiberglass takes a bit more effort to install properly, but even with this added labor it’s still significantly cheaper. I also take the time to staple the paper flat with the stud and overlap each piece of paper so there is a decent vapor retarder from that.
How are you guys justifying this additional cost to use mineral wool? And how do you sell it to your customers so they jump for joy when you mention it is double the cost and then some? I’ll be honest if I do another personal build I’d probably think it over twice to use the mineral wool since I know I can install fiberglass properly. It would be an easier decision if I had to count on someone else installing the fiberglass correctly.
Edit: Made typo, it’s R21 not R19 for the price.
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Personally, I like working with mineral wool better. I like that I can cut it around obstructions and it just slides into place. No fluffing. Doubtful that would mean much to your customer though.
Mineral wool is better for fire safety, it's better in terms of installation quality (usually), and it's more resistant to critters. That last one is actually why I started using it: I had to replace a bunch of fiberglass batts that had had squirrels nesting in them. The critters don't seem to like to burrow into mineral wool, maybe because it's more dense?
Bill
Fiberglass batts are available in both R21 and R23 to fit 2 x 6 framing from most, if not all fiberglass manufacturers. Both are 5 1/2" thick so there is no compression as there would be with R19 which is 6 to 6.25" thick. They are considerably more dense than an R19 and are as a result, easier to fabricate and work with. R23 is not available everywhere, but R21 is now a very common product. Available with kraft facing, or unfaced.
Mineral wool is fireproof, mold proof, and resistant to water (hydrophobic). It is generally almost double the price of fiberglass insulation for relative R-value. In my estimates I've seen spray foam become a small price increment to mineral wool (rockwool). So if air sealing is of interest that may be worth exploring as an option and could make the client excited for that option as well..
Ralph,
My take on this is that clients are either interested in energy improvements in which case the rock wool would be one of a number of measures you would propose, or they aren't and it makes no sense to put in a different insulation than is typically used where you build.
I have a hard time staying competitive with other spec home builders who do not have any concern for the energy efficient building process. I hate the saying of if you can't beat them, join them. But I feel it applies in this case to be able to make a living at the end of the day. In a comment I have below I mention this. It costs me $5 to $10 more per sq ft to make a home actually better quality in build, water proofing, and insulation. Any competition of mine does not consider these traits because they have trained buyers to just look at the sticker price. I hope I can find more custom builds and educate them and justify these extra expenses for them.
Ralph - a few questions: Are you or your client interested in energy efficiency or comfort? Do you typically test for air tightness? A tight house is easier to heat since the heat stays in the house, rather than leaking out through random holes, and it prevents the drafts that homeowners find uncomfortable. Using ZIP sheathing is a good start, but it can't do all of the work, and you'll need a blower door test to find out where the leaks are. If you sign up for Energy Star (it's free!), they will do a blower door test as part of the ES service. My suggestion is to start with an Energy Star application, have them check and verify the house and when they do the blower door test, have them leave the blower on for a while while you check the house for leaks. You can seal many with your foam gun; others might take more work, but it will help you to identify the problems areas. As far as your wall insulation, rock wool is better, but any amount of air sealing you can do is more valuable.
Around here it's very hard to sell people on energy efficiency. I guess they have more money or a lack of common sense? It's rough to stay competitive with other builders who do not care one lick about air tight or insulation values. I'm trying hard to earn the reputation of quality and energy efficiency but a realtor is as much to blame as a builder. They sell my spec homes just the same as any other spec home and when mine is $5 more per sq ft the buyer will pick the cheaper one. It's infuriating. That's why I am honored to do this custom build but working with the client....they just see initial cost and are leaning towards cheaper routes. Same with sheathing I could do zip or plywood and tape seams to be better air seal but they don't like the added cost (zip is also special order here like the thermafiber).
I did try putting a home on the market and listed that the lights were all air sealed, the walls and attic were super insulated, and detail was added to window and door flashing but that made no difference.
For what it's worth I am considering using the 3M tape on OSB walls just to help air seal some before housewrap. It adheres to OSB pretty well, not sure how it holds up but if I tape it a few days before house wrap I think it will be a good thing to try, and only add a couple hundred to total cost of material and labor.
Q: Have you thought about whether or not these upgrades could be partially recouped via savings on the HVAC equipment?
I can be an uphill battle. When I talk about a lack of comfort within my house the typical reply is "Put on a sweater, we're not spending X$ to re-seal the plenum".
There is never much savings on HVAC equipment. A 100KBTU/hr condensing furnace is only $200-300 more than a 40KBTU/hr condensing furnace, and a 3 ton AC isn't that much more than 1 ton AC. That's one reason why most HVAC equipment out there is more than 2x oversized for their actual loads- they don't do the math, and upsize "just in case" you might end up frying in hell, or hell freezes over.
It's more subtle to sell, but (even with ridiculously oversized equipment) the better building envelope package ends up being more comfortable.
Right sizing the mechanicals is sometimes even harder to sell even though it's noticeably more comfortable. What, only 30,000 BTU/hr? How can that POSSIBLY be big enough for a 2500' house? Those other guys have a 120,000 BTU/hr furnace in a house the same size, and aren't charging as much! What a rip-off!!
Like anything else, properly educating the customer is an important aspect of making the sale, and that isn't always easy in a "bigger is always better" world. Pointing them to Nate Adams' "Comfort Guides" books and videos (targeted at the retrofit market) might be useful for selling them on the concept of tighter, slightly better-R houses with smaller mechanical systems:
http://www.natethehousewhisperer.com/comfort-guides.html
>"This build is in Zone 5B? or 5A in Nebraska."
If it's anywhere in Nebraska it's in zone 5A:
https://beta.greenbuildingadvisor.com/app/uploads/sites/default/files/images/DOE_climate_zone_map.preview.jpg
>"Back to Fibgerglass: It’s R19 batts, paper faced, so no smart membrain.
$736 for a total."
R19 does not meet IRC 2019 code minimum, and only performs at R18 when compressed to 5.5" in a 2x6 cavity. Since it is NOT EVEN code-compliant, it has no place on the comparison list.
See TABLE N1102.1.2:
https://codes.iccsafe.org/content/IRC2018/chapter-11-re-energy-efficiency
See also:
https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2018/08/08062722/Compressing%20fiberglass_2-700x310.jpg
So rock wool would be an R5 improvement, a 28% uptick in center cavity R value (22% reduction of conducted heat flow in the cavities). R21 fiberglass would be an R3 improvement, a 17% uptick over R19s (a 14% reduction of conducted heat flow in the cavities.)
R20 fiberglass batts DO exist, and are the absolute minimum that would comply with IRC 2018. R21 fiberglass would be the closest performance comparison to R23 rock wool.
Dana, my apologies. It's R21. I mentioned R21 initially then put R19 absentmindedly there in the price part. I've fixed the typo, sorry you went to all that work to show code and calcs for one typo.
>"1128 SqFt of wall cavity after subtracting doors and windows – but not studs (16″ o.c.)
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>"That puts the mineral wool materials at $988 more."
$988/1128 =~88 cents per square foot. Is it possible to come up with is a better place to spend that on performance? Methinks "yes", even on the walls:
That 88 cents is enough to pay for a layer of R6 foil faced polyiso sheathing, typically 45-75 cents per square foot material cost even at box store pricing. (In Nebraska try Menards if it's more than $15/sheet at distributors). If you get it for $15/sheet or less the 88 cents would likely include the cost of installation, delivering a MUCH bigger bang/buck than another R2 center-cavity-only. R5 is the minimum needed for dew point control on R20 at the structural sheathing, so with R6 foam you could even skip the facers on the fiberglass if its' any cheaper. (Unfaced is definitely more inspect-able.)
The inch of polyiso would dramatically improve the moisture resilience of structure, and would cut the total wall losses (not just the cavity losses) by about 30%.
I've heard the same arguments that energy-efficiency improvements cost the builder, and the buyer doesn't care, just looks at the price tag. Comfort means nothing, lower utility bills mean nothing, resilience means nothing, quality improvements mean nothing.
A few selling points that lead me to prefer mineral wool to fiberglass:
- Mineral wool helps reduce sound transmission better than fiberglass.
- As mentioned, fireproof and critter resistant. What's that worth to you, the buyer?
- If you lose electricity or fuel supply mid-winter, the small improvements in the airtightness and insulation value in walls would help keep the place more comfortable. What's that worth to you, the buyer? How much would you pay in an outage?
- You are paying less for heating and cooling, month after month, year after year. Unlike most things in a house, insulation (if properly installed) requires zero maintenance, generates no monthly bills to pay, it just saves you some. You might do some calculations showing that the more comfortable energy-efficient home lowers their yearly utility costs FOREVER, and reduces the wear and tear on their heating and cooling systems too. Could at least run the number estimates to subtract from the price tag difference.
- If you decide later that you would like more insulation or better air sealing, the costs are far higher after the home is built. Really hard to justify then.
People generally make the same mistake with appliances. They buy the model that's somewhat cheaper, but the vast majority of the money they spend on the appliance is the electricity or fuel to run it for decades.
Until real estate listings or mortgage companies list and consider the expected cost for a decade of utility bills, HVAC system maintenance costs, and expected HVAC repairs and eventual replacement, it becomes difficult to justify other than "Quality-built homes cost 5% more." Do you buy the cheapest cereal, cheapest clothes and cheapest model car? Why would you pay more for something higher quality?
It can be difficult to sell quality, but here goes. For the extra $988 a client gets better thermal performance (since fg is rarely installed properly and doesn't perform well even in rare situations where it is properly installed) and better quality of life b/c rodents love nesting in fiberglass. So if they choose poorly clients will pay more each year in operating cost for a lower quality home.
After doing a couple hundred home assessments and seeing what fiberglass actually looks like in attics and walls after 10+ years, and seeing its thermal performance with a blower door and infrared camera, I don't think anyone in their right mind would consider using it. I wouldn't want it in my house even if they were giving it away for free.
It always amazes me that people buy houses all the time and they don’t care if the monthly bills are going to be $300 or $150. It’s like buying a car and not know whether it gets 5 miles to the gallon or 50!
If the savings of going to r21 fiberglass allow a budget for better air sealing measures, take that option every time. The difference between r21 fiberglass and r23 rockwool is just never going to be hundreds of $/month. Definitely trade to fiberglass to get taped sheathing.
Further, Dana has shown you could get in the ballpark of exterior foam (probably would actually be an additional some cost). Check pricing on R5 zipR, probably doesn’t make sense, but is a lot less work.
I like rockwool a lot, but I am using it largely because I can, it doesn’t actually make complete financial sense. R21 batts are much nicer to work with than old r19.
I just bought a few rolls of membrain from Amazon $58.55/8.3x50ft
The advantages of R23 mineral wool batts over R21 fiberglass are trivial. I just looked up some prices: R 23 Rockwool at Lowe's comes in at $1.53/sq ft. Johns Manville R21 unfaced fiberglass is 73¢/sq.ft. Manards has R 23 Thermafiber mineral wool for $1.12/sq. ft. and Guardian R21 unfaced fiberglass for 57¢/sq.ft. (save big money) Almost all the performance issues of fiberglass can be attributed to poor installation of low density batts. Bad cutting and fitting can ruin both mineral wool and fiberglass walls but R19 low density fiberglass. is hard to "fluff" to it's full thickness, which can lead to convection loops. The increased density of R21 fiberglass solves this problem. My living room ceiling was insulated with 40 year old high density fiberglass batts. When I converted to a cathedral ceiling the batts were in great shape and were easily refitted in the new ceiling.
Douglas Higden