The 2" XPS is better from a moisture standpoint than adding cavity batt insulation in front also. The first condensing surface- the inside face of the foamboard will be warmed by the room air. Batt insulation insulates/stops the room heat from warming, hence lowering the dew-point because the foam is now colder. The f.b. would require more R-value to make-up for the lost heat due to the cavity insulation. Basement insulation is about 1/2 the R-value of above grade insulation values due to no wind, rain, or exposure because the ground/concrete wall buffers. This is why minimum code for your Michigan location is EITHER R-10 continuous, or R-13 (cavity)= see footnote "c" under Basement R-value;
http://publicecodes.cyberregs.com/ic..._11_sec002.htm
You don't need both,
either will work, but cavity f.g. will not stop condensation from infiltrating/exfiltrating moisture vapor as foamboard does. Moisture wicking through the wall (looks dry now, wait until you block the room air currents with drywall) is constantly evaporating or having high relative humidity in the basement can also be problems. Also, as already said, summertime outside air condensing on the basement slab/lower wall sections due to the time lag, pp.4;
http://www.buildingscience.com/docum...g-your-basment
The interior rigid f.b. stops summertime condensation on the concrete. Without f.b., the wall concrete (at various heights) is at different temps throughout the year due to following seasonal temps of the air/ground. The temps are caused by the Geo-thermal warming of the earth, the deeper below grade, the warmer the earth. And, the deeper below grade, the longer the time lag of the basement temperature. 6' down is about 1-1/2 month time lag, average, depending on the soil type, solar exposure, water content of soil, snow cover, etc. At 6' down the soil is about 10-11*F warmer than surface temps. So, using this map, MI is about 42-52*F PLUS 10*F = 52-62*F----- 6' below grade,
on an average... not during the middle/end of winter.
http://www.epa.gov/athens/learn2mode...enrys_map.html
With your basement window above grade (2') and the "frost line" at 25-30", you have 4-5' you really need to insulate.
http://www.soundfootings.com/pdf/US_...t_DepthAVG.pdf
Notice the heat losses and insulating R-value of your CMU (block);
http://www.quadlock.com/technical_li...Insulation.pdf
Even with today's above grade/frost line temps of 27*F, why not insulate that 50-60% of basement wall, while the lower 3' is around last month's temperature plus 10*F;
http://www.sfgiowa.com/weather?map=us_soil_temps You could use 1/2 as much in the lower 1/3 but hard to drywall for the minimal dollars saved.
Gary
Linear Mode
