Sorry if this topic has come up before, but I am in need of some advice and a good reference if possible. I am in the process of having a home built in Iowa. It will have a hydronic radiant heating system in the basement(water to water geothermal). What is the proper amount of XPS (R value) to be placed under the basement slab? Is there a good reference/article that I can provide my builder so that he may understand where the recommendation comes from. Also, what should the basement foundation walls be insulated with and to what R value? The plan is for 1 inch XPS and fiberglass batt insulation for the basement walls. Thanks for your responses.

Here's one idea.  Everyone has their own, of course... the PassiveHaus people can put 18" under a slab! 
http://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/how-much-insulation-needed

check out buildingscience.com too.

Here's a map of deep well (the circles) and average annual ground temps (the contour lines)  Assume the sub-soil  2m down, (where your basement slab will be) is going to be somewhat colder in mid-late winter than these temps, and work that against the winter average temperature of the slab for the heat loss you'd expect to see.

Soils themselves vary in thermal conductivity depending on type & level, so your heat loss will be lower than if it were an aluminum heat-sink under the slab.

In most of the upper midwest R10 (2" of XPS) is the minimum you'd need, but R15 wouldn't be crazy.  For the walls, anything above grade down to about the frost line should be at or above the same insulation levels on the first-floor.  (If it's 2x6' with R19 batts and R5 of insulating sheathing, give yourself 2x6 & R19 batts in the basement in front of that 1" of XPS.)  Be sure to let the foundation wall dry to the interior (no foils, poly, vinyl, or vapor retardent paint) or you 'll end up with mold in your studwall, and possibly spalling on the exterior of the foundation wall, even sill rot (unless you have an impermeable sill gasket.) 

Price it out, but sometimes it's just easier (and higher performance) to glue thick unfaced EPS beadboard against the foundation wall, held permanently in place by furring strips long-screwed into the foundation which also serve for mounting the gypsum.  (R20 would be ~5" of EPS, R16 would be ~4"))   In my own home (central MA, similar sub-soil temps to IA if the contour lines mean anything)  I used recycled R20 fiber-faced ISO roofing insulation (3" thick).  I recently hacked out a section to move a drain- no hint of mold or moisture damage was evident in the fiber facing against the concrete, so it's drying to the interior quite well.  Be sure to avoid foil or poly facers though, since that will add to the moisture burden in the concrete, allowing it to wick up and damage the sill or effloresce/spall on the above grade section.  (If the latter happens, a coat of sacrificial parging would protect the foundation from slow structural degradation over the decades.)

BTW: If you're heating with a hydronic slab, you're obviously not anywhere near PassivHaus insulation standards (you have a substantial heating system, after all), so insulating the slab to PassivHaus standards would likely be FAR less cost-effective than adding more insulation above-grade.

Also, if the foundation walls aren't yet poured, you'd get excellent performance out of using insulated concrete forms, which are cost-competive in installed-price to post-insulating the foundation walls. (My addition has 2" of EPS concrete form on the exterior, 2" on the interior for ~R18-ish performance.) They come in a variety of thicknesses/R-values.

I appreciate the responses. Dana1, I am insisting on 2" of xps. Am I to understand that I should have them glue 1" of xps up all exposed foundation walls? The insulation contractor was planning on using fiberglass bats in a 2x4 studded wall in the basement.

My parents have radiant floor heating in their basement and they have 2" rigid foam under the slab. Surprisingly, the basement stays at a very comfortable temperature without the floor heat even on, and the basement is a full walkout with a lot of windows. I think just having the foam under the slab is a huge benefit. Their basement is almost finished and is 9' deep by Superior Walls (Great Lakes Superior Walls in Michigan). My dad had closed cell spray foam added between the concrete studs and the bond was also spray foamed.
I also have a Superior Walls basement and wish that I would have put 2" foam under the slab to keep the cold from coming through the floor... the next house I build will have it.

Jere

Jere, Thanks for the response. I am definately going with 2" of XPS under the basement slab. I am looking for advice on insulating the foundation walls in the finished basement. As of now, there are no studs, so I still have flexibility. We didn't go with any kind of insulated concrete form. Wish I would have thought of it.

Is it possible to glue foam to the concrete wall and then glue drywall to the foam (I'm asking, not suggesting)?

Jonr,

I saw on tv a few months ago, I believe it was This Old House, where they took rigid foam tongue & groove sheets and glued (adhesive for foam & concrete) them to the concrete wall. They then used 1x3's or 1x4's and anchored them horizontally, with one row at the top, one row at the bottom, and two rows equally spaced in between. If I remember correctly, they used a self tapping screw for concrete.  Then, vertical rows of 1x3 or 1x4 where installed, either 16" or 24" o.c., over the horizontal rows. Electrical boxes were installed, might have to use shallow depth or cut some of the foam from behind the box to get it to fit.

Jere

It will not perform reliably as a code-compliant thermal barrier if only glued in place, since glues will soften & fail under the heat then the drywall comes down in whole sheets.  With furring strips through-screwed to the foundation, and  drywall screws holding the gypsum to the furring strips it passes muster.

dolphin:  Yes, putting the XPS against the foundation wall (with sealed seams), not just a studwall+ FG is important.  By putting the foam as a thermal break and semi-permeable retarder against the colder-damper foundation, the cold-side of the FG & edge of the stud stays both warmer and drier, reducing the likelihood of mold & condensation.  Use unfaced batts only- we need to maximize the capacity of the assembly to dry toward the interior.

Read this carefully to get a better handle on it:

 http://www.buildingscience.com/documents/reports/rr-0202-basement-insulation-systems 

System #6 or #4 on the price/performance listed on P17 are approximate what we're proposing here as an acceptable solution.  

Ok. I will look over that article. Thanks.

Ok. Very interesting article. Thanks for the reference. Data is key!!! I am going to strongly recommend that my builder use 1 inch XPS with a 2 by 4 stud wall and unfaced fiberglass batts. I think that should do the trick. I do have another question related to the information in that article as well as the information in the article on Understanding Vapor Barriers (Building Science). If my insulation contractor is applying 3/4-1 1/4 inches of closed cell foam, followed by R-19 fiberglass batt, does there need to be a vapor barrier on the inside? I think not. There is going to be stone and vinyl siding on the exterior, followed by a house wrap, followed by OSB, followed by 2 by 6 studded walls with the foam/batt combo. I think he mentioned putting "plastic" over the fiberglass before drywall. This seems wrong! Any comments are appreciated.

At less than 2" of thickness closed cell foam is only a class-III vapor retarder (semi-permeable).  I'm assuming your proposed stackup is:

stone & vinyl || housewrap || OSB ||  flash foam ~1" thick  || ~4.5" fiberglass ||  poly (or not) || gypsum || paint

The quad-cities are smack in the middle of zone 5:



...in which case you need a class-II (semi-impermeable) vapor retarder on the warm side of the structure.  You could probably get away with vapor retardent paint (class-III) on the interior layer, as long as it's airtight.  If the foam were applied to the gypsum, not the OSB you could get away with out any other vapor retarder (it too is class-III). 

The vapor retarder's function is to keep the water vapor on the warm side (the interior, in heating dominated climates) from infusing into the cooler depths of the insulation.  If the vapor retarder is only placed on the colder side of the structure, vapor diffusion into the wall from the warm side allows it to find the depth into the insulation where the dew point temperature occurs, where it condenses, creating potential mold & rot issues. 

Poly is a class-I vapor retarder (highly impermeable)- it does the trick (a requirement most Canadian climate zones.)  It's not a terrible idea for zone-5- take that recommendation unless you have a well-thought out model that says you can do better.

Even a small hole is worth a whole 4x8 panel of class-III diffusion, or a whole HOUSE of class-II diffusion.  That interior layer needs to be as absolutely air-tight as possible!

More info/recommendations:

http://www.buildingscience.com/documents/information-sheets/3-water-management-and-vapor-control/info-sheet-310-vapor-control-layer-recommendations

Also, going with high density R21-R "cathedral ceiling" batts instead of R19 standard density reduces the rate of convection within the cavity (that occurs with all fiber insulations), and less transport of water vapor to the cold side where it can condense.  Better yet, BLOWN insulation (nearly) eliminates gaps voids, and further reduces convection. The actual R-value of the batts as-installed would be about R14-R15, since it'll be compressed.

My first choice on the fiber would be wet-spray cellulose, followed by (sulfate free) dense-packed dry cellulose, followed by blown fiberglass.  The blown FG will yield a slightly higher steady-state R-value, but the cellulose will wick any condensation away from the timbers, adds thermal mass, and will have substantially less convection.  (Convection degrades performance at higher temperature differences, meaning the R-value falls with temperature. The less-dense the fiber, the greater this factors in.)

It's nearly impossible to install batts perfectly around plumbing & electrical, and it's impossible in the sub-1 inch micro-cavities that occurs around doors/corners/windows in real-world framing, but wet-spray cellulose does a pretty good job at filling it all in.  If you can fill the micro cavities with 2lb foam that's even better, but would mess up window & door frames as it expands.

Great information as usual! You are truly an asset to the Greenbuilding Talk community. I will not "rock the boat" and allow them to use the poly on the inside. I will discuss the issues as they relate to the high density R-21-R "cathedral ceiling batts. The builder is "old-school" and really is not a fan of wet-blown anything! I am struggling with getting him to open to the idea of a "newer" way of doing things. But, I will continue to educate myself and try to educate him as well. Ill let you know how things turn out.