Determining correct target R value for wall
Midwayman
| Posted in General Questions on
I have a large building (zone 4) where the bottom 54″ of the wall is a concrete stem wall, while the rest of the wall (18ft tall) is a steel framed all. I am trying to determine a good target R value for the wall above the concrete.
The plan is to insulate the concrete stem wall with rigid insulation before installing a veneer, but I only have the space on the outside to go so far (roughly 1″ or R5).
Is there a good rule of thumb for R value targets in situations like this? In case it matters, the way I am currently building the wall with rigid exterior and cavity fiberglass insulation, I expect to get to about R12.
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Replies
If the plan is to heat and cool the building 24-7-365, the current R5 is not code complaint and will have you spending a lot of money buying fuel.
Putting insulation in between the steel studs is almost a pointless waste of materials and money.
The way I see it when you selected the steel studs you also chose exterior insulation as your only option. Suck it up make room for R20 or so of insulation.
If this is someplace to keep the tractor dry the don’t heat the space and there is no need for insulation.
To the title of your question, it depends on your point of view and usage plan your current fuel price and your projected future fuel price.
The code inspector has a number from his book.
The property flipper will never spend a dime on anything hidden in a wall.
If you are all about the lowest cost of ownership you need a computer model with lots of numbers.
If you have drunk the Passive house Kool-Aid their craze math will give you a number.
Walta
Walta,
Not a living space, but a space I want to keep >50F during the winter and possibly even cool/dehumidify during the summer as needed. I just didn't know if there was a rule for diminishing returns when you have an appreciable amount of the wall with a relatively low R value. I didn't know if there was a "good enough" value for the wall, then focus more on ceiling/roof insulation.
It is true that the insulation between the girts is largely negated, but consider this: I can get R19 unfaced fiberglass insulation from a local bulk vendor for around $0.35/sqft. Doing the wall calculation, I found that the R19 ends up being R7 due to the metal framing. Sounds like a waste, but considering that 1" of XPS (R5) goes for $.60/sq ft and requires eating up interior space, the fiberglass in the cavity still seemed worth the money.
First off let me congratulate you on what is shaping up to be a very affordable build.
I say stick with your plan. It leaves a good deal to be desired, but it will work for the way you intend to use the building.
If you were planning on conditioning the building year-round to typical indoor temps then you would want to handle things the way Walta has laid out.
Depending on where you are located your local codes should dictate if insulation is required and if so, how much.
If there are no codes that apply then you are looking at it from a financial standpoint. In order to do this typically the construction industry will look at this in terms of payback period. You take the upfront cost of the insulation (materials and labor) and divide it by how much money you save each year on the energy used to heat and cool the space. This will tell you how long it will take to pay for itself also known as the payback period. If the payback period is less than 5 years then it is a good investment for sure. If it is greater than 5 but less than 10 years it's probably still worth while. Anything between 10-20 years is questionable and a judgement call. Anything over 20 years is not a good investment.
Usually to determine the annual energy usage based on different insulation levels we create an energy model. We use software to do this but there are likely hand calculations or rules of thumb that could also be used. One free software I've used before is eQuest but there are others available as well.
For a semi conditioned building on zone4, there is probably no ROI on going above what you propose. The steel is a big thermal bridge, so that section of the wall with batts would about an R5 assembly. If you have not already built it, I would swap the steel for wood which would double the assembly R value there for pretty much free.
The important detail is getting a solid air barrier over the building. A good air barrier generally requires minimal extra materials and a bit of elbow grease but has the biggest impact on heating load. In your case, I'm assuming that would be the rigid, make sure to tape all the seams with a quality tape and seal the rigid to your foundation with acoustic caulk on the bottom. Not sure what your ceiling will be, but the wall air barrier should be continuous across your walls at the top to the ceiling air barrier.
I appreciate all of the feedback. Yes, the exterior layer of foam is glued/taped continuously to give a good air barrier. Also, on top of the XPS is a layer of house wrap just for good measure.
From the feedback, it sounds like my targets are reasonable. It is tempting to try and put some kind of layer of foam on the inside of the wall girts before the wall board (be it plywood or sheetrock) just to get me to around R17. Not sure it is worth the hassle and money.
Why not install the rigid on the outside? You can install it over the house wrap as is.
Akos,
I do have 1" of foam underneath the steel siding and outside of the girts. Is that what you are referring to? Or are you suggesting putting a second layer outside?