Unbalanced R Values
MickButler
| Posted in Energy Efficiency and Durability on
Hello everyone, I am building a house in climate zone 4A. Due to structural constraints my truss manufacture specifies a 27″ deep parallel cord truss. I am using a smart vapor retarder on the underside and a WRB on the top side of the trusses therefore the entire parallel cord truss must be filled with insulation (cellulose). Shooting for .25ACH50.
My question is will there be uneven temperatures inside of the building due to disproportionate R values within the assembly? The R-values are as follows. R5 sub-slab R24 basement walls R23+6 above grade walls R5 windows and R100 roof?
Thanks in advance for your comments.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
To some degree, yes, but in most cases it shouldn't be enough to notice. It would be most noticeable at thermal bridges.
Someone will probably try to talk you out of using so much cellulose. It's good insurance against potential climate changes and energy costs.
I would use less if I could but I want to have a primary and secondary air barrier and the only way to do that is to fill the entire cavity with insulation. That was my understanding.
Deleted
You'll have the usual "light on the R value" windows, but everyone has that, so not a big deal. I'd try for R10 under the slab myself, but I'm used to colder climates. I doubt you'll really see much noticeable variation in temperatures though, assuming your HVAC system is properly designed.
I suppose I'll be the one to point out the issues with R100 worth of cellulose, besides cost (I usually go for R60): the big one for you will be the strength of whatever you use on the underside of those trusses. Regular drywall alone is usually not rated for that much weight. I'd have to consult my little drywall guide to be sure, but memory says a bit over 2 pounds per square foot is about max for 1/2" ultralight, you'll be up around 3.5 pounds per square foot or so at R100. I'd consider putting some thin plywood or OSB up under the trusses to handle that weight, then some 1x or 2x strapping, then the drywall. This prevents the drywall from having to handle the weight, and provides some room for the thin support structure to sag a bit without bearing on the drywall mid-span.
Make sure you have enough access to get that cellulose fill in place during construction. Working in confined spaces can be tricky, so you may need to plan to insulate in stages as you close things off. It would be very difficult to insulate that entire ceiling space from just a central access hatch, for example.
Bill
Bill, thank you for your reply.
I plan on using 2x strapping to attach the ceiling to, most likely drywall. I though that the intello and 2x strapping would hold it but you make a good case for some plywood to help.
Good points, Bill. I've learned from forum members in and near CZ4 that their balanced heating and cooling needs mean that even uninsulated slabs can work there.
As for cellulose, I have the New Englander habit of assuming everyone "straps" their ceilings; it's normal here to install 1x3 or 1x4 16" o.c. across the underside of ceilings. That is enough to hold at least 36" of dense-packed cellulose, I have found.
Installing cellulose that deeply requires techniques that installers in CZ4 are almost certainly not familiar with. The bays should be divided into two or three "cells" using insulweb, with each "cell" insulated separately, and to a higher density than the 3.0 lbs/ft³ or so that most installers use. It should be installed at close to 4.0 lbs/ft³, which will feel like an extremely firm mattress, not a fluffy pillow. If not insulated to that density, it will settle over time. It's not possible to dense-pack 27" deep bays effectively in a single pass.
The rafter bays should also be vented, but the insulation density will crush most rafter vents. I have found Accuvent's 1.5" "high performance" option to be ideal. Smartbaffle can also work. Or make your own out of plywood.
Michael,
Thank you for your reply.
I am venting over the trusses just like this. https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
Mick, I see, that's my favorite way to vent roofs. Most builders won't do it so I don't think of it much these days. But it works great.
I like 1/4" wafer board for site built baffles that need some strength to them. 1x2s tacked to the inside edges of the rafters up tight to the sheathing go in quick with a finish nailer, then the waferboard can be tacked to those furring strips with the same finish nailer. It's quick and relatively easy, and uses cheap materials. I just described that baffle construction technique in another thread on GBA today actually :-)
I find that 1x2s and 2x2s are great for "framing" stronger baffles when and where needed. There usually is no need for anything stronger than those when used with 1/4" wafer board. I've done the same thing to build a sort of "false floor" above drywall when reinsulating with much deeper loose fill than originally installed too. Doing this prevents the drywall from bowing. The older 1/2" drywall (pre-ultralight) can't handle as much weight as the new stuff before you start to see noticeable bowing.
Bill
Bill,
The reason I do not want to use baffles is because I want to have an air barrier on both sides of the insulation.
The baffle would be an air barrier if you seal it with canned foam. Also, if you do the plywood/strapping/drywall I suggested, you'd have a primary (drywall) and secondary (plywood or OSB) air barrier on the underside of the insulation. It's not really necassary to have an air barrier against the "top" of the insulation here, especially when using cellulose, if your purpose for the air barrier is to limit convection currents within the insulation. Convective airflow is mostly a problem with low density batts in walls, where it's more important to have an air barrier on both sides of the batts.
Having vent channels is going to make your roof assembly much more robust and less likely to have any moisture issues down the road.
Bill
Bill,
Thanks again for you insight.
I am not as concerned with preventing convection loops as I am with getting a .25 ACH50 and want to make sure I have two continuous air barriers all the way around the assembly for belt and suspenders.
Even seasoned pros fall into old house thinking
One drafty room in an old house gets cold.
The percentage of heat that is retained in a modernish house is so high that you will never notice the difference between various R values.
To exaggerate very slightly:
You will never ever notice the difference between an R20 roof and an R100 roof.
You will likely see the difference between an R20 roof and an R50 roof in your energy bill
R50 and R100?
Unlikely
Why do it?
Cause we are trying to save the planet.
A small percentage increase in insulation, paid for once, repeated over and over with your neighbors and on and on makes a difference.
Math will lead to the truth
No I am doing it because I need to fill the entire cavity with insulation and structural requirements dictate the trusses are 27.5" deep.
Mick,
Like Michael I think what you are doing makes sense - and that assembly in Josh's article will be very resilient to any moisture issues.
If the truss slope wasn't too steep (less than 5/12) I'd be tempted to put in less cellulose and leave a space between it and the membrane above. Functionally I don't see any difference between that and a roof with the bays filled all the way.
Malcolm,
Thank you four your reply. The roof is 6/12. I was under the understanding that insulation needed to be in contact with both the top and bottom air barrier. Is this not necessary?
If it is the case that I can leave an air gap between the cellulose and the top membrane then I would not need to be as picky when installing the dense packing because if anything settles overtime it would not be a big deal.
Mick,
The advice to have insulation enclosed on all six sides is to reduce wind washing, but blown cellulose doesn’t really suffer from that - which is why it is almost always left exposed on attic floors.
The problem with a 6/12 slope is the cellulose may settle towards the eaves leaving gaps or less near the peak, but as you say - if you have more than you need you don’t have to worry if some settling occurs.
I agree with Malcolm; wind-washing affects the top inch or two of cellulose at the most. It's not good to have interstitial spaces that are not vapor-open, but your exterior membrane will be vapor-open so it shouldn't matter.
One way that I and others have insulated roofs without having to fill the cavity is to include baffles--not the normal kind, but dams installed horizontally and perpendicular to the slope, to retain loose-blown insulation that wants to slide toward the eave. It's not perfect, as the insulation will still slump between baffles, but it's still effective. As is filling the bays.