Insulation overkill or not
I am in the later stages of finishing a 2500 sq ft 2nd floor attic cathedral space under the roof above a barn/garage. The construction has 2×8 inch rough sawn purlins located horizontally 24 on center between vertically sloping 12 inch LVLs supporting the roof. I am planning to fur out the purlins with on-edge 2x4s set vertically (and a 1/2inch spacer) so that I can finish the whole ceiling with horizontally applied 1 x 12 shiplap on the same plane as the LVLS.
The current insulation has 2 inches of sprayed closed cell foam under the roof deck with 5 inches of ‘hybrid’ open cell spray foam between the purlins. I believe that gives me an approximate R-value of just around R32? Sounds low to me., perhaps?
My questions are threefold:
Is that insulation enough to minimize the heat/cooling load on a mini split heating and cooling system in Zone 5. I am considering adding R15 Rockwool or is that just overkill? I keep thinking I need more insulation, but my spray foam contractor insists that what I have is more than enough and that adding more would be deep in the zone of diminishing returns.
What do you think?
Or would a more cost effective solution be to simply add inch or 1/2 inch of foam board attached to the exposed 2inch purlins to minimize conductive heat loss to the roof deck through the 2×8 purlins?
Maybe the answer is to do both. If I did this, would it still make sense to add insulation, be it Kraft fiberglass (more cost effective) or Rockwool? I guess this approach would mitigate thermal bridging from the purlins except for where the vertical 2x4s intersect with the purlins?
I am looking to balance upfront cost vs future operating cost.
Many thanks for your comments/suggestions
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
The minimum roof insulation level recommended in the IRC for climate zone 5 is currently R-60: https://codes.iccsafe.org/content/IRC2021P1/chapter-11-re-energy-efficiency#IRC2021P1_Pt04_Ch11_SecN1102. Previous iterations required R-49. Your proposed R-32 (over time it will be closer to R-30) is a fraction of what is required for "the worst house you can legally build." Spray foam proponents argue that its air-sealing abilities mean you can use less insulation. That is false; R-value and air-sealing are separate issues, and any insulation can (and should) be made airtight.
In an unvented ceiling assembly in climate zone 5, you need at least 30% of the total R-value to be in the closed-cell layer, or else you are at risk of moisture accumulation (aka condensation). https://codes.iccsafe.org/content/IRC2021P1/chapter-8-roof-ceiling-construction#IRC2021P1_Pt03_Ch08_SecR806.5
I recommend designing a vented roof with materials that do not contribute significantly to global warming, one of several weak points of closed cell foam. If you are set on using foam, at least meet code-minimum requirements for total R-value and for the ratio of closed-cell to open-cell.
For more information, check out these articles and the related ones listed in the sidebars:
https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Michael,
I'm curious about how this works from a code perspective? You wrote: "The minimum roof insulation level recommended in the IRC for climate zone 5 is currently R-60". I realize that your building codes and enforcement vary by region, but typically when doing upgrades like this is there a requirement to insulate to certain levels, or just a recommendation?
Is that insulation enough to minimize the heat/cooling load on a mini split heating and cooling system in Zone 5. I am considering adding R15 Rockwool or is that just overkill?
These are two distinct questions:
1. Adding insulation will reduce the heat/cooling load.
2. Adding insulation to existing R-32 is overkill and you can skip it. If you calculate out the cost of the energy you're saving (i.e. going from R-32 to R-60 in a place with 6000 HDD saves you about 2100 btu/year/sqft, which discounted over 30 years at 5% is about 32kbtu. That's about $.38 in my market, and Rockwool will cost maybe 4-10x that per sqft, installed).
To show the math a different way, your roof surface area is probably about 3500 sq.ft.. Climate zone 5 can have anywhere from 5400 to 7200 heating degree days, so let's Paul's low side of average, 6,000 HDD.
Going from R-32 to R-49 in your roof will save about 5.5 million Btus per year. Energy costs are currently averaging anywhere from $16 to $34 per million Btus, depending on location and fuel type, which translates to an annual energy savings of $88 to $187. Over 30 years that's $2,640 to $5,610.
That's assuming energy costs never go up. They will go up; you can use a net present value calculator to figure out the eventual savings. But even if costs remain constant, over 30 years the insulation will pay for itself (unless it's high-cost foam), you will have better comfort and your HVAC systems will not have to work as hard, and may even be able to be downsized.
If it costs $2/sqft to get from R-32 to R-49, you're paying $83/MMBtu assuming 5%, 30 years, and 6000 HDD. That's steep, almost $8/gallon of heating oil. If 30 years is too ambitious, 15 years is $123/MMBtu, almost $14/gallon of heating oil. Seems more prudent to buy the energy than save it.
Will it be more comfortable? Maybe! It should only drop your heat loss about 2,500 btus at 0 degrees outside.
Paul, it would be helpful if you explained your math in more detail rather than using shorthand. Assuming 5% what? How do you get $8-14 heating oil?
I think what you're trying to say is that the cost of the insulation over time would need heating oil to be $8/gallon or more to "pay off" the insulation investment. I agree with Michael that you need to describe your process a bit more to be more clear.
Comfort is almost assured with more insulation, especially if an existing structure is starting out on the light side in terms of installed R value. I think it's also safe to assume increasing energy costs over time -- even just over the past year natural gas prices have almost doubled in my area, and it's higher than a year ago everywhere. If you're thinking of reasonable levels of insulation, you can think of it as a "it will probably pay off, but if energy costs don't increase, then I just bought an insurance policy in case they do". It's very difficult to accurately project out costs over a 30 year period. Future worth calculations assume interest rates, but don't account for much else. I tell my customers that it's hard to predict things over a 5 year time horizon, and much more than that is fortune telling.
If the question was "I'm thinking of going from R49 ro R100", then it's probably not going to pay off anytime soon. Going from around R30 to R49 though is a much more reasonable upgrade, with lower costs and a higher liklihood of working out financially over a reasonable timeframe too.
Bill
Sorry Bill and Michael, hopefully this attachment makes it clear. The gist is that something today (energy, money, otherwise) is worth more to us than something tomorrow. So spending money on insulation today has an energy cost associated with it, and is generally pretty high, assuming these is some existing insulation, as in this case. So in the attached scenario 2, insulating at $2/sqft would be equivalent to agreeing to a 30 year fixed cost oil contract of $9.20/gallon, but obviously interest rate, term length, etc. are all customizable. Shorter terms and higher rates make insulation less favorable, and vice versa. I used oil because it seems to be the most expensive heat right now, which should favor the insulation. I personally wouldn't agree to a oil contract with these terms (my electric heat pump burns the equivalent of $1.35 ish oil), but everyone's situation is different.
It would be easier to follow if you showed the row names (letters) too.
That doesn't look quite right to me, but I kinda see what you're doing. When I look at these kinds of problems, I consider the interest in some ways to cancel out. The reason is that if we assume a constant rate of inflation, that same inflation applies to the fuel too -- so the cost of the input BTUs increases over time. The comparison with future worth / present worth is basically "how does spending money on [some improvement] compare with just investing that money in some kind of index fund", where the investment would grow, presumably at least at around the rate of inflation. If the improvement has a better return, it's a better investment. If putting the money in the bank is better, than the improvement isn't finacially justified, basically.
The issue with insulation is that if you just invest the money, it AND the cost of fuel BOTH go up over time, but if you buy the insulation NOW, it's cost is FIXED. Fuel costs increase over time, which essentially cancels out what would have been the return if you just put the money in the bank instead of buying insulation with it.
I like to look at these things as a value over a project lifetime, which would mean cumulative fuel costs assuming similar heating BTUs per season. Would that cumulative cost savings pay for the insulation over the time period involved? If it would, then you're better off buying insulation instead of putting the money in the bank.
Maybe you did try to allow for that and I missed it in your spreadsheet snippet, but it looks to me like you're doing a usual present worth calculation for a fixed asset, and you're not allowing for an ongoing cost that will also increase by at least the rate of inflation over time.
Bill
Hi there Paul and Michael,
Thank you for your thoughtful replies. Your responses made me realize that I omitted a couple of details that might make answering my questions easier. (You're right about the area being around 3500sq ft.)
Firstly, the space is a retro fit of an already existing structure. The spray foam insulation is already done. So I'm really asking is it worth the extra material cost to install R-13 or R-15 insulation between the vertical 2x4s? I do the work myself. So labor is not a huge factor and so far as I can see from vendors, R-13 Kraft faced runs about $0.65 per sq ft.
The other point is that I expect the use of the space to be occasional at best as a hang-out space for kids in summer and winter and/or as a fitness or art studio. When unoocupied (more than 50% of the time) I expect it need be no warmer than 50 degrees and in summer cooler than 73, say when occupied.
I'm thinking that when installing the insulation, I'd need to try to minimize gaps between the new insulation and the existing spray foam - is that right?
I'm wondering how this new information would adjust your point of view. I am on a budget and don't believe I can afford at this point to get to R60, but I think if I used R-15 or even slightly compressed R-19 insulation, I could get close to R49 without spending the earth.
Does that sound crazy?
Spray foam is almost never flat, so it will be easiest to install lower density fiberglass batts that are more compressible. I would try to make the installation easier as a priority here rather than trying to squeeze out the maximum possible R value for the assembly. I think the kraft facer acting as a vapor retarder is a plus here too, since I'm a little concerned about the ratio of closed cell to "other stuff" in that assembly. A vapor retarder will provide some insurance against possible moisture issues in the future, and it will simplify the install a bit too by supporting the insulation as you install it before the finished ceiling goes up.
While I would normally advise to try to hit code minimum levels of insulation as a minimum, in your particular case it's probably not worth going to extremes to try to hit R60 if you can only get up to around R49 or so in the existing structure (which is what it sounds like here).
BTW, Michael is absolutely right about the spray foam people's claims about their product being "special" in terms of R value are marketing and little more. R value is R value, air sealing is a different thing altogether. Spray foam R value is not superior to any other material's R value here.
Bill
It sounds expensive to me for a little occupied space and that's $2400 without your time factored in. What's your heating fuel?
Berkshirecarrot, I missed that your spray foam is already in place. Although adding insulation will save energy (and makes for an interesting discussion on what should and shouldn't be important), another issue is that when you have a thin layer of closed-cell foam, which is vapor-closed, and the rest of the insulation is vapor-open (open cell foam, mineral wool, fiberglass, cellulose) the vapor-open insulation keeps the interior surface of the closed-cell foam from indoor heat, keeping the surface of the closed-cell foam cool, but allowing water vapor to pass through from the building interior and potentially condensing when it reaches the cool, closed-cell foam. In climate zone 5 you need at least 30% of the total roof R-value to be in the closed-cell foam layer, so in your case I would not recommend adding insulation, especially since it's not a heavily used space. I would put any attention into air-sealing; although foam itself is usually air-tight, there are a lot of edges and transition areas that don't have foam. Seal those up and you'll get a lot of bang for your buck (and time).
Sorry Bill, attached is a more useful screenshot. Another assumption I forgot to mention is that I believe insulation improvements have no resale value, which I think it pretty safe since it's a code requirement for new builds (meaning it probably wouldn't happen otherwise and often is poorly enforced). A key distinction between insulation and another investment that earns the same amount of interest is that a CD/Savings account/Index fund is easy to get out of - the only way to get cash out of insulation is to stay in the house. It's like having a gold tooth vs. a gold coin. That friction is worth money! Even paying off a mortgage is a better investment even if the % is the same - that's guaranteed, while energy prices are not, and often fall for years at a time. I think insulation improvements should be evaluated with a much higher interest rate than 5%, personally. I don't think the rate should just be the inflation rate, as this is more complicated with more risk than your typical investment.
More insulation, at least beyond what is required for basic comfort levels, isn't really going to do anything for resale value, you're absolutely right about that! If you watch any of those "renovation" shows, you do get a decent idea of what increases value, and it's all appearance stuff. Paint is a big one! How often do you see them doing anything with mechanicals or insulation? even for structural stuff, it's usually either to "open things up" (which is visual again), or to fix some glaring defect. Resale value is primarily based on what people can SEE.
I don't know if I'd consider insulation to be a high risk investment though, since it's never going to lose money on it's own, it can own lose money in comparison to putting that money something else. I also think code minimum levels of insulation are starting to get into the area of diminishing returns already, especially in some areas (like basement walls). It's a lot easier to justify going from R30 to R49 than it is to justify going from R49 to R100, for example. I do think little cheap upgrades like going from R30 to R60 may be worthwhile if you were already planning to go up to "at least" R49 though, simply because it's very cheap to add a little more with loose fill.
Your number still seem off to me though, but it's probably from my being used to looking at things differently. If I see tens of thousands of dollars of savings over 10, 15 years, I'm thinking that insulation was a pretty decent investment, because you could be doing other things with those savings over time. Insulation is a one-time deal, buying energy keeps going forever. I would not expect energy costs to go down, either. The overall trend has been higher for quite some time and is unlikely to change over the long term.
Bill
Buying energy only goes on as long as you stay in the house, which the median American stays in for 13 years. I agree, the shinier the home improvement, the more likely it seems to payback. Stainless steel refrigerators seem to be the best payback.
For this example: 3500 sqft x 1561 btu/sqft saved per year /1000000 = just 5.5MMBtu/year. That's only $67/year for me, about $250/year if someone is in the unfortunate situation of $5/gallon heating oil this winter. A 10 year payback if you can do it for $.71/sqft and oil averages $5/gallon for 10 years, but that's quite the parlay, especially since fuel switching is probably feasible in that scenario. It'd be a 37 year payback for my rates.
Play with the spreadsheet some, let me know what you find!
Paul,
At the risk of this sliding into a wider discussion on conservation (which based on some of the recent blogs may or may not be fruitful...), is ROI the main metric we should be using to evaluate levels of energy use? I agree it's an important one, but concentrating on it alone may mean considerations on how much energy houses should consume for other reasons aren't factored in.
No, of course not. That said, berkshirecarrot did say budget was a consideration and it frees up money for more impactful green investments. I think we all agree on this point, since no one suggested going to R-200, so it's understood that some levels of insulation are mostly useless, financially, comfort wise, etc.
Paul,
For sure.
I have been thinking about ROI a bit differently recently. Our electricity is currently so cheap here it's hard to justify any improvements based on ROI. But I look at what has happened recently in Britain. Their housing stock is for a variety of reasons (age? a hide-bound construction industry?) very energy inefficient. Fuel prices have also been low enough not to provide a huge incentive to improve it. Now they have seen increases that are both individually and collectively economically devastating. A bit of relatively inexpensive future-proofing looks better and better to me.
Some of Europe has seen natural gas prices up 600%, so definetely devastating. A lot of that is from a "get it over there instead of here" problem, which causes unreliability of supply. For whatever unfortunate reason, sometimes there is a misguided "green" idea that getting it from "over there" is somehow better than getting it from "here". Maybe it's NIMBY (Not In My Back Yard), a sort of "out of sight, out of mind", or maybe it's a version of the "don't tax me, don't tax thee, tax the guy behind that tree". Regardless, it's a problem. I've never liked the idea of "buying carbon offsets" either, since it's a sort of way to cheat if you have the money, and lets people feel good without really chaning their ways at all.
Anyway, without getting too far down that rabbit hole, ROI is part of things, but probably not all of it. Code minimums these days are pretty good in terms of keeping overall energy efficiency under control on the insulation side of things, so going much over those get's into poor ROI territory. If you're renovating, things are different. Old R19 requirements can be hugely improved upon. Air sealing can make a REALLY big difference, since it wasn't really much of a concern in the construction world until relatively recently. This all means insulate and air seal projects on older houses have a much better return than they do on newer houses, where the low hanging fruit stuff has pretty much already been done.
I like to think of insulation as cheap insurance with a premium you only have to pay once. As long as you're not going crazy (like R200 :-), you're probably going to be OK. Try for R49-R60 in an attic if using loose fill on the attic floor, put some reasonable effort into air sealing, use exterior rigid foam that doesn't get into the "thick" range of over 2" or so, and you're going to be in pretty good shape without really pushing the limits on the budget side. For most regular houses, I think those are pretty good goals. If you want to try for net zero, superinsulation and stuff like that, that's OK too, but the financial ROI isn't really there. It all comes down to what your goals are for any particular project. I personally do think putting some extra into insulating and air sealing is a good idea though, since you will always save some money on energy, the initial investment is usually relatively small if you're keeping things reasonable, and there are noticeable comfort improvements to be had as well.
Bill
Doesn't matter wall or roof, in zone 5, most of your energy is saved with an R25 assembly. Above that you quickly hit the law of diminishing returns.
Your assembly (2"ccSPF+5"ocSFP between purlins) works out to about a R26 once you factor in the thermal bridging from the purlins, so you are pretty much there. Much more R value only makes sense if it can be done for very cheap which isn't the case here.
If you add more insulation, you also run into condensation control ratio limits. Your 2" of ccSFP is only good condensation control of R19.5 of permeable insulation in Zone 5, your 5" is ocSPF is pretty much there.
Thanks again to everyone for the comments. Very interesting and helpful indeed. I think I'm going to end up going with R-19 Kraft faced insulation. At 6 1/4 inches thick, I recognize that it'll be somewhat compressed in front of the purlins losing some of that R value, but in the bays between the purlins there is mostly an extra inch or so. Having the extra thickness will help fill any cavities there.
Hopefully this is a decision I won't later regret! All I do know is that it would be very frustrating to do all the work of installing the shiplap final wall/ceiling covering etc only to find the place is a constant bear to heat or cool!