Ideal R-Value For Basement Wall Before It Becomes Overkill - Insulation - DIY Chatroom Home Improvement Forum

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Old 02-18-2014, 02:30 AM   #1
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Ideal R-Value for basement wall before it becomes overkill

I've already started with 2" xps on the poured foundation wall. I currently plan to framed it out with 2x6 16 "o.c. about 1" away from xps and fill in studs with roxul . My question is, how much of an r-value is good enough before it becomes overkill and a waste of money? I planned on using roxul and there are 3 types available in my area:

r15 $38 60sq' (r25 w/xps)
r23 $43 40sq' (r33 w/xps)
r30 $47 30Sq' (r40 w/xps)

My region is northern midwest Chicago area. Climate all winter has been well below -0 for the most part. It will be a finished basement for recreational use. heating will most likely be gas furnace.


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Old 02-18-2014, 06:47 AM   #2
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Originally Posted by jim20j View Post
My region is northern midwest Chicago area. much of an r-value is good enough...?
No such thing as "overkill" if they'll keep taking your Visa card

I've already started with 2" xps...
plan to frame it out with 2x6 ... and fill in studs with roxul.
The 2x6 and that extra inch between is probably at the overkill end of things.

Otherwise, having the framing right up against the XPS works better on several levels. The roxul ratings are based on their depth (eg: 3.5" @16"C/L = R14).



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Old 02-18-2014, 04:59 PM   #3
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Probably could have run the framing tight to the foam but at this point you probably want to fill the entire cavity (i.e. don't leave an air space).
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Old 02-18-2014, 05:20 PM   #4
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Just keep in mind that for a basement the soil behind the walls is far warmer than the outdoor AIR temperatures in the winter. As an extreme, here (MN) it can be -20F for days (air temp without winds) and the soil is about 50-55F about 4' down, so a basement does not see the same conditions. There is the lateral conductive heat flow that is quite slow. The soil temperatures in the soil can reduce AC costs in the summer for conditioned spaces if they are below grade. - That is common here.. Keep in mind that a basement has more square footage exposed to Mother Nature than to the air above grade. Just get rid of the obvious drafts in the livable space.

Basement are different than above grade because lightweight frame construction has little(if any thermal mass storage). - It seems ironic that people build large frame homes and then use geothermal for heating and cooling.

Too bad the "scientists"/"specialists" that study the decimal points and details in R-values for individual materials do not address the same effort when it comes to a whole structure instead of micro-managing small air flow paths that have little effect on a daily basis. If I had a frame home on a basement, I would spend my time in a basement, close the doors to the upstairs if the power was out for an extended period and then drain it and let it freeze.

The details are much more fun to talk about and speculate on.

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Old 02-18-2014, 11:55 PM   #5
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Thanks for your replies.
My reasoning for 2x6 is that the R23 is 5 1/2". The 1" gap is because 1/4 of my foundation wall has a 6" waste line running across it, so to clear it, I would need 7" off the wall. Now that I think about it, I forgot to factor in the 2" of xps, so I need 5" off the wall. Even with 2x4 frame, I would still need an additional 1 1/2" to clear the waste line. The 1" gap is for the framing alone,the roxul will be pushed against the foam.

So, for the waste line wall, if I go 2x4 framing against xps and leave a 1 1/2" gap to make 7" but push roxul to the foam and fill up the rest of the 1 1/2" with split layers or somehow, I believe that would suffice?

According to my calculations assuming they are correct, R30 roxul would be $562 more than R23 for the entire basement, so I will go the r23 route.

Thanks again!
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Old 02-19-2014, 12:41 AM   #6
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Just curious. What is the reason for the Roxul as opposed to fiberglass or even spray foam?

I am not sure but for the cost of the Roxul, you might be able to get a DIY spray foam.

Also, do you ne to frame all of teh walls out or just one with the waste pipe?
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Old 02-19-2014, 05:19 AM   #7
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I currently do not have any moisture issues, however I also do not wish to have any in the future. I plan to be here a long time and it's more of a peace of mind knowing if excessive moisture were to occur. And from what I have read so far, it also serves as a fireblock. Sprayfoam seems to be an over budget route. it's not totally out of the question. I intend to frame out and finish the entire basement. 1500sq ft
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Old 02-19-2014, 10:34 PM   #8
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Overkill is adding any cavity insulation to the foam board. With an empty cavity, the inside face of the FB would be at about 58*F and safe to 73%RH, the HVAC in basement (68*F) would take care of any moisture. The more cavity fill insulation in front of the FB, the colder it will be/lower the dew-point temp, Fig. 3a- without cavity fill------ Figs.4a,4b each with additional insulation;

R-15 +R-10= 45*F inside face with dew-point there at 43% Relative Humidity in basement at 68*F. For Chicago- 3 lowest months averages;

R-23 +10= 41*F FB w. 38%RH---------- R-47 + R-10= 36*F FB temp and safe to only 30% RH basement air till condensation.

PS. Some AHJ's require fire-blocking every 10' (per code) between the FB panels, check locally.
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Old 03-07-2014, 10:49 PM   #9
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I'm late to this thread but I'd like to ad, in this area ( I'm an 1hr + 1/2 west of Chic. )
you should consider having a dehumidifier in your basement for the times of the year when you need it.
Winter I'm putting in humidity through the hvac ( April air) , summer the Ac takes care of it , but Spring rains ( high indoor relative humidity)with still cold frozen ground could spell trouble condensation wise ( in your basement walls).

I went with 1"xps with R11 stud batts and the basement never got below 63f with heat vent barely open . Canned lights ( there's plenty and they actually help to heat) and vents all open ..64-65-66f in no time.
Ceramic tile floor hurts me for better temps but I'm comfy.


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