Hey all! First post here so bear with me if I am forgetting something. I am building a new home in Raleigh NC and need some advice on insulating the basement. It is a walk out basement...with the front portion of the house being mostly underground and the back half above ground. There is poured concrete foundation and on the back half it is framed on top of the slab. Now...we are planning on having this finished down the road, but that is YEARS away. I will be using the basement as my "man cave" and will have my computer junk down there. I will be spending a fair amount of time down there. The rest of the house is spray foamed. Now...what to do down in the basement? The builder was leaning towards spray foaming the ceiling of the basement and leaving the walls/etc alone. Getting spray foam out when we do decide to finish seems like a pain, so I am thinking that is why he is leaning this way. I was thinking of putting in some XPS on the framed sections just as a DIY project down the road. That being said, it isn't too late to do this correctly. What would you do in this situation (without breaking the bank). Thanks all!

If your exterior framed basement walls can dry to the outside I would put up EPS foam on the inside basement walls. Tape the seams and fur it our for electric and then drywall. You need the drywall for fire code but it does not need to be finished.

Compared to spray foam on the ceiling this should be a very cost effective solution.

A well-insulated basement is well worth it in your climate. We are in upstate SC, which is similar. Our basement is ICF, so fully insulated. There is no HVAC, but the coldest it gets in the winter is 60, and the hottest in the summer is 78, which is a pretty tolerable range. I was required to insulate the ceiling despite the walls being insulated; I assume you will be too. Having an insulated basement will also reduce the HVAC load and improve comfort on the main floor as well.

Icf. I know that Build Block is manufactured in Butner NC so shipping is easy. It will save you steps with what you are trying to accomplish

Spray foaming the ceiling of the basement seems extreme if you plan on using the basement as a conditioned space.

Here in VT our builder will do a stud wall with blown cellulose on walk out walls.

Otherwise he uses Dow thermax on the inside concrete walls. Drywall isn't needed on top of Thermax, to my knowledge at least. (we don't have any).


A heat pump hot water heater in combination with a very minimal sized HRV supply and exhaust duct keeps our basement really dry.

Yea I don't know if I will count the basement as "conditioned space." It's going to have a bunch of mountain bikes, a mountain bike "garage" and my computer shoved in a corner so when I'm playing games and yelling my wife is far away...aka "go to your basement to play games and work on bikes and get out of my hair." Half of the time we are actually using the space the back door will be wide open so bikes can get shuffled in/out (more of a underground garage).

We are doing a heat pump hot water heater to help with humidity control down there.

The spray foam on the ceiling appeals to me for sound insulation as well...my computer area will be directly below the master bedroom.

I was out there yesterday when it was in the low 40s and it was bearable with zero insulation anywhere. Probably in the low-mid 50s. I'm thinking once things get zipped up tighter it will only improve.

So yea...it's kinda a weird area so I'm still not certain what I want to do with the insulation down there.

I suppose another option is to frame up the small area I will use for my computer and insulate it, stick with the spray foam on the ceiling as in the contract, and not worry about the "garage" area.

Do these details help clarify the situation at all and change any recommendations?

Thermax the walls, insulate the ceiling with glass or mineral wool and use 5/8" drywall for sound and comfort.

I foamed everything but the basement walls and the joist spacing, including a hot roof, but foaming below floors is rarely called for unless your home unless you wish to isolate the air flow to the next level.

foam doesn't provide much sound attenuation

Raleigh / Durham is on the warm edge of US climate zone 4. An IRC code-min wall in zone 4 would take a hint more than 1.5" of Thermax, but 2" wouldn't be insane. Any brand-X polyiso is good enough if you cover it with half-inch wallboard. A standard solution is to strap the 1.5" polyiso to the foundation with 1x furring through screwed to the foundation with 3.5" masonry. The polyiso is rated about R9, but with a foil-facer pointed at the 3/4" gap it brings it up to a code-min R10 performance. Tape the seams with a pretty-good temperature rated foil tape with an adhesive that goes the distance (eg Nashua 324a, found in most box stores.) You can leave the wallboard unpainted for now if you like-it's serving it's function as a fire rated assembly. Only Dow Thermax has a fire rated facer, and it's fire rating is not sufficient for all applications. If it's a space where you'd be hanging with a computer, go with half-inch wallboard.

Spray foam in the ceiling does squat for sound abatement beyond the air-sealing aspects. R13 kraft faced batts that are not full-depth installed a couple inches from the subfloor and a couple inches from the ceiling gypsum works pretty well. If you want to do better, use Green Glue (tm) on the joist edges when installing the ceiling gypsum.

Be sure to air seal and insulate the band joist and foundation sill before putting up the ceiling. The biggest untreated air leak in most existing homes is at the foundation sill and band joist, usually bigger than all window & door crackage combined.

XPS is the least green foam available, since it's blown with HFC134a (automotive AC refrigerant), with a 100 year global warming potential about 1400x that of CO2. As it leaks out over a handful of decades performance drops to about R4.2 per inch, exactly that of EPS of equal density. EPS is blown with pentane at a 100 year GWP about 7x that of CO2, most of which leaks out of the foam at the factory, where it is recaptured and burned as process heat. Polyiso is also blown with pentane.

Any polystyrene foam (EPS or XPS) absolutely needs to be covered with a thermal barrier to be fire-safe. It has a lower kindling temp than polyiso, and as it burns it melts, rather than charring in place the way polyiso or polyurethane would.