Questions about insulating a basement wall in 100 yr old house in Massachusetts – GreenBuildingAdvisor

Hello,

I will be insulating my basement walls to add some basic livable space and was wondering how I should approach it given recent code changes, best practices etc. I have read many of the articles on here also – thanks!

I am planning on submitting a permit for this work and I will be doing the work myself (except for electrical). The inspector may have his own opinions/requirements, but I would like to start somewhere.

21 and 24 IECC look to be the same for basement wall R value.

– Basement wall R value for Zone 6: “15ci or 19 or 13&5ci” – “…. means R-15 continuous insulation on the interior or exterior of the wall, or R-19 cavity insulation on the interior side of the wall, or R-13 cavity insulation on the interior of the wall in addition to R-5 continuous insulation on the interior or exterior of the wall.”

I’m planning to do R5 foam board on wall + R 15 rockwool in 16″0C wall butted up against foam board + sheetrock which would just exceed code.

(Don’t want to use sprayfoam or fiberglass batt.)

I was thinking of using one of the following: (1) 1″ Foil faced polyiso at R6, (2) 1″ Unfaced XPS at R5, or (3) Foil faced EPS at R 5.8. The price is essentially the same for each at the local big box store.

Questions:

1. Basically, which insulation board should I use?

From reading the forum, it looks like polyiso performs less at lower temp, XPS off gasses/loses R value over time, and EPS is the most ‘stable’.

My preference is to use option # 1 foil faced polyiso. (Especially given ‘offgassing’ of XPS which I don’t like the sound of given kids playroom which was one of the main reasons to avoid sprayfoam – perhaps unfounded fear).

2. Although each option meets IECC code, are there any concerns about condensation with just using R5 on the basement wall? If for example, I increase the wall to R10 would that give sufficient thermal break or is this a non-issue?

3. Follow on from Q2. From reading the forum I am assuming that I don’t need a plastic vapor barrier between rockwool and sheetrock? Is that correct in all cases?

4. Is there any other benefits or negatives to using the foil faced EPS or polyiso boards Vs unfaced XPS?

I’m probably overthinking this but appreciate any input.

Thanks