# R value of basement insulation vs. regular wall insulation



## rmcfall (Aug 8, 2009)

I've been researching recommendations for basement insulation and seen that R-10 of rigid foam is pretty standard.  What I am wondering is, does the recommended R-value change for basement walls that are above grade?  It seems that the R value for a basement wall above grade should be comparable to any wall above grade.  Or am I looking at this the wrong way?


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## DBoon (Aug 9, 2009)

The reason that R-10 may be ok for below grade is that the below grade temperature is much higher than the ambient air temperature in the wintertime.  If some of your basement walls are above grade, you should insulate them much more heavily.  

I just redid my walkout basement last year, and I added two inches of thickness to the above grade walls and offset the studs, then used foam.  I have about an R-21 foam wall above grade, and this keeps things much cozier than before.  I highly recommend insulating to this level.


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## rmcfall (Aug 9, 2009)

About half of my basement walls are above grade, so I was trying to figure out how heavily to insulate them.  I was thinking about using 4 inches of foam board, which would achieve  R-20, similar to what you did.


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## DBoon (Aug 9, 2009)

I'm a little confused - is the foam board going to go over a concrete or concrete block wall, or are you going to try to put it between studs?  

If over a concrete block wall, then R-20 with no thermal bridging is a good number to aim for.


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## Gooserider (Aug 10, 2009)

I've been sort of puzzling over the same thing...  We have essentially an 8' high basement foundation, measured from the slab to the top edge,  so not including any footings...  It is made of 10" poured concrete, with about 4' below grade, and 4' above grade, so I'm not at all sure what the right way to insulate it thoroughly is.

Currently most of the space has (I think) 2x4 studs fastened to the concrete w/ fiberglass batts between them, and individual board panelling over them.  The furnace / utility room has bare concrete walls, and one of the other rooms appears to have sheetrock under the panelling.  The basement is ALWAYS cold, especially in the winter, and while I'm not certain that I have punched the right numbers into my heat-load calculations, it appears the basement accounts for almost 1/2 the heat loss in the house....

Incidentally, I think a large part of the reason the basement only goes down that far is that the water table appears to be at or slightly above the level of the slab during the spring thaw or other very wet times of year.

As best I can tell from looking at the Building Science website, it appears that what they suggest for that sort of setup is to use two different sorts of foam, a vapor permeable foam below the grade line, and a non-permeable one above.  The idea is apparently that the below ground part of the wall should always be considered "wet" so you want a permeable insulation below grade so the wall can dry to the inside, as that is the only available path for it to do so.  On the above grade parts, you want a non-permeable insulation to keep interior vapor from penetrating the wall and condensing inside it, and moisture in the concrete isn't an issue as it can dry to the exterior...

Another aspect that isn't as clear is whether or not there would be an advantage to applying some sort of insulation to the EXTERIOR part of the above ground walls - theory being that it would help to prevent the cold from getting into the concrete in the first place, which would bring the wall temperature closer to below grade level, and lower the need for as much interior insulation...  Downside of that seems to be protecting the foam, and the potential for the foam being used by insect pests as an invasion path.

Gooserider


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## rmcfall (Aug 10, 2009)

D-Boon - the basement is 8" thick concrete block.  About half of the basement walls are entirely above grade.  

Gooserider-  I bet you are about right that almost 1/2 of the heat loss occurs due to cold air coming in through the basement and pushing warm air out.  I saw that Building Science article where they used foil faced above grade, in particular in situations where the wall may not have a thermal barrier (i.e., drywall).  They also show using spray foam for the entire wall (requiring thermal barrier), which would I imagine would be comparable to using sheets of extruded polystyrene.  I think the important issue is to NOT use foil faced foam board below grade for the reason you mention.  I don't see where non-foil faced foam boards would be a problem above grade, as it would simply allow the walls to dry to either the interior or the exterior.  You mentioned having fiberglass batts in your basement walls.  Hope you don't have any issues with mold.  

Applying foam board to the exterior would be ideal, but then the issue as you mentioned is finding a material to cover the foam, putting a drip edge along the top, etc...

Check this link out for some interesting numbers....  http://www.woodstove.com/pages/basement_install.html

I'm thinking at least R-20 would be a good number to aim for with the basement walls above grade.  Maybe just R-10 for those that are below grade.  




			
				Gooserider said:
			
		

> I've been sort of puzzling over the same thing...  We have essentially an 8' high basement foundation, measured from the slab to the top edge,  so not including any footings...  It is made of 10" poured concrete, with about 4' below grade, and 4' above grade, so I'm not at all sure what the right way to insulate it thoroughly is.
> 
> Currently most of the space has (I think) 2x4 studs fastened to the concrete w/ fiberglass batts between them, and individual board panelling over them.  The furnace / utility room has bare concrete walls, and one of the other rooms appears to have sheetrock under the panelling.  The basement is ALWAYS cold, especially in the winter, and while I'm not certain that I have punched the right numbers into my heat-load calculations, it appears the basement accounts for almost 1/2 the heat loss in the house....
> 
> ...


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## Wet1 (Aug 10, 2009)

I'm mostly insulated on the inside to R-15, but I have a couple of areas I still need to address.  I'd also like to install some above grade exterior insulation on the 20" or so of my basement walls that are exposed to the elements (this part is brick).  I would go below grade, but my foundation turns to stone (and flares out) just below the grade, making it tough to use panels.  When I cover the exposed brick, I'll probably use this product in 1".

http://www.styro.net/FoundationInsulationPanel.htm


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## Wet1 (Aug 10, 2009)

LADYGO DIVA said:
			
		

> i,as an ant, love the comfort of foam in winter & summer=thanx


Well then please come over and enjoy hanging out around my house, just watch the Chlordane rich soil!


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## DiscoInferno (Aug 12, 2009)

We just insulated our MI basement, R10 extruded polystyrene on the three mostly buried walls and R15 on the exposed fourth wall.  All walls covered with sheetrock.  It's still not code in this area (not that anyone here cares) but I don't plan to actually heat the basement, just keep the pipes and furnace condensate pump from freezing (like it did this past winter).  A lot folks around here insulate the outside of the basement instead/also, but we weren't that forward-thinking.


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## DBoon (Aug 13, 2009)

I can't profess to be an expert on this topic, so I won't profess to have the answers to a particular situation.  However, I'm apt to believe that the "open-cell" versus "closed-cell" foam debate is mostly marketing b.s.

I used the open-cell foam (Icynene) for my a basement remodel.  The basement is a dry basement - no moisture problems on walls or floors.  I used Icynene throughout on concrete walls and above-grade walls.  Four years later, everything seems fine - no mold smell or problems of any kind. And the basement is much warmer than before.   My heating bills for this place were about 35% less after the remodel compared to before, and it feels much warmer.  

In a different house, I used closed-cell foam for above-grade walls, and nothing on the below-grade concrete walls.  Everything has worked out well.  I had the outside foundation walls waterproofed to eliminate some moisture problems, and that helped mitigate moisture through the block walls.  I wish I had thought of adding foam below grade when I had this done - too late now.  In any case, the basement is now 4 degrees warmer than before in the wintertime. 

So, I'm not answering the core question here, but just giving two examples where the foam did it's job.  

I would recommend in a cold climate creating a non-thermally bridging wall for any above-grade wall.  It's not really hard to do, and it makes the most of a foam insulation job.  My R-21 wall is really R-21 - no bridging.  Ultimately, this is a better performing wall than one with the same thickness of foam but with thermal bridging.


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