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Using rigid foam insulation to acheive R-19 in true 2x4 exterior walls

Using rigid foam insulation to acheive R-19 in true 2x4 exterior walls


Old 12-07-09, 06:40 PM
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Join Date: Dec 2009
Location: Minnesota
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Using rigid foam insulation to acheive R-19 in true 2x4 exterior walls

I am in the process of completely remodeling the interior of my 1896 Minnesota home. I will be gutting down to studs. Minnesota requires R-19 in exterior walls. I would rather not furr out my studs to 2x6. Can I (according to code) acheive R-19 by stacking two, 2 inch, R-10 panels in the stud cavities? Would this work?

This seems like it would work logically. But I can't find a single similar example on the internet.
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Old 12-07-09, 07:05 PM
Tolyn Ironhand's Avatar
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Location: Twin Cities, MN
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Welcome from another Minnesotan!

It might be a bit more money but your best option would be using spray in expanding foam. This will give you the R-value you need and fill all gaps that you would never be able to do with rigid foam. There are some DIY kits out there or you can hire it out. For some reason $2 a square foot of wall rings a bell but I could be wrong.
Old 12-07-09, 07:07 PM
Join Date: Mar 2005
Location: USA
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You could have a time proving it without any teats

I have not looked at the MN requirements lately, but they may be prescriptive and just specify combinations of materials that will be acceptable.

Unfortunately, the pink panther is behind this because R19 in a wood stud walls will never give you an R19 wall because of the thermal short circuiting through the studs. It will probably be R16 or R17 if they are wood or either R11 to R13 for steel studs. - This does not include the infiltration that comes with any batt insulation.

Cramming insulation into the joist spacing just decreases the insulation value of a bat dramatically.

You can fir out the studs to meet the code minimums, but that would not give you a R19 wall if it ever was tested, buy you might be able to fool someone.

Using some type of rigid foam (on the interior) may be an alternative. 1" of polyisocyanurite (usually yellow with foil on one side) could add R7.5 to the R13 batts to get you up high enough and cut infiltration also. - Just buy some spray foam to fix up around the typically leaky windows since they are rarely installed correctly.

Last edited by Concretemasonry; 12-07-09 at 08:13 PM.
Old 12-08-09, 01:16 PM
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Join Date: Dec 2009
Location: Minnesota
Posts: 12
I have considered the spray foam option. But, I would like to save as much money as possible. So, do you think it would pass inspection to use rigid foam. Remember, this house has no existing insulation. This would be rigid foam between the studs only, not over batts.
Old 12-08-09, 01:34 PM
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It might pay to talk to the inspector first. Go over your options with him.... should prevent any nasty surprises later.
Old 12-08-09, 06:37 PM
Join Date: Aug 2009
Location: USA
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Definitely talk to the inspector before hand. I know this is something done very commonly on the outside of the house. I don't see a reason why the inside should be a problem. Just make sure your electrical boxes are properly extended.

I don't think I would use two 2 inch panels. I think you should still batt he cavities and add a panel to get the total you need.

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