New construction basement insulation

[DoItMyself]

I have a walk-out basement and need to get R-25. I was hoping to fur out the 12" thick block walls with 2x4 but not sure how I can get the R-25 (or in fact what the best way to insulate block walls is without worrying about condensation). Does the block wall count for anything or do I need to find a way to get R-25 just from insulation?

[concretemasonry]

Why in the world do you need an R25 for a below grade wall?

Even if it is partially below grade, any reasonable code should recognize the moderating effects of the soil and the mass of the wall.

It sounds like a prescriptive standard for a small part of the wall that is being grossly improperly applied to the majority of the basement. There is no way you can justify the cost of the excessive insulation compared to a cost or comfort measure.

Even in our cold climate, that is really not justified or logical.

Even fiberglass with an R25 rating will not give you a R25 wall. That is the problem with prescriptine R-value requirements.

If you want a good insulation for a basement fiberglass is really not a good choice.- Look at extruded (not expanded) foams.

[bkrahmer]

I agree that this sounds fishy. I'm very happy with my QuadLock ICF walls in my basement. They're about R-22, and require very little heat to heat my basement in the winter, and no cooling in the summer.

[homebild]

Energy Codes as well as Building Codes recognize 'walk out' walls as above grade walls, and they must therefore be insulated just as above grade walls.

Basement walls that are less than 50% below grade are also considered above grade walls and must be insulated the same as above-grade walls.

The only time the 'mass' of basement walls are considered in reducing the need for insulation is when they are truly 'mass' walls.

Poured concrete walls are 'mass' walls...as are rubble walls, log walls, and solid block concrete masonry unit walls (CMUs)...and are given a credit of about R-5 regardless of thickness, but hollow-core CMUs have no R-value at all regardless if they above or below grade.

That said, where do you live that you think you need an R-25 for an above grade wall?

Most Building and energy codes in the US only require a maximum of R-21 for walls even in Northern Michigan, Wisconsin, Minnesota, Montana and all of Alaska.

Are you in Canada or some other nation?

Otherwise, most 'cold climates' in the US only require R-19 (or less) for above grade walls and only R-10 (or less) for below grade basement walls.

Please provide more information.

[DoItMyself]

This is located oddly enough in sunny Southern California, but in the Southern Sierra Mountains at 6,000 ft elevation. Plans state "walls to be filled with R25 insulation", but doesn't differentiate between which walls. This is a walk-out basement, 12" filled concrete block for two of the walls, SIP panel walls (R25) for the other two walls that see daylight. The block walls are a rather massive retaining wall, so plenty of dirt behind them.

The basement will be finished, and I will fur out the block walls, but not sure if I actually have to insulate the furred walls with R25 (or anything, for that matter), and if so, seems the only way to accomplish that is with 2x6 instead of 2x4 or 2x3, with extruded foam, as nothing else seems to get me to R25.

[homebild]

This is located oddly enough in sunny Southern California, but in the Southern Sierra Mountains at 6,000 ft elevation. Plans state "walls to be filled with R25 insulation", but doesn't differentiate between which walls. This is a walk-out basement, 12" filled concrete block for two of the walls, SIP panel walls (R25) for the other two walls that see daylight. The block walls are a rather massive retaining wall, so plenty of dirt behind them.

The basement will be finished, and I will fur out the block walls, but not sure if I actually have to insulate the furred walls with R25 (or anything, for that matter), and if so, seems the only way to accomplish that is with 2x6 instead of 2x4 or 2x3, with extruded foam, as nothing else seems to get me to R25.

Even in your location, the maximum R-value for above grade walls you would ever need is R-19. The SIPs may be coming in a standard R-25 format which may be why they are specified that way.

Your below grade walls will still need to be insulated even though the hollow block will be filled with insulation. R-10 continuous or R-13 cavity insulation is is still required.

Block walls and dirt do not provide as much insulating ability as most presume. You get a small credit for having walls below grade and/or if they are filled with insulation (about an R-5), but that's it.

The R-10/R-13 values assume this R-5 credit, so you still need to add R-10 or R-13 to the block walls depending on how you plan to insulate them.

[DoItMyself]

That makes sense, thanks for your help, I appreciate it.

I FINALLY heard back from the company that sold me the plans and SIPS panels, and they confim what your saying. They indicate 2x4 with fiberglass batt or blown in cellulose for a range of R13-R19.