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Another basement insulation question

1606 Views 7 Replies 4 Participants Last post by  Windows on Wash
I have read over numerous posts on here regarding basement insulation as well as checked out building I have poured concrete walls that are water proofed on the exterior. I have never had any signs of moisture I'm also in zone 5. My originally plan was to use 2" XPS on the walls and frame right up against the XPS, but leave the stud cavities empty. This would provide me R-10 continuos insulation. My local building inspector says I need to have a minimum or R-11. He suggested to frame a 1/2" off the concrete walls and use faced or unlaced insulation. No mention of vapor barrier, but my research has suggested that is an old way of thinking. So now I'm thinking of framing 1/2"-1" away from the concrete wall and using Roxul and then 1/2" drywall. Anyone see any problems with this or have any suggestions?

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The studs should be filled anyway.

Your 2" XPS option is the way to go.

Make sure you seal all the band joists as well prior to stick framing the walls.

Fiberglass and Roxul are cheap. You should always fill the stud cavities.

Sorry about the game. That was tough to watch.
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" My local building inspector says I need to have a minimum or R-11. He suggested to frame a 1/2" off the concrete walls and use faced or unlaced insulation."---------- he does know there are two R-values for basements, R-10 continuous, and R-13 cavity insulation, correct? Notice in IRC 2009 which you are under, read footnote "c" in basement wall R-value;

And you are also under 2009 IECC, which has the same requirements in the chart;

Unless you have local requirements, perhaps he is thinking R-11 for cavity insulation... which they stopped making a while ago when codes called for greater minimum R-values; 3rd line up from bottom ;

I hope he meant that R-value as the minimum for the wall cavity and knows there are two choices. IF required to go with f.g. and a gap, we can help you understand that wall insulation application better.

What is the nearest large city near you (below the map)?

Gary, Thanks for your reply.

In my original inquiry to the inspector I indicated that my intentions where to use the 2" XPS thus giving me a continuos R-10 rating. I his response telling me that I had to have a minimum of R-11, he stated that if I could find foam board that was R-11 they would approve it, but I needed to show documentation and the board must have R-11 stamped on it. I guess I assumed he knows that there are two choices, but I have not questioned him. By the way I'm just 20 miles from Indianapolis so Im in zone 5A.

Is holding off the foundation wall a 1/2" and using Roxul and then 1/2"drywall going to give me any vapor issues?

In terms of XPS type material, there is no version of that material made in 2" thickness that is going to be R11. They are R10. I believe polyisocyanurate has higher R value than XPS. I don't think you want to use the polyisocyanurate version in a basement because (I believe) it does not like to get wet in case of a basement moisture event. I think XPS is the best unless you are going to use spray polyurethane. Spray polyurethane will be very expensive if you contract it out but I believe you get something like R7 per inch. And while you can buy a DIY kit, think very carefully before you do so.

So what do you do in your case? How about doing the whole perimeter in 2", R10 XPS. When it comes to inbetween the stud bays, either fill the studs with 1-2" XPS or use Roxul. I believe that fiberglass does not belong in a basement due to potential moisture issues. I have read on these forums that WRT basements that it is not a matter if they willl leak but when. This way, behind the studs there is only R10 but inbetween studs the R value will be higher. Will the bldg guy compromise to something like that? I don't think that you will have to worry about the vapor barrier. Provided that you seal up the XPS really well, should not be an issue.
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I guess if Im honest with my self, Im trying to find the most economical way to do this. 2" XPS @ $29.00 per 4'x8' sheet plus the Roxul is really going to drive the cost up. I would need 33 sheets of the XPS and 14 packages of the Roxul R14 for wood studs.

So that's why I'm trying to figure if I could just do the Roxul in the stud cavity and no XPS.

On the upside Roxul won't mold but it does not do much for sealing up the concrete. It is when you get a cold foundation and warm interior that meet up and cause condensation on the concrete. I would worry about musty smelling conditions.

There is a thread here about finishing basements. Yes I know, another forum to go through.

There are a couple types of foam that could be used. XPS and EPS. One is extruded poly (pink or blue stuff) and one is expanded poly (like styrofoam cups, etc). You may get away with EPS as it is cheaper. But you will need to look at R value, permeability, etc to really determine if it is right for this application. If you want the above article on basement finishing PM me with an email address. I will send it to you.

Also you will need to provide a fire barrier (drywall or concrete board) over the foam of choice. Factor that cost into the overall cost as well.

I think the bottomline is that using FG or roxul may work but if mold/mustiness develops later, you will have to redo it unless you are doing a quick renovation to sell soon.
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Although the blowing agents are less nefarious, EPS is too vapor open in my opinion if you are in a cold climate like algored2deth mentioned.

If you do a really good job sealing up the walls, you can use fiberglass in my opinion. If you wanted to be really diligent about it, you could put rockwool at the bottom 2' and the rest up as fiberglass.

It is much more likely to grow mold though if it gets wet.
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