GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

JM Cladstone exterior mineral wool board?

Bridgetdee | Posted in Green Products and Materials on

Hi all,
Found a better price, lower cost AND greater R value (see below) on John Mansville Cladstone mineral wool but wondering if 4.5 pcf is dense enough for screwing 1 x 4 furring strips in without considerable compression.

Thanks in advance,


Allied building products
2″ r-value 8.6;
4.5pcf 16″ HAS 42.67 SQ FT PER BAG AND COST WOULD BE $45.87
2″ 6 pcf 16″ HAS 32.00 SQ FT PER BAG AND COST WOULD BE $43.20

For a comparison, Roxul 1.5″ was $60/48 sq ft.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Ideally, you are aiming for a product with a density of at least 8 pcf. But if you are patient and careful, you may be able to make a product with a density of only 4 pcf work. Here's what researcher John Straube had to say on this topic:

    “We tested a range of higher density Roxul product. The 6-pound-per-cubic-foot (pcf) and even the 4 pcf products did amazingly well, but the 8 pcf product was easier to handle for people used to the stiffness of foam and it also worked the best.”

    For more information on this issue, see Installing Mineral Wool Insulation Over Exterior Wall Sheathing.

  2. Bridgetdee | | #2

    Can you or anyone provide further details on what challenges may come up and how to prepare in advance for them. It's a significant savings going with the lower density but may not work out that way if it takes longer to install.

  3. GBA Editor
    Martin Holladay | | #3

    The problem with squishy mineral wool is that the squishiness makes it harder to get the furring strips co-planar. With patience you can get there, however, by regularly checking the furring strips with a long straightedge (horizontally and diagonally), and by tightening and loosing the screws holding the furring strips until everything is hunky-dory.

  4. Bridgetdee | | #4

    On the grand scale for a 3200 square foot surface area how much extra time is this going to add?

  5. Expert Member
    Michael Maines | | #5

    Bridgette, it's hard to say how much longer it will take. Probably something on the order of 10-20 person-hours for a project the size of yours. Maybe more, maybe less. It depends on how squishy the insulation is, the weather, the skill of the installers, whether they're doing it on the deck or after the walls are erected, etc. I haven't done it myself but have done similar leveling, by getting the two ends straight using a straightedge, then using a stringline and 3/4" spacer block to adjust the field.

    On the 2016 Fine Homebuilding ProHome we spec'd 2" Roxul Comfortboard 80, which the builder installed while the walls were on the deck--more info here: I don't recall how long it took him; he wasn't looking forward to it but after it was done he commented that it didn't take as long as he expected. He countersunk the screws; alternatively you can get self-sinking screws made for this purpose, called Heco Topix, from Small Planet Supply.

    I'm not sure how thick you're planning to install your exterior insulation, but you might want to consider using a low-conductivity standoff such as this:

  6. Bridgetdee | | #6

    The Heco Topix might be worth the added expense. I I wonder if I could use three per 8' batten to get it in the correct plane and then filled in in between with decking screws to meet smartside's manufacturer requirements.

  7. JC72 | | #7

    I'm suspicious of those R-value claims. It don't think it's possible for material of different densities (i.e. 4pcf / 6pcf) to have the same R-value/inch.

  8. Jon_Lawrence | | #8


    That is much more than I pay for Roxul ComfortBoard 80. Pricing from my local vendors, including Allied, is around $.60/bd ft which would equate to $43 for 48 sf of 1.5" material.

  9. Bridgetdee | | #9

    Thanks Jonathan. Wow, I really need to shop around more. Who else besides Allied?

    John, will check with technical at JM on density variations and R-value.

    Michael, how crazy is sinking a 2.5" small head screw .4" deep on the back side of each furring strip centered for a quick and easy blocking....more time that just checking, maybe?

  10. Jon_Lawrence | | #10


    With respect to national retailers, Allied, HD and Marjam were able to quote me ComfortBoard. Pricing was consistent among the 3 with HD being slightly higher and Allied having a minimum pallet requirement (15). I was also able to get pricing from 4 local (NJ/NY) supply houses. I wonder if Allied mistakenly quoted you the 110 instead of the 80? The 110 is roughly 25% more than the 80, but overkill for a residential project.

  11. Bridgetdee | | #11

    Thanks, I didn't get my quote from Allied it was from a couple other smaller local suppliers.

  12. Jon_Lawrence | | #12

    I see the JM was from Allied.

  13. Bridgetdee | | #13

    Lowest price was from Home Depot.

    .91/sq ft delivered for Roxul Comfortboard 80, 1.5" thick

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |