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RegLearning 10-14-2013 07:13 AM

Basement insulation
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Hey guys,

Here is a pic of my basement exterior wall. My current plan is 1" of xps then roxul comfortbatt r14 in stud cavity.

What should I do in the area above the foundation wall thats framed?

What should I do in the rim joist area?

Should I run the xps all the way to the floor joists?

Should there be a vapour barrier between drywall and roxul?

Out of curiosity could you just build the wall away from the foundation a 1/2 or so stick comfort batt in (no organics) and then vapour barrier between drywall and studs?

RegLearning 10-14-2013 07:16 AM

Btw I'm in Canada and does anybody know the actually R value code here for a finished basement? The codes I have read are R-12 but I think it was moved to R-20 recently?

Windows on Wash 10-14-2013 07:58 AM

Ton of threads out there about this stuff.

SPF is easy for the rib joists or cut and cobble rigid foam and seal it to the sides.

No vapor barrier but air tight drywall.

In the areas that are framed, I would want to pull out the existing insulation, seal the exterior sheathing against any air leaks, and install batting.

No need to run roxul unless you really want to. Once you have sealed the cavity against air movement, HD fiberglass batts will do just fine.

Gary in WA 10-15-2013 07:53 PM

Describe the sheathing/siding, any foam board out there?


Gary in WA 10-17-2013 01:23 PM

Are you still around? Poly wrapped fiberglass batt is bad;

Poly sheeting is alright for some areas Canada;

Air space is bad letting basement air condense on cold concrete wall, as is fiberglass (air/moisture permeable) alone, per links.


concretemasonry 10-17-2013 03:07 PM

Watch out for the imaginary red line somewhere on the side of the foundation that is created to make you think there is real deifinte temperature change that the outside air temperature can effect on day by day, hour by hour basis to overcome the huge thermal storage of the soil arounf AND under the basement.

Even R19 can give you only R9 or R10 in many cases IF it is dry and 1% moisture reduces that.


RegLearning 10-18-2013 07:07 AM

My thoughts are 2" xps inside rimjoist and false foundation area that's framed above the concrete. Then fiberglass. Then glue 1" foil faced eps to all foundation walls, tuck tape all seams. Then frame 2x4 with sill gasket under and fill with r12 fiberglass then drywall no vapour barrier. Does all this make sense.

Windows on Wash 10-18-2013 05:08 PM

Whats the point of the foil facing in that capacity and which direction are you pointing it?

I would rather see XPS on concrete in most applications as compared to EPS.

RegLearning 10-18-2013 07:14 PM

Cost is the main reason. Eps with foil face is 1/2 the cost of Xps. Durfoil from Hd is much cheaper than xps from Hd. Trying to remember that although I want to build above the minimum, money still matters!

Windows on Wash 10-19-2013 10:02 AM

A radiant barrier, without air space, does not provide any benefit in term of R-value. It is a class I vapor retarder though so be cognizant of that.

At the end of the day, you have to have the right amount of R-Value in that wall assembly so while I appreciate the fact that things cost money, even more expensive is having to do it twice.

Gary in WA 10-19-2013 03:44 PM

EPS is a lot more "green" than XPS, by a long way. You will need to use more to get the same R-value to stop basement air condensation on it. Especially if you fill the cavities in front of concrete/fb with fibrous insulation- requires thicker fb; pp.16;

If using the foil faced EPS and an empty cavity (double bonus- for vapor barrier and reflective radiation), the foam would need to thick enough to meet local code and prevent condensation on inside face, so R-value is important.

What is the R-value and Type (for density/perms) of foam board? Eg.

Then I could figure the minimum thickness fb required (with cavity insulation or without?) to prevent condensation for your location...


Windows on Wash 10-19-2013 08:03 PM


EPS is devoid of the blowing agents used in XPS.

That being said, you need the right around of R-value no matter the substrate and EPS does require some sort of facing in most cases as it is not nearly as much of a vapor retarder as XPS.

Last I recall, vapor permeance ratings on EPS are 1.5'ish to 6 depending on the type and formulation.

They are getting better about the blowing agents being used.

Why not try ISO?

Gary in WA 10-21-2013 11:52 PM

Just get foil-faced and don't pin-hole the face, Tables 4,7;


RegLearning 10-22-2013 11:12 AM

Amazing post thanks Gary!

Gary in WA 10-23-2013 12:19 AM

You're welcome, we are here to help!

Just to mention need for an egress (escape) window or door while making new living space down there...

Try some "search" in the white box, top right corner; eg. "basement foam board";

Canned foam/XPS the rims, 1" fb under the bottom frame wall plate for thermal/air/capillary break, ADA the drywall, mastic/tape fb seams, canned foam under XPS, fire-stopping every 10' horizontally and at top plate/joist cavities above, glue fb on in 1' square grid pattern to limit air infiltration to same, air-seal all wiring/plumbing holes through floor above (I don't envy you from pic....) foil tape your HVAC supply pipe joints to get all air sent to register, have links for all my answers....


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