Furring out interior of stud 1.5″
I am about to order my mineral wool insulation from menards. They have two options I am interested in. I have 2×6 stud walls 16″ o.c.
They sell Thermafiber UltraBatt 5.5″ x 15″ x 47″ R-23 Mineral Wool Insulation & Thermafiber UltraBatt 7″ x 15″ x 47″ R-30 Mineral Wool Insulation
According to my calculations it would only cost $365 more for the 7″ deep insulation.
My question is – should I opt for the deeper insulation knowing I will have to fur out every single stud 1.5″ to get the extra insulation in there, or is it not something worth the hassle? Main floor is 30 x 44 and that’s the one I would be using this for.
Will the 1.5 furring strips be cumbersome to work with and install electrical boxes and drywall to?
Comments, any input? Thanks!
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Forgot to mention: Zone 5B - and for what it's worth...I am leaning toward the extra insulation for the extra $365 because it seems feasible on my size of house as a DIY build.
If you're going to fur out the framing, fur it out with 1.5" rigid foil-faced polyisocyanurate cut into 1.5" wide strips rather than 2x2 lumber. The 1.5" polyiso will put an ~R10 thermal break on the ~R6 stud, significantly improving the overall thermal performance of the wall over what it would do with wood furring.
Assuming 16" o.c. spacing you'll have about a 25% framing fraction, and with wooden furring that would be an R8.4 thermal bridge through your R30 insulation, and more than half the total heat transfer of the wall would be going through the wood. With an R10 thermal break over the 2x6s the framing fraction losses would be cut by about half.
That means you'd need longer fasteners for the wallboard, but it's not an awkwardly long length.
So in laymans terms, if I keep the wood I am losing 50% of my heat through the studs...or only losing 50% of the particles making contact with the wall? I can't comprehend how 50% of energy is lost from the thin studs versus 16" of insulated space. If I use foam, I am not sure I can cut the polyiso that thin. Maybe being polyiso it's easier to work with. But electrical boxes fasten to that last 1.5" edge of the stud, and in this case it would be foam :P -- more info please? :) Don't get me wrong...I do understand thermal bridging limits the wall and is a reason exterior foam is so beneficial. But in my situation with current circumstances, what would the numbers be and the hassle be with that foam strip? Thanks!
16" OC walls consist of 10.6% wood over the area of a wall.
Shane,
That would be true if walls didn't include built-up posts, backing for interior walls, headers, etc. Dana's 25% is probably a pretty good rule of thumb.
Soo 25% of my conditioned air is lost through thermal bridging or just 25% of air against the wall...? I don't see thermal bridging allowing 25% of the total volume of air to be lost at a constant rate.
Nicholas,
While a typical wall might have a 25% framing fraction -- meaning that 25% of the area of the wall consists of framing, and 75% consists of insulation -- more than 25% of the heat flow through the wall occurs in the studs, because the studs have a lower R-value than the insulation. That's where the 50% figure came from.
Even if 50% of the heat flow through your wall occurs at the studs, that doesn't mean that 50% of the heat flow through your thermal envelope is going through your studs. You also have heat flow through your windows, ceiling, and floor; moreover, there is heat flow due to convection (air leaks). To determine the effect of a change in your wall design on your energy bills, you would need to do some energy modeling of your entire house.
If you decide to follow Dana's suggestion, attaching your electrical boxes would be complicated but not impossible. Visit a good electrical supply house; there are lots of kinds of electrical boxes, and not all of them require solid wood adjacent to the box for attachment. You can also use blocking where necessary to attach your boxes.
I thought of installing a 6" or 12" piece of blocking on every stud located 16" above the floor for example where each electrical outlet would install. Anything less than 6" I imagine would split when I hammer those two angled nails from the plastic outlet box into the "modified stud". The kitchen I may skimp on foam and just use wood blocking due to cabinets needing a good backing. Another idea would be to install 3/4" foam sandwiched between the stud and then a 3/4" piece of wood so there is still 3/4" of wood for drywall to screw into. I can't imagine working with 2.5" or 3" drywall screws for every exterior wall...
I will install foam over my headers though, since there isn't anything that fastens into them interior-wise and since insulation can't normally cover it.
I have already ran energy modeling but I am not sure what my wall's true r value is with R30 cavity and 7" thick studs every 16" o.c. for a 30x44 house compared to 5.5" stud + 1.5" foam.
Could you run a strip of foam tape under the 1-1/2" furring to provide a thermal break within the "stud" while keeping enough wood to screw drywall into?
A strip of foam tape is good for what, R0.05?
With standard siding 1/2" CDX or OSB and 1/2" gypsum, the whole-wall R of a 2x6 wall with R23 rock will is about R15.2, or R16 with generous allowances for interior & exterior air films.
If you bump the depth to 7" with 2x2 furring and compress some R30s (designed for 7.25" cavities, or about R29.5 into the stud bays) it comes to about R19.1, (R20-ish with air-films), an increase of about R3.9 over the straight 2x6 wall.
If instead of furring you add 1.5" polyiso over the framing fraction and install the R29.5 compressed batts you end up with a whole-wall R of about R22.1, or ~R23 with air films.
That's an increase of about R6.9 over the base-case 2x6 w/R23 wall, and R3 above the wood-furred wall with the same compressed R30s.
If you did full coverage 1.5" polyiso it would be about an R9 increase in whole-wall performance. By thermally breaking the framing you're getting a bit more than 3/4 of the enhancement of a full-iso layer using only 25% of the amount of rigid foam.
Nicholas - Kind of a pain, but a hybrid of Dana's,Martin's and your ideas: 2" polyiso strips over framing( as per Dana) except where you electric boxes go. For those areas, use 3/4 polyiso with 3/4 wood screwed through the poyiso to the studs (your idea). You could add 3/4 plywood to the edges of the 3/4 wood spanning past the poyiso strips to the studs- 6-8" high pieces where the electric boxes go, nice nailers for the boxes,though screws would probably work better. You'd have to figure out a clever way to carve the rock wool around these, but you'd have to do that anyway even with just 2x6's. Kind of a pain for running the electric, but so will any of these options other than the pure wood add-ons.
Dana, where are you getting your calculations with the Stud's R value and true wall or 'whole wall' r value?
I will use MemBrain by Certainteed on the interior.
I really don't like the idea of installing drywall to poly iso or using screws long enough to go through it all. It would be quite time consuming and make the wall feel spongy. What about when I go to hang anything on the wall...long enough nails to go through to the stud I suppose.
Nicholas,
So far the discussion has centred on what to use to fur out the wall for the extra insulation, but there are lots of alternate variations that would avoid the problems you are worried about and still yield good results.
One is creating a Mooney Wall by running the furring horizontally. That greatly reduces thermal bridging while still providing solid backing for boxes and finishes.
There are lots of others. Taking a step back and looking at other ways might be a good idea.
Running furring horizontally would mean it interferes with insulation, and insulation would need cut around each furring strip I assume.
Nicholas,
Mooney Walls are usually insulated by either dense-pcked cellulose or sheets of rigid insulation between the furring. One of the chief benefits is you can use a smart vapour barrier on the face of the studs, which is protected from mechanical damage, and keep the electrical boxes on the warm side.
I'm not really championing the Mooney Wall, I'm suggesting you started down a specific path, and might benefit from widening your options, rather than choosing the unpalatable choices it entails.
I may have misses this in the discussion. Is this an existing house, or if a new build what stage are you at?
Thermal bridging has nothing to do with 25% air loss or air leakage- it's simply the conducted loss through the ~25% framing fraction.
Regarding whole-wall framing fractions, this has been studied to death by both state & federal agencies. Typical 16" framing on real houses comes in at about 25-27% of the total wall area, higher- up to about 30% where mid-wall firestops or seismic blocking is required. With 24" o.c. stud spacing the framing fractions are typically 20-22%, less if you go all-out with AF framing. If you need some web-published references to look at to believe those numbers, I can dig some up for you.
For roof assembly whole-assembly estimates I typically use 15% framing fraction, though it can be as low as 12% in many cases, or as high as 20%. But we're talking walls here, with doubled-up top plates, jack studs window & door framing, etc. It adds up.
Regarding the whole-wall R, I use a simple arithemetic 2-D parallel path model (using standard spreadsheet tools like Excel, etc.) based on first-principles of the R-values of the material. Unless the species is specified I use R1.2/inch for the framing timber, which is typical of the most common framing species (hemlock, dense pine, etc.) though some species like douglas fir are closer to R1/inch. Half-inch gypsum + half inch OSB or CDX with vinyl siding adds up to about R1, some siding might add appreciably to the R value.
I don't normally include the interior & exterior air films, or the additional air films or the furring-R of rainscreened siding- though they do add something, it's at most a single-digits percentage difference in whole-wall R.
For real houses where the actual framing details are known there are more accurate 2-D and 3D models that factor in the higher thermal bridging of corners and the nonlinearity of temperature within the cavity fill near the framing, but the errors of doing a simple 2-D parallel path model is usually smaller than the differences in quality of fiber insulation. Higher accuracy is warranted for the building science nerds (who would also factor in the non-linearity of R value with temperature, thermal mass issues, etc.) but from the practical world point of few the parallel path method is good enough to be predictive. The model I use comes in reasonably close (within a few percent) to the whole wall numbers in Building Science Corp documentation or the Oak Ridge National Lab's (somewhat limited) whole-wall/clear-wall calculator.
http://buildingscience.com/documents/bareports/ba-0903-building-america-special-research-project-high-r-walls/view
http://web.ornl.gov/sci/roofs+walls/AWT/InteractiveCalculators/NS/Calc.htm
Using a Mooney-wall approach with rock wool, put R23s in the 2x6 cavity, and split R15s compressed into the 2x2 cavities. In a Mooney wall stackup it's worth installing a broadsheet air-barrier (or use MemBrain) between the studs & furring rather than on the interior since that's easier to keep air-tight over the long term than the air-sealing at the gypsum. With ~R6.5-R7 rock wool between the MemBrain and the gypsum the RH at the MemBrain will run slightly higher, increasing it's average vapor permeance, but with the R23 on the exterior it won't be a problem.
Nicholas - Another reference for whole wall R-values: " Practical Residential Wall Systems: R-30 and Beyond", by Aldrich, et.al. 2012. Their numbers are a bit more generous than Dana's, but not much.( they say R-17 for a 2x6 wall with nominal R-20 insulation and a 20% framing fraction). One thing that is really interesting is that when the framing is discontinuous from inside to outside, whether it be from foam or double wall, the amount of framing doesn't matter that much. The whole wall R for a double stud wall that is 16% framing fraction vs. 25% is R-35 vs. R33. Having continuous insulation somewhere in the assembly makes a big difference.Good luck to you with your project.
Kevin: At a 20% framing fraction with R20 (not R23 rock wool) cavity fill my model comes up with R16.1 if counting the air films, R15.3 without.
But a 20% framing fraction is pretty rare at 16" o.c. stud spacing on any house with windows & doors. It's not always possible to hit 20% even with 24" o.c. stud spacing in a house with lots of corners bump outs. (The more corners, and bump-outs the footprint has, the higher the framing fraction ends up being.)
For typical framing fractions, see p2 (p11 in pdf pagination) of this document, just one of many:
http://www.energy.ca.gov/reports/2002-09-06_500-02-002.PDF
This might be instructive on how to calclulate this stuff:
http://www.taitem.com/wp-content/uploads/2011/01/TT-NC-Calculating-U-values-Nov-2008.pdf
(note, they also use the ASHRAE assumption of a 25% framing fraction, but they're using R1.25 / inch for the framing, which is slightly high.)
For a simple rectangle with optimized wall lengths, AF corners and optimized window & door placement it's POSSIBLE to get the framing fraction under 20% with 16" o.c. spacing, but that's pretty rare. Using the ASHRAE survey's ~25% is a safer bet until/unless you are analyzing a particular framing plan where you can just do the math.
Thank you for the information and articles.
Here's a bit more information to answer questions and mull over.
Yes, this is a new build. I have already installed: 7/16" OSB Sheathing and 2" x 6" x 8' studs. I am leaning very heavy on installing the claimed R-30 Mineral Wool insulation and also MemBrain on the interior side of my wall.
I feel any splitting/cutting of insulation will be quite a chore over the already tedious work of furring the wall out. I do like the pros of poly-iso (I assume polyiso as in the foil faced sheets) cut with a long razer knife in 1.5 x 2" strips 8' long to install over the studs. I do not like the cons which are using significantly longer screws for drywall, the squishy sense of foam right behind drywall which is already not a solid surface, the extra detail to cut the foam and carefully install that narrow of strip. I do intend on using poly-iso (or XPS) on the inside void of my headers to prevent that thermal bridge. That will reduce some of the thermal bridging at least.
My current thoughts are to see how much time I can allot for this part of my build (DIY Build on a construction loan with an expiration date of early quarter next year) If I have time I will consider a rigid foam sandwich, or simply rigid foam. I think a compromise is to install wood furring where outlets and switches go and foam elsewhere. No matter what I can not guarantee I will remember or know every electrical box location. I do not want to use rigid foam on the furred out section as described for mooney walls. Interesting idea to split the mineral wool and compact it in the void. I would need strapping install 16 or 24 o.c. & I have no idea how I would 'split' mineral wool to be consistant for that 1.5" or 2" gap. Compressing a 7.1" R-30 piece sounds like an easier method. I dislike it may compress since it's 7.1" - but it will only be a fraction of an inch. If I had time and budget I could even use 2x4's ripped to get 7.25" of cavity for insulation.
Will using MemBrain over this Mineral Wool product by Thermafiber have any issues? If I end up installing a Mooney Wall with 5.5" insulation versus 7" deep insulation, will it be beneficial to have the MemBrain between the stud and furring strip as noted earlier? I imagine I have to tape every cut in the MemBrain and it will be harder to tape between two layers of insulation versus being on the outside face.
Nicolas,
have you thought of perhaps applying a layer of rigid mineral wool to the sheathing instead?
Sharpening the edge of a 3" or 4" putty knife makes it a very handy tool for cutting edge strips of rigid foam.
On the Mooney wall approach, splitting a 3.5" batt roughly in half is pretty easy, and it's unlikely either half would be substantially less than 1.5". It's easy to compress into a 1.5" void. If the R23 in the 2x6 are installed correctly they will have been tucked into the corners and edges, then gently pulled out to where they're 1/4" proud of the 2x6 edges anyway, for a compression fit on the next layer. You'd have to really shred the split batts to make half of it too thin for a compression fit. A cheap 10" bread knife (pick one: http://www.webstaurantstore.com/2827/bread-sandwich-knives.html?filter=blade-length:10*-inches )works great for both trimming and splitting 15" wide batts. If going 24". o.c. with the furring and 23" batts it might be easier if you used a longer tool for splitting the R15s, but it would still be doable with a 10" knife with a bit of care.
MemBrain can be used over any fiber insulation product.
I want to keep the outside conventional. So how are electrical boxes installed when using a Mooney wall?
What are the three true wall ratings for R30 with all wood studs, R30 with 2x6 + 1.5" rigid foam, and R23 + R15 split in Mooney Wall?
Nicholas,
Here is an exhaustive photo essay of a house build by Mike Smith who uses Mooney walls. You will find details for every aspect of the construction.
http://forums.delphiforums.com/breaktimeclass/messages/?start=Start+Reading+%3E%3E
Go to "Photo Gallery" and look at "PV Dutch Colonial"