The Benefit of Insulating Just Two Basement Walls
verm
| Posted in General Questions on
Hi, Im in NY in zone 5. Im looking to update my basement to a game room and do some insulating. My home was built in 1927 has an exterior foundation with awesome stones embedded in them that I dont want to cover up with rigid foam.
However, one side of the home is covered by a porch and another side a deck. You cannot see the foundation here on either side. Does it make sense for me to add exterior insulation on just these two sides that are not visible? Would it help a little? Or would it just be a waste as heat may just escape to the other sides?
I just got new double pane windows and will insulate the rim joists. Getting a ductless minisplit also. Id rather not build out basement walls.
Thank you
However, one side of the home is covered by a porch and another side a deck. You cannot see the foundation here on either side. Does it make sense for me to add exterior insulation on just these two sides that are not visible? Would it help a little? Or would it just be a waste as heat may just escape to the other sides?
I just got new double pane windows and will insulate the rim joists. Getting a ductless minisplit also. Id rather not build out basement walls.
Thank you
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Imagine a wall 10ft x 10ft. If I insulate 1/2 the wall to R100 while the other half is R0, what is the total r value of the wall? It's R0. Similarly, if your insulation result is 10x greater R value on the insulated walls vs non-insulated, the total improvement will be negligible.
I don’t agree that the math would be R0, it would certainly lower the effectiveness of having some insulation but it wouldn’t revert to 0. What do you mean both foundations are covered? Are they underground? if you can’t access or see the exterior you can insulate walls from the interior in the whole basement or on just those 2 sides.
The foundation walls on front/back of house cannot been seen due to being underneath the porch and deck. I would have access to apply rigid foam.
The result wouldn't be R-0, but it also wouldn't be the average of the insulated and uninsulated sides, because masonry is a good conductor of heat--it will move horizontally, short-circuiting the insulation to some degree.
Can you insulate the entire interior instead of part of the exterior?
> it also wouldn't be the average of the insulated and uninsulated sides
It never is, even without a masonry thermal bridge.
Jon, I'm not sure I follow--if there were two walls insulated to R-20 and two walls totalling the same area insulated to R-10, the effective R-value would be very close to R-15.
Michael, not exactly. You have to use the UA approach. Heat loss is proportional to the U value times area, U*A. U value is the inverse of R-value. So, if the walls are 10x10 or 100 sf, and one wall is insulated to R-10 (U=.1) and the other is R-20 (U=.05), the average U-value is
U = [100 (.1) + 100 (.05)]/200 = .075, or R-13.
In Jay's post (#1 above), the total R value is undefined because he assumed R-0 for the uninsulated wall. In practice, there is no such thing. But, if we assume an uninsulated masonry wall at around R-1 with half insulated to R-100 and half uninsulated, the total R-value comes in around R-2. That's not very good. The first few inches of insulation make the most difference, and it's way more effective to add a little bit of insulation to the whole wall than a lot of insulation to part of the wall.
Peter,
There is no way that a large basement with two of the walls insulated with foam to R-100 and two left uninsulated experiences an aggregate resistance to heat loss equivalent to R-2.
Edit: Peter you are of course right. I was thinking of it in terms of the thermal bridge the concrete provides between the two differing walls, as opposed to the negligible R-value of half the perimeter.
Peter and Jon, thanks for setting me straight. Malcolm, I'm sorry to say that their math is correct, which highlights why it's important to consider thermal bridges, as they skew the total R-value (or U-factor) disproportionately.
Yeah, it isn't R2 , it is 1.98!
But one mistake is that acting like R2 is a bad thing, it is half the heat loss of R1
But that calculation only works for a tent, not a concrete wall. If concrete walls worked that way every basement would be an iceblock, and they are not.
Of course one wouldnt insulate to R100 because it is a waste of money and resources on a masonry wall, but R10 or 20 would yield 40 or 45 percent heat loss reduction, while R100 would give you 49.5 percent reduction[assuming a tent and equal wall sizes]
Deleted
> R-20 and ... R-10, the effective R-value would be very close to R-15
Run through an example, calculating the heat loss and then an effective R-value. 1÷((1÷10)÷2 + (1÷20)÷2) = an effective R value of 13.3.
This is why it's more effective to have average R values everywhere vs high R in some areas and low in others.
Another way to think about upgrades is that the heat loss elsewhere has little to no* effect on the heat loss on the area that you are considering insulating. "I'm not going to insulate this wall because the other wall has little insulation" isn't logical.
* - minor exceptions for thermal bridging or reduced interior temperature
Edit: Peter beat me to it.
Yes, all this math about effective R values is distracting from the simplicity of the fact that the losses are independent. Insulating two of four walls provides half the savings that you'd get insulating all four. If it's half the cost, the cost/benefit is the same.
Not sure I am agreeing with the physics here
While you will see [with a thermal camera] due to the thermal mass of the concrete the edges of the wall going from equal to the uninsulated portion at the ends to warmer in the center, you have not fundamentally changed the square footage of insulated vs lesser insulated wall. Certainly it is less than ideal, but it is certainly worth doing.
If this is really the basement, consider doing the foam board on the inside and covering with sheetrock. There is a certain fascination hereabouts with building out a full stud wall that I do not understand, perhaps because the cost of interior space is so high. I would just bond it to the inside and cover, or sleepers and foam/rockwool and sheetrock.
Point being that if you were willing to do it inside, that would be the more complete solution to insulating that the exterior version, due to aesthetics, would never be
Keith,
I think part of the preference for basement stud walls comes from the necessity of having a cavity to run services in.
I'm curious - one can't route/melt a 1" channel in 2-3" EPS and use this for services?
Jon,
I guess it would depend on how your inspector interpreted the protection requirements for wires in the electrical code. Either way you would still have to hog out a large area of foam at each box or switch and somehow find solid attachment for them.
Well, they tax by exterior square footage around here, and I will be danged if I am going to give up a square foot I don't have to for a stud wall that doesn't hold anything up, no less to give space for a couple wires. Better to run down from the ceiling and put a nail plate over.
The stud wall is also typically used for those that want to beef up the insulation. Like use fluffy insulation in front of foam board. To run services you could stack 1 inch foam board and run services in front of first layer. I didn’t trust glue alone in my situation or just one connecter at top. I ran 1x4 on the flat as furring strips. That’s only 3/4 of inch of lost space. You could also run 2x4 on flat which is still thinner than a typical stud wall.
Keith,
It's not just the wires, it's the boxes too. And most basement walls are also not perfectly straight, smooth or plumb. Then there are the complications around fastening the drywall and trim.
I'm not advocating using stud walls, just saying it an understandable preference.
Depending on the situation, it could be _highly_ effective to insulate two walls. There will be some horizontal conduction of heat, but assuming your walls are "long", that doesn't change the calculations.
Let's say your walls are 30' and 40', by 7' high. So the area is 490 sq ft. The U-value (BTU/(ft^2 * hr *degF)) is the inverse of R, and lets say the concrete/stone wall has R=2 and 2 inches of foam R=10 (R=12 for foam + wall).
for a 50 deg temperature difference inside to outside, BTUs/hr heat loss for the insulated and uninsulated walls are 2042 and 12250;
for 40 deg diff 1633 and 9800
30 deg 1225 and 7350
20 deg 817 and 4900
If you're heating your basement, my interpretation is that these are pretty big numbers, and you'll see it in your energy bill and feel the difference. The difference is a factor of 6 (R12/R2). So if you make the existing wall R1, then the differences in energy use between insulated and uninsulated will be even larger.
While I agree with "worth doing", lateral heat transfer does change the numbers. The THERM software would do it right.
It's true that R values don't really average. It's also true that a wall insulated half R100 and half R0 is going to perform quite a bit better than if the entire wall was R0. The reasons are that insulation anywhere does slow the rate of thermal energy moving through the wall, and masonry, while a poor insulator, is not a perfect conductor of heat either.
If you can only insulate two walls, I say go for it -- you likely WILL see a noticeable improvement, just not as much as if you insulated all the walls. It's also true that doubling the insulation on the walls that you DO insulate isn't enough to "make up" for the walls that you DON'T insulate.
Allison Bailes wrote a pretty good article explaining why you can't do simple averages when working with R values. A quick bit of google mojo and I was able to find his article here:
https://www.energyvanguard.com/blog/Flat-or-Lumpy-How-Would-You-Like-Your-Insulation
Bill
Well, the math is the math. But what it really means is that heat loss to R value is not a linear function.
IF you have an R1 roof 1000 sq ft with a delta T of 50 degrees it will lose:
50,000 BTU at R1
5000 BTU at R10
2500 BTU at R20
1000 BTU at R50
500 BTU at R100
So that step from R1 to R10 actually did the heavy lifting if you will, and all the R value added over that is really about heating[or cooling] many houses over many hours.
It is not really some weird calculations required because of partial insulation or concrete doing something funny
Verm,
Northern Star and Building America have developed a technique for exterior insulation using hydro-vacuum trenching for the excavation, which can allow you to insulate under patios and decks by "cutting in" from the sides. Basically, it uses a high-pressure spray of water to cut the soil away and a vacuum to suck up the resulting mud. This leaves you with a narrow trench (minimum 4") that you can fill with pourable spray foam or insulating concrete.
You can find the initial study here: https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/excavationless_exterior_found.pdf and the follow-on field study here: https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/excavationless_exterior_fountain_study.pdf
All, thank you very much for your input. I didnt think it would be such a debate! I think what I got from this, is that if I insulate only 2 walls it will help over having none insulated.
Thank you
It the internet!
there will be debate on what color black is
I mean, that's the fun, right?