Unless you KNOW that you have good capillary breaks between the concrete and foundation sill AND between the footing & foundation wall you would be taking a very real risk of rotting out the foundation sill going full-height on the foil faced iso rather than just the upper half.  

Yes, with a top-half-only solution it's half the foam, and if you use fire rated goods you can skip the interior gypsum, but if you use semi-permeable reclaimed foam it both cheaper and more effective than a half-foundation job. It's even less money-  reclaimed foam costs about 1/4 that of fire-rated iso at any given R value, and the cost of furring/studs, Tapcons (if furring), and half-inch gypsum just isn't very huge.  (If you use full height fiber faced iso leave an inch or two of space at the bottom, or add a capillary break, since the exposed edge of bare-iso should rest on bare concrete or it could absorb moisture directly from the slab than through the wall & fiber facer, or even become water-logged at the bottom if you had a 1/4'' flood or water spill that didn't get cleaned up for weeks. This isn't a problem with EPS or XPS.)

To make the foam stick to a concrete wall using foam board adhesive, apply  a ~18" on center grid of hazelnut to walnut sized blobs to the foam and hold it in place under pressure for 10-15 minutes.  I wouldn't count on it hanging in there for 25 years using just the adhesive though, even if it's legal from a fire-code point of view.  If you REALLY want to skip the furring/studwall, there are polypropylene fasteners designed for exactly this sort of installation.  For pictures & descriptions see:

http://blog.energysmiths.com/2011/0...685b1b970c

Note that the house in that blog is recent construction- it has a sill gasket protecting the sill, even if the status of footing-to-wall capillary break is unknown.  Give yourself at least a foot of exposed wall at the bottom to allow at least some drying toward the inteiror- it's unlikely that you'd ever need a 4' band unless the footing is at the water table and you have efflorescence coming out of the wall at that height.

But the polypropylene fasteners make little sense if you're putting up a studwall or furring.

The 50% vs. 70% number for half vs. full height on heat uloss reduction is a very squishy and not very relevant number. A lot depends on how much of the foundation is above grade, what is the subsoil temperatre (for you it's ~48-50F, but in Tennesee it might be 58-62F), and the ratio of wall area to uninsulated slab, etc.  Insulating a 60F basement from 50F soil through R1 of foundation would be a cost effective cut in heat loss with R15 between a 65F basement and 50F soil.  At the slab you get the insulating effect of the air-films of the pooled cool air to help out, but not on the lower half of the foundation walls.

On the cold edge of US zone 5 (where you live) or higher the cost effectiveness of a full height treatment at R15 is quite rational for new construction, even when a contractor is doing it with virgin-stock using union-scale labor, and the heating is natural gas or heat pump.  As a DIY using oil as the heating fuel it's pretty easy to make the case for R20 or higher, maybe even R25 if using reclaimed foam. Whatever that fraction of heat loss you fix by going full height, it's still going to be worth it for you on a per square foot basis, but maybe not for retrofitting a house in TN heated with a mini-split.  Your cost per heating BTU would be about 3-4x that of the TN house, and your heat loss per square foot of sub-grade foundation wall more than 2x.   Top-half-only insulation would make sense if you were planning to flip this place in 2-3 years, but I suspect you have longer time horizons than that if you're talking about a new boiler.

Read at least the first chapter of this document , and note the values for zones 5 (& 6) in table 2, p10.

I've yet to meet anyone who has regretted putting in too much basement insulation, but I've met several who kick themselves for not going high enough on the first round, making more work for themselves when boosting it further.  Don't take a half-measure when you can cost-effectively take it all the way. Use the table in that document as your guide when retrofitting any part of your house, and until you're heating primarily with high-efficiency heat pumps consider the zone 5 recommendations as a minimum. Even zone-7 recommendations may make financial sense for oil-burners in your neighborhood.

Dana, thanks as always for the detailed reply-- I'm starting to feel like I'm taking advantage of you with my questions, so I'll slow down. I get your point about how picking up used insulation makes sense. Tried googleing but couldn't find any recycled foamboard companies near Nashua. Does it make economic sense to order used foam over the Web?

I've never done a job like this so I'm sketchy about how to apply furring and attach the foam to it. Would the furring go under the foam boards or between the sheets (see how dumb I am about this?)? I have some handier friends who can probably offer some advice. Doesn't seem plausible to apply glue and lean against the wall for 15 minutes to get it to stick. I did check out how that guy used the poly fasteners to attach the foamboard-- seemed like a sensible option. Do you need to apply two layers and offset them to get rid of air leaks... or can you apply one layer and tape the seam? Would foam 1.5 or 2" be thick enough, or should I go with a 4" piece? (You said R15 or R20... so that's at least 4", right?)

Well, I'll try to research this further....

Dana said: "“your floors will be warmer, more comfortable”
People in the northeast are used to cold, damp basements; that's just the way they are. Thing is, it doesn't have to be that way. When I mention insulating basements, people's eyes glaze over, but living in a house with an uninsulated basement is analogous to wearing shorts in 10o weather - you can do it, you can wear extra sweaters to stay comfortable, but it makes more sense to cover your self head to toe with warm clothes. Almost no one has an insulated, weatherstripped tightly sealed basement door, but basements are an integral part of todays houses and should be insulated as such. Makes the whole house more comfortable. Forever.

The furring goes over the foam - the foam MUST be tight against the concrete. The furring is placed over the foam and fastened through it into the concrete. you can buy foam adhesive in HD/Lowes which will "grab" & hold pretty well without difficulty, but the furring will hold better long term. Either one or two layers, but two may be easier & provide good seal. Either way, foam between, above & below so you stop any air leaks behind the foam panels.

It's funny, but I don't think of our basement as being cold and damp. Somehow, it always remains dry and is usually not very cool down there. The wood door that gives access to the bulkhead stairs is hollow core, so a few years ago I glued a piece of foamboard to it. Last week I spray-foamed the boxes by the sills and floor joists. I understand that a lot of heat is lost through the basement, but don't actually feel it, even though our first floor is not insulated.

The heat loss we're getting is due to the fact that the coolness of the basement is sucking away the upstairs heat through the floor, right? The floor doesn't feel that cold to me when I walk on it barefoot (it's mostly hardwood)... maybe a bit cool, but not that cool. If the basement were very cold and damp, I'd be better able to appreciate the heat loss.

When we put in a new boiler, its 3-inch insulation will significantly reduce its heat loss to the basement, I'd assume.

Posted By Bob I on 16 Jan 2012 05:56 PM
The furring goes over the foam - the foam MUST be tight against the concrete. The furring is placed over the foam and fastened through it into the concrete. you can buy foam adhesive in HD/Lowes which will "grab" & hold pretty well without difficulty, but the furring will hold better long term. Either one or two layers, but two may be easier & provide good seal. Either way, foam between, above & below so you stop any air leaks behind the foam panels.

Ahh, I get it! 

But what would be the advantage of the furring over the poly holders Dana mentioned?  You would have to drill pilot holes in the furring and then power-drill some sort of masonry screw through the foam and into the concrete walls, right?  The furring would act as strapping to press the board to the wall.  I can't see how you could use furring with two layers of boards, since you couldn't apply it until after you put up the second board (what would hold the first board up in the meantime?)

Even if you use furring, you'd have to place the wood boards in a few inches from the edge of the foam so you could tape the seams, is that right?

Posted By Amazer98 on 16 Jan 2012 05:57 PM
It's funny, but I don't think of our basement as being cold and damp. Somehow, it always remains dry and is usually not very cool down there. The wood door that gives access to the bulkhead stairs is hollow core, so a few years ago I glued a piece of foamboard to it. Last week I spray-foamed the boxes by the sills and floor joists. I understand that a lot of heat is lost through the basement, but don't actually feel it, even though our first floor is not insulated.

The heat loss we're getting is due to the fact that the coolness of the basement is sucking away the upstairs heat through the floor, right? The floor doesn't feel that cold to me when I walk on it barefoot (it's mostly hardwood)... maybe a bit cool, but not that cool. If the basement were very cold and damp, I'd be better able to appreciate the heat loss.

When we put in a new boiler, its 3-inch insulation will significantly reduce its heat loss to the basement, I'd assume.

So, if it's not very cool down there, that's an even greater argument for insulating the walls- the heat loss of a 65F basement through an R1 foundation wall is dramatically more than in my 50F basement sketch analysis.  At 60-65F room temp the earth coupling at the slab has turned into a net negative, and the losses through the above grade portion of the wall is nearly double what I'd sketched out. If you simply insulate the walls, that huge distribution & standby loss of the beastie boiler would now accrue to the first floor- the floor would warmer even though the burner's duty cycle would be reduced.

With a better insulated and smaller boiler with heat purge controls will have lower losses from the boiler to the basement, and your basment temp will drop, taking the barefoot comfort on your first floor down a couple of notches.  Insulating the walls will keep the basement room and first-floor temps up closer to what you've been accustomed to, but at a net lower operating cost.  If you then insulated between the joists to warm up the floor you'd have but a modest benefit to comfort, and an almost unmeasurable effect on the heating bill. (In many cases it's even more surface area, more material.) And keeping the basement cooler will also increase the mold potential on anything you store down there.

Unless you have R6-R8 pipe insulation on the heating distribution plumbing (including returns) that is and will continue to be heat loss from the system into the basement. It's worth adding where it's easy to do so, but is less critical than insulating the foundation.

You can use adhesives to install the foam prior to taping the seams, put the furring up later. If you use fiber-faced goods (recommended) rather than foil you can use duct mastic to air-seal the seams, but with any you want to use expanding 1-part foam to seal the top/bottom edges to prevent convective loops. Snug up against the concrete the convection won't pass much heat, but can move more moisture than you might think.

The advantage of furring is being able to mount an ignition barrier/finish wall gypsum easily. You can tack or hold the furring in place and use a hammerdrill to drill the whole thing in one shot. Finding concrete screws long enough to work gets to be a problem with foam more than 2.5" thick, since the big box-stores don't tend to carry them longer than about 3", but they exist in lengths up to 6", which is good enough for hanging 4" foam + 3/4" furring. You can always buy them online. See:

http://www.concretescrews.com/tapcon-prices/blue-standard-tapcon.aspx

It goes up reasonably quickly even as a 1-person DIY, but it's quicker as a 2-person job. Spacing the furring & concrete screws every 24" probably makes code, it makes for a flatter better looking wall to shrink that to 16". The furring can run either laterally or vertically. If laterally it's good to leave a few inches of clearance to the floor for ease of drilling, and it avoids any wicking issues of being in contact with the concrete. Just make it low enough that you'd be able to install a kick board if you ever decide to finish the room more completely. (I used composite PVC kick boards and left a 2" clearance between the gypsum & concrete due to a history of high-water table and minor flooding during power outages during the high-water season when the sump pumps can't run. YMMV.)

You might gather that all insulation strategies are highly subjective. First, a proper heat load analysis since no cost benefit analysis is valid unless you know how much heat you are losing. Comfort will be improved however modest the insulation since AUST will be elevated.

The four foot reference is better understood by knowing what the typical frost line is your specific area (as apposed to the mear earth temperature which is valid but harder to grasp). Insulating to the frost line does increase both comfort while decreasing
energy consumption, but as Dana points out is not practical.

Here in Minneapolis we have experimented with various treatments depending on the construction, age and condition of the basement walls. In most cases the best application is to frame the wall and foam them rim joist to floor (older homes) and foro newer or in my own case 1921 hand poured cement walls) to use 1.5" Thermax followed by 2x4 stud wall and R13 glass.

Every home has to be evaluated independently, especially for the source of existing moisture (look to the gutter first). Most of the homes we specify insulation for are also finishing the basement for future increased use and radiant floors follow with some insulation below a new slab or on top of the old with PEX sandwiched in between insulation and new floor.

Insulation walls will improve comfort but the floor will still make the space less than livable if the naturally cold basement floor is not addressed with insulation and a radiant panel.

Thanks Dana and Badger,

Now I understand how the furring, screwing and installation of foamboard is supposed to be done!

I'm having an energy audit done in two weeks and will look forward to getting his feedback, given the specifics of our house. Will report back!