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econger 11-20-2011 06:08 PM

Finishing a basement
I purchased a home in Central New York that was built in 2006. The 2000 sqft basement is 12 courses of block with all but one or two blocks below grade. The blocks have been filled with CoreFoam insulation that has an R value of R-12.25. The basement is unfinished at this time. The walls appear dry, but I do need to run a dehumidifier. There is a french drain around the perimeter that is always dry. There is a sump with a natural drain (i.e. no sump pump).

I want to finish the basement for as a rec room. I have done a fair amount of research, including the Building Science website. I understand that one acceptable method of finishing the walls would be to install 2" of XPS, stud the walls and sheetrock. There are variations, but bottom-line, you need to install the XPS to provide enough insulation (about R-10) to prevent condensation and to allow the moisture to slowly move to the inside of the room. Of coures, you still need a dehumidifier. That's the basics of what I've learned and I don't believe it necessary to rehash the strategy of the Building Science website to ask my question.

The cost for 2"x4'x8' XPS is close to $29 a sheet. Since the CoreFoam filled block walls have an R-12.25 rating, do I need a full 2" of XPS or can I use a thinner sheet? I see it comes in thicknesses as low as 3/8". Would my current block walls already prevent condensation from forming? Do I need the XPS just to slowly release any moisture into the interior? I'm willing to install the XPS, but if 2" is overkill under my circumstances, then I'd like to save some cash. Any thoughts would be appreciated.

Windows on Wash 11-20-2011 09:04 PM

I am highly suspect of the quoted R-Value on the core foam.

Even if the interior of the block is insulated, this has little to no impact on the R-Value where the block is solid to the outside and the thermal bridge is not interrupted.

I prefer ISO board to XPS but there may be issues with its compatibility with moisture.

I think you would be well served to stay within the accepted and tested wall construction methods.

Paging GBR :whistling2:

Gary in WA 11-21-2011 04:02 PM

Welcome to the forums!

I agree with WW, above, especially on thermal bridging and the shape of the block leaves un-insulated areas from the outside in. These would be similar to wood studs changing the whole-wall R-value:
Scrolling down the list, notice your 8 ? concrete block is a little over R-1 for its thickness between the insulated cells. Looking at the inside cell section you may have 3-1-1/2 areas of solid concrete that is not insulated, per 16 ? each. So 25-30% of your total wall area is insulated to R-1.

After reading these results of a study with foam thickness, scroll to the main page for more reading: http://www.buildingfoundation.umn.ed...timum-main.htm Compare your Heating Degree Days to the cities listed.

Did you notice page #9 at BSC:

Did you catch page 10, on thickness here- quite similar:

The thickness of board counts, and also where it is installed:

No air gap to insulation:

My guidelines without sites (unless questioned, lol);
Air (sill) sealer under p.t. bottom plate (as per code)- h.d.galv.nails, air-tight drywall (as per code), rim joist foam board with air seal, fire-block top plate to floor joist cavities and every 10 horizontally (as per code).


econger 11-22-2011 07:00 PM

Thanks for all the input. Makes sense. I know what I need to do to do it right.

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