Detailing Bottom Edge of Mineral Wool in Exterior Application
I will be adding 4″ mineral wool to the exterior of my 1991 Cape Cod in zone 5a in western NY. This summer I am starting by insulating the exterior of the concrete block foundation.
I plan to dig a trench 2-3′ below grade with the overall goal that the top 4′ of block are covered with insulation. I will be following the “Detail 03” diagram in Rockwool’s install instructions here, including the drainage mat: https://www.rockwool.com/north-america/syssiteassets/o2-rockwool/documentation/construction-details/residential/constructiondetails-woodframe-construction-lightweight-horizontal-cladding.pdf
I planned to backfill with the existing clay soil and ensure grading directs water away from the house.
The part I have not found any guidance about is what to do at the bottom edge of the mineral wool to prevent water from being directed to the face of the block. Adding gravel at the bottom of the trench seems like it would direct water TO the block. Would angling the dirt and bottom of the mineral wool away from the house provide any benefit for the labor it would require? Do I simply ensure the bottom is as flat and compacted as I can, tightly press the mineral wool to it, and rely on the surface grading to direct bulk water away? I’m leaning toward the latter.
Thank you for any insight and suggestions. GBA has been such a helpful resource as I embark on this adventure.
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Replies
Dan,
I think you'd want to protect the block with a waterproofing membrane of some sort, be it sheet or fluid applied. Make sure that there's a cant at the interface of the block and footing, and have the membrane come down over it if possible. Then the appropriate drainage, back fill and compaction. The drain needs to lead somewhere else of course.
Compacting the back fill as you go up is a challenge to do in an efficient manner. Especially with a low manned crew. Pile the dirt a little higher than you think you ought to next to the foundation, and mash it down as best you can. It'll settle down over time and probably be at just the right slope.
Hi Kyle, thanks, yes I do plan to use a fluid applied waterproofing membrane on the areas of the block that get uncovered. However I should have been more clear on my post - I'm not bringing the insulation all the way down to the footing and that is where my research is coming up short. I'm only bringing the insulation down 4' from the top of the block which is 2-3' below grade. All the resources online I can find about adding exterior foundation insulation talk about bringing it to the footer - nobody seems to only go part way down. I'm doing this for cost and DIY ease while still providing a benefit.
My current thinking is to ensure the bottom of the trench is as flat and compacted as possible, tightly press the mineral wool to it, and then kick out the bottom of the drainage mat away from the foundation like in the attached picture. What do you think?
Dan, that does help! I was imagining something different.
I'd have to ponder that one for a few minutes. On one hand, the perimeter loss is highest as you get near the surface. Having that insulation there should do quite a bit to limit the temperature gradient from reaching down so low. On the other hand concrete is quite conductive thermally, and I wonder if having that much exposed to earth would short circuit the insulation, and effectively conduct the temperature upwards.
I'll try to dig up a few reports that ring a bell and get back.
I'm mainly thinking about the long winters (not as long as they used to be) here outside Rochester, NY. Keeping the above grade and couple feet below grade of the foundation warmer through the worst of the winter I think will have a big effect on this house. Viewing the interior of the wall with a thermal camera over the winter, there was a temperature gradient from floor to ceiling where it was coldest on top and warmer toward the bottom. I'm hoping to flip that on its head.
In a perfect world I would go to the footing but I don't think the cost and effort is worth it for this retrofit.
Any info you could find would definitely be appreciated. Thanks.
This was the article I was trying to find earlier. It seems to support your idea of a limited depth insulation as a good strategy.
https://www.greenbuildingadvisor.com/article/getting-slab-edge-insulation-right