Insulating top half of basement wall
user-7653783
| Posted in General Questions on
Hi
I am considering insulating only the top half of my basement foundation wall in the unfinished portion of my basement. The bottom half has waste, vent and water pipes attached to it which makes it difficult to insulate. There won’t be any drywall in front of this half insulated wall since this area of the basement is unfinished. questions:
1) will insulating the top half of the wall help with energy efficiency? About 2 feet of this wall is above grade
2) will this cause moisture issues?
thanks
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Is your rim joist insulated? I guess I also wonder how far you would get with a super insulated rim joist without the rest of the wall insulated. Would it make a huge difference between having just the rim joist or no insulation at all.
No my rim joist is not insulated in this area because it is parallel to the ceiling/floor joists and very hard to access
What climate zone are you in? It seems likely it'll help with energy efficiency. This site http://www.sensiblehouse.org/nrg_heatloss.htm has a good discussion of heat transfer, including through soil, and it's has the info you need should you want to estimate heat loss.
I don't know about moisture - you'd presumably be reducing the temperature of your wall, thereby increasing the likelihood of condensation. Moisture won't hurt a concrete wall (is your wall concrete, CMU, or?) but might promote mold. Perhaps a good idea to insulate with impermeable foam (e.g. faced ISO) and seal the edges. I'm guessing the experts on this site will ask you about relative humidity, humidity controls, and any historical water problems in your basement before attempting an answer.
Generally the rule is that insulating part of a wall is like waterproofing part of the bottom of a boat.
If you have a wall that is insulated to R2, and you up the insulation on half of the wall to R20, the average combined insulation is R3.6. Which is better than R2, but not close to the R20 you're paying for.
But basements can be tricky, if part of the wall is above ground insulating that part already has different heat loss characteristics.
I would think about ways to insulate around and behind the pipes.
You essentially have three "zones" in a basement wall:
1- The part ABOVE grade. This is where you lose the most heat
2- The part BELOW grade, but ABOVE the frost line. This is the next big heat loss area.
3- The part BELOW the frost line. This area never goes below freezing, and while you still lose heat here, it's less than the other two zones.
You get the most benefit by insulating the parts of the wall that are above the frost line, since those are the parts of the wall that see the largest temperature differentials and thus have the largest heat loss. Since you're going to be insulating all of the part of the wall that is above grade, you'll get a big benefit, especially since you said there are two feet above grade. If you can extend down to the frost line, you'll get a big chunk of the benefits that you'd get by insulating the entire wall.
It's definetely worth insulating the upper four feet of this wall, you'll see a big improvement there. If you can insulate down to at least the frost line everywhere else, that's worth doing too. Insulating the entire wall is obviously optimal, but not required to see improvement in the overall heat loss from the basement wall.
You won't have moisture issues, especially if you have a capillary break between the top of the masonry and the wood sill plate. Leaving part of the wall uninsulated actually allows for more drying potential and less chance of moisture wicking it's way up the wall.
As others have mentioned, make sure to air seal and insulate your rim joist as part of this project. That area will have similar levels of heat loss to the above-grade portion of your basement wall, and probably a lot more air leaks too.
Bill
Thanks all
Does it fit out to be an exact math problem of heat loss, I’m sure there are a lot of variables. This is certainly not optimal but if you obtained the highest r value you could reach in your rim joist assuming average depth. How would you do it and what would be your expected r value overall? As expensive as it may be if you somehow ended up with a average r value of slightly over 3 that is like you adding an 1in of eps to your entire wall.