Replies

1. Expert Member
   Dana Dorsett | Mar 08, 2019 01:50pm | #1

   For rubble / fieldstone / brick foundations it's usually better to touch up the mortar then install 2-3" of closed cell foam to air seal, insulate, and moisture seal the foundation, then build a conventional uninsulated studwall in front of it.

   A co-worker did his uneven not-too-flat CMU basement wall last year by installing a 1.5" deep steel furring /stud framing about 3" in front of the foundation as a depth guide and installing 3" of HFO blown closed cell before installing the wallboard. Some rubble foundations are flat enough to use that approach, but very few fieldstone walls are.

2. 
   rhl_ | Mar 08, 2019 03:58pm | #2

   What is the solution when you don’t want to use spray foam (rigid is OK) and you also don’t want to waste needless space with a framed wall? (But just want it to look nicer)

1. Expert Member
   Dana Dorsett | Mar 08, 2019 04:21pm | #3

   Don't use EPS with a big cavity between it and the foundation- the potential fire spread issues are pretty severe.

   Start with a serious round of mortar repair, and maybe even a sacrificial parge of lime mortar if there is a lot of efflorscence. Set an exterior-grade steel studwall as close to the foundation as possible, with a 0.75-1.5" continuous layer fire-rated polyiso on basement side, seams taped. Build your 2x4 fiber insulated wall tight to the polyiso, with a 1" layer of EPS under the bottom plate that is also under the cut edge of the polyiso to prevent potential moisture wicking. Ideally the R-value of the polyiso would be at least the amount sufficient for dew point control in your climate zone on a fully above-grade 2x4 wall.

   The joist cavities above the steel studwall & gap will need an air sealed fire stop, and the joist bays need to be insulated as far inward toward the basement fully over the 2x4 top plate.

1. 
   rhl_ | Mar 08, 2019 05:42pm | #4

   Thanks, When you say mortar repair, what do you mean?

   Also we are in Zone 4A. We will be doing some exterior insulation on some of these walls..

1. Expert Member
   Dana Dorsett | Mar 08, 2019 06:00pm | #5

   >"When you say mortar repair, what do you mean?"

   Rubble foundations are usually built with mortar between the chunks, not dry stacked. Over 90 years some of that mortar has likely fallen out or been degrade by efforescence, etc. Replacing any missing or compromised mortar should be done now, while it's still inspectable and do-able before boxing it off ne'er to be seen again until somebody guts the basement walls.

1. 
   rhl_ | Mar 09, 2019 11:10am | #6

   Right ok, otherwise known as repointing. That’s what I thought.