derate for sloppy batt insulation?
JohnWalker
| Posted in Energy, Heating & Insulation on
Hi
I used the calculator on the ORNL site for a 2×6 @ 16″ with R19 fiberglass batt, drywall and wood siding to come up with clear wall R17.48, whole wall R13.65.
On another site I was reading about derating the “R19” value depending on the actual workmanship of the installation job. That is to say the “R19” value is based on a perfect installation without gaps, compressed areas etc.
Anyone care to hazard a guess at what the derate might be for spec homes? I am thinking perhaps R1.5 less.
Any insights appreciated.
John
Replies
Depends on the level of sloppiness IMO. I've seen jobs where the insulator stuffs a full batt into e.g. a 12" space and expects a 16" batt to 'make do' in a 18" space (or cuts it way small). Some don't even put the cut 'plug' behind outlet boxes and other obstructions. I think some old tests would say that 1.5 derrate on a bad job is too generous; Many would derrate by R-5 or more, I think.
Personally I wouldn't accept a sloppy job, so I wouldn't derrate it much. Some energy codes simply state that all voids must be filled. When a contractor starts a job, I explain the level of workmanship that is required. I encourage cutting batts accurately +1/4" -0". Cut it like sheetrock, I say. Push into the cavities fully and fluff back out to the surface of the framing. If the insulator routinely doesn't do this, you could end up w/ two significant air spaces at each framing bay that will significantly reduce the R-value.
If you have no control over the sub, that can be difficult. Inulation is often the bottom of the skill barrell and one of the worst jobs (handling that stuff). Staff turnover can be high, training is minimal. It's a part of the job requiring no 'journeyman' status. It is ultimately hidden, so who gives a rip? It's up to those in the industry that understand the value of a better job that need to push for the best workmanship. It's a job like any other and we should expect people to be doing a good job.
If you have control, spot check the job and hold the payment over their heads. Be a good businessman to start with by expressing your expectations simply and upfront. A reputable insulator knows the [R] value of a good job. We need to raise the bar on good craftsmanship in insulating.
Some don't even put the cut 'plug' behind outlet boxes and other obstructions.
I will often cut a good size piece, matbe 12x12 and split it, place it behind the out let or box and perhaps staple the top two corners to keep it from sliding down.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." — Sherlock Holmes, 1896
If you cut it too large and then place the full batt in place you end up w/ a void. If you staple it in place, you also create a less than desireable situation. It's just as easy, if not easier, to take the entire plug you cut out for the box and compress it slightly and place behind the box. The compression keeps it in place and provides a higher insulation value than cutting or splitting a piece for the correct depth. The full batt goes in and fits like a glove. You waste no insulation out of the piece, either.
Try spraying a little foam behind the electrical box. Forget putting fiberglass behind the electrical boxes.
Hmm ... a LITTLE foam? I think that would be worse, even though the concept is good. That is a lot of foam, IMO and then you have to either trim it square or again, possibly have some gaps in your insulation. Sounds like a lot more skill involved as well putting in enough, but not too much. Admitedly, no easy answers/solutions.
More I think about it, the more I like the cellulose BIB or whatever they call it these days ... moist applied and it fills cavities better. Messier install w/ lots of cleanup, though (although minimized if the netting is used, I suppose).
To me, insulating is another trade like any other. There is skill involved. The details ARE important. Doing a good job is important.
Thanks all. What I am trying to do is establish a real world baseline value for a typical spec house of that construction (not actually hiring these guys).
Does anyone else think a derate of R5 is in order? That would push the clear wall down to R12.5, and the whole wall down to R8.5 (roughly). This derate is fairly key to my calculations.
Ultimately I want to design for "code +50%" (got that from the "Green from the ground up" video and it seems about right). I would modify the calculation though to be "real world baseline + at least 50%". That's roughly clear wall R18.75, whole wall R12.75
I am really sold on wet blown cellulose (the kind with a binder that hardens). It shouldn't be too hard to find a wall assembly design that meets the required clear wall/whole wall R values on ASHRAE, ORNL, NRC, etc.
Thanks
John
John,
For more information on installing fiberglass insulation and "de-rating" sloppy installations, check out this article:
http://www.greenbuildingadvisor.com/blogs/dept/musings/installing-fiberglass-right
Do I/we know the average penalty for sloppy insulation in the real world? I certainly don't. My fiberglass installer did a good job. I'd derate by only 5% maybe, but even that is simply a guess. But according to the average install in the article by the other poster, the problem can be significant. I'm guessing the derating factor can vary substantially from say R-3 to R-5 or even more. What kinds of calculations are you trying to do?
Seems that without actually testing some sample sloppy installs (which may exist, by the way), you can't get a feel for this. Even if you do, for your calcs, how sloppy are you going to assume YOUR typical job might be? That may require a statistical sampling which I think would be difficult to come by.
BTW as the article pointed out by the other poster .... a good install IS code ... but I sense you know that. Care to share the mystery of your analysis?
Also, I'd never do 2x6 @ 16oc unless necessary, which most times it isn't. It's a waste of material, money, labor/effort, and thermal performance.
I would think the heating calc tables must take this into account somehow, since they must deal with real-world situations, and a miscalculation results in a home you can't keep warm. OTOH, I suppose they're fairly well padded -- 30-50%.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
A heating calc can or should account for possible variations, but most calcs assume a fairly consistent install. The bottom line equipment choice will likely go to the next higher size which most often leaves room for 'error' (e.g. you calc 38KBtuh, but the next available size is like 45KBtuh).
FYI Most energy codes limit arbitrary gross oversizing, though ... although probably not often checked (although in the NW we did check). Not sure how the IRC handles this. I've seen limits of 125% of design size for the max oversize.
<Does anyone else think a derate of R5 is in order?>Yes, I think that's in the ballpark, unless you're somewhere that has an insulation inspection as part of the building process. Even then, not tucking corners back properly, gaps at the top and bottom plates ... ends of joist bays ... bumpouts...j
A colleague of mine used to be a small scale developer working odd lots in larger projects. He has seen some horror stories. One in particular where he actually witnessed contractors working on another house remove insulation that had been inspected (prior to drywalling) and install it in the next home. There's some real shady stuff out there.
I guess the "donor" walls would be derated 100% !
5% doesn't seem so bad now.
John
<remove insulation that had been inspected and install it in the next home.>Yikes, I thought I'd seen some stuff but never that. Vapour barrier continuity is inspected at the same time as insulation where I am, so it'd be hard to re-use batts. However, with poly in place over insulation it's hard for the inspector to check odd cavities, behind electrical boxes, corners, plumbing, and so on. It can all look pretty seamless on the surface but lots of voids hiding near the exterior of the wall. If you're using paper face or bare batts in your climate it would be way easier to check the details. j
If new construction, make sure the studs are accurately placed so that you don't get gaps. On old construction without good stud placement, the you can lose almost everything if you don't have a good, solid air barrier. FG doesn't do anything for infiltration, which is usually more important. If you have leaky walls, the FG could just serve to trap moisture, but have little insulating value because you need to trap the air for it to be useful. It would probably add problems in that case.
Watch your barriers, too. If poly has penetrations, you'll let moisture in, but keep in from airing out. I'd only use unfaced FG against foam. For FG to be effective, stop air movement, and let it breathe if in case it gets wets.
For open studs, I'd go high density spray foam. No infiltration and no vapor problems at all. The initial cost may be high, but the return on investment is usually short.
Watch out for exterior foam then FG: you need at least 2/3 of the insulation to be foam or you could get condensation on it, and don't use a vapor barrier on the FG or it will be sandwiched between two vapor barriers.
Dense pack cellulose will give you a good infiltration barrier if you have a retrofit.
---mike...
Interior XPS?
Definitely leary of the dew point and dual vapour barrier issues. If two thirds of the insulation's R value has to be outside the sheathing that's a no go for me.
Does anyone do INTERIOR 1" XPS in combination with say wet blown cellulose? I realize the joists ends would not be ideally thermally broken but all the rest of the framing would be.
As an owner-builder it would remove finicky insulation from the close-in or "lockup" schedule critical path.
Thanks
John
I just thought that maybe joist ends could be thermally broken in new construction. Just hold them back from the face of framing 1" and install some rigid insulation there. I don't that I've ever seen this detail, but don't know why it wouldn't work.
1" XPS over rim joist only
Clewless
I like that detail and it seems like a good use of material (i.e. a relatively small piece of XPS to thermally break the biggest problem area).
However, in our area I am afraid not being able to overlap the plywood sheathing 2" over the rim joist and nail it together would compromise the seismic shear wall too much.
John
good point ... maybe it could be strapped across the rim w/ Simpson straps to the wall (or mud sill) below. Not an uncommon detail, really. In the PNW, the 2 inches wouldn't mean much structurally, I don't think. There are other ways to tie it together, too, I think (e.g. bolt through the bottom plate into the top plate of the wall below). As with anything, it's the details that become important.
i'd guess 30% derate for bad jobs
Thermal performance
John, I have been anaylyzing thermal performance of structures of all types for decades. Typically, frame walls with the nominal batts installed by the lowest bidder/contractor (i.e. sloppier work that most DIYs can manage) is good for 70 - 75% of the batt R-value and that accounts for the framing members as well, for a composite thermal resistence for the element. Of course you need to account for GWB, sheathing and siding, but the most important eneveloping property of the structure is more usually air in/exfiltration. If the sheathing installer does an excelent job and the insulator does a poor one, you're much better off than vice-versa.
So your R-19 2x wall (16" oc) will yeild an average of R-14.25 for the studs and insulation. Add drywall, sheathing, siding and surface effects to determint the real composite thermal resistance.
I am not familiar with the ORNL "whole wall" calc, but I would expect it takes foundation losses into accout. I do that separately. Any real calculation invloves judgement. You have to even the ledger where you're real conservative and where you're not conservative.