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2x6 vs. 2x4 construction

mikeymoran's picture

    Has anybody worked out a cost-benfit analysis for 2x6 vs. 2x4 construction in ,say, the midwest, for, say an 1100 sf 2nd floor addition to a home? at what point, if ever, do i recoup additional construction costs in energy savings? They say natural gas prices will jump 36 percent this year. thanks in advance...


(post #109676, reply #1 of 30)


I recently asked the same question of several building and HVAC professionals, with a great deal more experience that I have. Never got an answer.

There are many variables to consider. How much of the walls are occupied by glass and doors? How well insulated is the rest of the house? In new construction, the difference in insulation can affect the size and first cost of the heating and cooling equipment, as well as energy costs. It all depends on the scale. In an addition or a remodel, the difference could be the difference between being able to utilize existing HVAC equipment or having to upgrade.

However, since no one could give me a simple answer, I have started a side project with a basic house design to run energy use calculations on the house with various efficiency levels of HVAC equipment, various wall constructions, window insulation values, etc. At this point, I'm just gathering data, and don't have any real numbers.

(post #109676, reply #22 of 30)

I found this discussion particularly interesting.  First of all, I should start out by introducing myself because although I have read and browsed the discussions on several occasions, I have never posted.  My name is Andy and I am presently an Architectural and Civil engineering student at CMTC.  Although only nineteen, FHB has been my favorite publication for years and is much of the reason I am where I am today.  When I started to read this discussion I happened  on more than I had anticipated for a number of reasons.  Presently In my building systems class, we are designing heating systems.  Literally two hours before reading this posting I was doing calculations on the very things this discussion is about.  There is a lot of good information here that you all have posted and some that isnt quite as accurate, at least from what my instructor, an AIA registered architect has presented to us.  Although, at this point I wish it was, calculating such things as R values and BTUH losses accurately is not as simple as it may seem.  There are many things involved such as finding the movement of outside air and inside air, adding up the r values for everything in your wall (yes everything, even moisture barrier and vapor barrier. they all have values believe it or not) infiltration coeffiecients based on slab perimeter, whether you have a basement, crawlspace, attic, number of exposed walls with openings.  As far as the percentage of the wall taken up by studs being 10 that is accurate in some ways close to accurate, but in a wood framed wall, headers and such are included and it brings the number closer to 25% of the wall.  As far as the R values per inch of softwood framing material it ranges from 1-.89 for southern pine to west coast woods such as cedar in the 1.48-1.1 range for an R value per inch. Most of this information is Available from ASHRAE and they do have a website.  The books we use are well over a hundred dollars.  Although not simplistic in method, these topics arent that hard to understand especially when you take particular interest in the subject.  Another great user friendly source of information that we also use is called the Heat loss calculation guide (h22) from the IBR/gama hydronics Institute.  It is an 80 page booklet that is step by step help in figuring out all these coefficients and values to get an accurate idea of how much these things matter when It comes to energy efficiency.  Although i did pay for it, I dont believe it was much and would encourage anyone to look into it.  I hope I havent wasted too much of your time, I just found this opportunity to share my education too irresistable.  Good luck.  I will try and find time to check this discussion again in the future in between designing AC and Electrical (plumbing is already done). Thanks.

(post #109676, reply #23 of 30)

hey, andy.. welcome...

everything you stated is true, but don't get lost in the details.. a detailed analysis  may be ok for a large commercial bldg, with repititious wall assemblies..

but in residiential insulation, it's better to have a strategy than an analysis.. and the strategy should account for the human factor of the installers and mechanics  also..

Mike Smith   Rhode Island : Design / Build / Repair / Restore

Mike Smith   Rhode Island : Design / Build / Repair / Restore

(post #109676, reply #24 of 30)

Welcome Andy,

While you may only be nineteen, it's apparent that you're already playin' heads up ball. There's always room for folks like that here and I, for one, hope to hear more from you. You just might very well think of something that on one else has. Post away but be ready to get ignored or shot down from time to time.....everybody does...and don't take it personally. Just climb right back in the saddle.

Enjoyed your post. You certainly didn't waste anyone's time.

Knowledge is power, but only if applied in a timely fashion.

Knowledge is power, but only if applied in a timely fashion.

(post #109676, reply #27 of 30)


Glad to see that you are interested in the details of how these things are determined.

I am a mechanical engineer, and I determine energy losses and gains, size heating and cooling equipment, and design distribution systems for a living. It is very interesting. Every job is different and I learn something on every one.

You mentioned ASHRAE. Excellent source of the right information on determining heat losses and gains. They also provide a great deal of texts and handbooks to students at a significant discount. If you are interested in HVAC systems, I can recommend some very good ones to you. Many other non-ASHRAE excellent sources as well. I would recommend that you join, as a student member and get as much stuff from them as a student, as possible. You will save lots of $$$$.

I learned how to calculate energy losses and gains by hand, (before computers were prevalent on desks) and can tell there are many ways to get to the same answer. There are 3 different ways to calculate air infiltration for instance, and if done properly, all yeild similar magnitudes of flow. I use the air change method.

BTW, where is CMTC?


(post #109676, reply #2 of 30)

While we haven't discussed this in a while, it has been beat to death over the past few years.

A few minutes with the advanced search engine should get you planty of reading.

Not to speak ill of another requires only your silence.

(post #109676, reply #3 of 30)

Hi Mikey,

The 2x4 vs 2x6 question is all about insulation.  The funny thing is we seem to choose one over the other and overall insulation is left behind.  On a typical wall framed 16 inches on centre, 32 feet long, the 2 inch lumber will add up to 37.5 inches or just under 10%.  This means 10% of your wall will have a R value of 2 or 3 while the balance of the wall is either R12 (2x4) or R20 (2x6). 

I suggest you consider 2x4 wall with 2 inches of styrofoam SM applied to the outside.  SM has the equivelent of R5 per inch thus 2 inches of SM gives you R10.  If you add the 2x4 wall into the equation, 90% of your wall will be R22 and the other 10% will be R12.

I have left out windows in these rough numbers as they add a dimension to the R value that cannot be easily discussed.  Windows give you an R value in the range of aproximately R2 to about R5-6. 


(post #109676, reply #7 of 30)

"This means 10% of your wall will have a R value of 2 or 3..."

FYI, dry softwoods, i.e spruce-pine-fir, have an approximate "R" value of 1 per inch. Makes a 2x4 good for 3 to 4 and a 2x6 good for 5 to 6.

(post #109676, reply #8 of 30) is an interesting test of the thermal efficiency of alternative wall types.

(post #109676, reply #9 of 30)

Looks to me like the piggy in the straw house was smarter than we thought...

We do Fast, Cheap, and Reliable work.

Just pick any two.

(post #109676, reply #10 of 30)

As a practical matter, when calculating the overall R value for wood stud walls, the standard is to use 80% of the insulation R value. This approximates the measured values from the test pretty well. Something the test did not show, that I could tell, though my review was brief< was the effect of decreased air infiltration with increased insulation thickenss. Using a design standard 7.5 mph wind, the air inleakage for 2x6 with r-19 fg batts, compared to 2x4 with r-11/13 is about half.

(post #109676, reply #11 of 30)

I've been talking with ORNL because we're interested in getting good numbers for the dome walls. 22-mil vinyl/polyester, 3" continuous polyurethane, and 4" 4000psi shotcrete should test well, but until tested, it's only a guess. Their tests come in three flavors. Testing a representative wall segment (8' x 13') in a hot box to get a whole-wall value. Running a fancy computer simulation on the assembly with a variety of climatological data. And running some elapsed-time tests in a climate controlled room (from <0 to >100 degrees) with a 10' x 10' mini-house of that construction. Each test costs about $8K to $10K to sponsor, so haven't yet done it. But their methodology seems good, and what they've done has made an appearance in the code book in the energy requirements area.

(post #109676, reply #12 of 30)

I understand the hotbox test, and I think it is a very good method to test for overall thermal resisience. I don't know how they could do it, but not accounting for air infiltration makes the results incomplete, as far as the total benefit of different insulation thicknesses. I, among other thing, perform heat loss and heat gain calculations on a regular basis. For heating, air infiltration rates are very important (the energy required to heat air from -15 to 70 can be greater than heat loss through wall, several times over for spaces with more than one wall exposed). In structures with 2x6 construction, I reduce the infiltration rates, based on experience and what works. It would be nice to see that borne out in testing.

(post #109676, reply #13 of 30)

Their third test, in the climate simulator, is where they do the blower test for air infiltration and any moderating effects of thermal mass.

(post #109676, reply #15 of 30)

Great idea to use the rigid insulation on the outside to stop any thermal bridging at the studs. We use 2x6 wall here and then add 1/2" to 1 1/2" rigid on the outside, (here being Alberta, Canada). On your estimate of the R value of softwood framing lumber it is valued at R 1.3 per inch, this would give a total of R 7.15 for 5 1/2" stud. We are also going to get some price increases this winter it sounds like, but when was the last time you heard of utility prices going down?

Jeff Up North

(post #109676, reply #16 of 30)

Structurally I don’t like the idea of putting 1 1/2" of air between the sheeting and the styrofoam In my opinion is, it weakens the building weak considerable. Instead of having to have a great force to sheer nails or rip up the sheathing to cause the house to rack. With the 1 1/2 of air (styrofoam) between the sheeting and the 2x4 or 6 studs it takes a lot less force for the house to rack the nails twist and bend not sheer off.


Now I may be wrong about the reason why the building cods went to 2x6 studs back in the 80s.

For a while you only need the 2x6 studs if you built two stories. There was a few reasons that I was told for it.

1. The 2x4s are not 2x4 and allot of them coming off tree farms are weaker.

2. The water beds and Jacuzzi were to heavy and were causing walls to fail.

3. The square footage was going up and the customer wanted no load bearing wall in the middle of the house. Again causing the walls to fail

4. Was to get more insulation in the walls.


(post #109676, reply #19 of 30)

I agree with what you are saying about the "space" between the sheething. The preferred method is to sheath the wall with structural sheets (OSB, ply ) and apply the rigid insulation on top of that. The cost of the jamb extensions is very small, at least up here in Alberta, all the window manuf. will do it right at the plant, just send in the measurement to the supplier when you order your windows. I have seen a few buildings with rigid insulation on the studs and no structural sheathing but the guys did a "let in brace" at a 45 degree angle at all the corners. This is for the racking strength you were mentioning and it is also code here.

Jeff Up North

(post #109676, reply #25 of 30)

Over here in Quebec Canada we also frame using 2x6 lumber. I think the primary reason is for the insulation cavity. The winter are pretty cold and the cost of energy offsets the extra cost of the 2x6. Doors and windows here are manufactured to fit into these wall thicknesses and it is harder to  get doors and windows to fit into 2x4 walls than 2x6, no new homes here are framed with 2x4s. the only homes that maybe framed with 2x4 have a layer of extruded polyurethane on them for extra insulation. This extra insulation is of course  installed after the 7/16 osb or the 7/16 temptest is applied to the walls.

The only problem I've had with installing the foam on the outside wall is when it is done to a home with a lot of gable walls. Maybe someone has a good technic they could share. The problem I see is if I add 2" of foam to the wall I have to somehow add 2" to the gable truss for it to be in line with my wall so my siding will install all the same. The foam sheets are expensive and if you have a 12 in 12 roof you are adding a lot of cost to insulate the attic that is not heated. What would be the most efficient way to add that 2" thickness to the 1st gable truss?


(post #109676, reply #26 of 30)

The only thought that wandered through my mind was to find a way to cantilever the gable truss out 2" beyond the wall.

Bumpersticker: Honk if you see something fall off.

(post #109676, reply #28 of 30)

I have never added 2" usually 1 1/2" max. On the house I am building right now I had a flashing made to go over the 1 1/2" styrofoam. Then you could do a number of things such as change up the siding at the transition to vertical siding, shingles, shakes etc. Or have a band of alum. (different colour than siding) cladding over a 2 x 4 at transition then change up siding colour or installation. Strapping might be an option to build out the rest of the gable to a flush wall, I liked the other suggestion here for a projected gable end...The "textbook" technique is to recess the floor and wall frame to allow for the foam. This takes planning for it right at the beginning of the drawing stage and gives you the overhang at the gable scenario.

(post #109676, reply #20 of 30)

Those are interesting ideas about the code changes from 2x4 to 2x6 const. Again, here in Alberta and indeed across Canada our codes still allow for 2x4 construction throughout. It's pretty common to build with 2 x 6 for years now, and the main reason is to provide more of an insulation cavity. There are lots of other methods to get the R values in a wall into the 20 plus range but one of the easiest ways was to just give your framers 2x6 and they can go at it as they always have done.....I have seen 2x8 stud walls at 24" o.c. , not sure about how that will work out but some guys are trying some different stuff.........I have seen some recent articles about research being done with studs of TJI and LVL materials. That gives walls some 9 1/2" plus thickness......

Jeff Up North

(post #109676, reply #21 of 30)

I never considered putting studs at 24" centers.  What does that do to your sheetrock?  I'd be concerned it would make things pretty springy.

(post #109676, reply #4 of 30)

The 2x6 framing is code in many upper midwest cities. That ought to tell you something about whether its a good idea. I say "Yes".



"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1927

Regards, Boris "Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934

(post #109676, reply #5 of 30)

boris.. that assumes that code writers  know something about what  works and what doesn' , i think 2x6 wall conts is dumb.. and archaic.. did it from '75 to about '90... wouldn't dream of it now..

our standard wall is 2x4  @ 16" OC with dens-pak cellulose.. if we want an energy wall we add 1" of EPS foam to the interior and strap the wall horizontally with furring to move the elec. devices away from the exterior sheathing..

but hey, whadda i no ?

Mike Smith   Rhode Island : Design / Build / Repair / Restore

Mike Smith   Rhode Island : Design / Build / Repair / Restore

(post #109676, reply #6 of 30)

check out this site gentlemen:

"Don't take life too seriously, you are not getting out of it alive"

(post #109676, reply #14 of 30)

I built mine with 2x6 walls, the only thing I would do different is to use 24" centers verses 16" which would reduce thermal bridging effects via the studs.  Another big advantage to 2x6 construction is that 2x6's are typically much straighter and resist twisting more then 2x4's.  In addition if you are using 24" centers for your trusses your roof won't sag from heavy snow loads.  I have a friend who is an Architect who does work for Insurance companies around the country.  He sees a lot of sagging trusses between studs on homes that are subject to unusual wet snow falls like we get in Michigan.  I used 2x6 for everything including attached garage.  For a 2300 sq. ft. home and 2 1/2 car garage only cost $500.00 more, which would have been less if I had used 24" center's.

(post #109676, reply #17 of 30)

"In addition if you are using 24" centers for your trusses your roof won't sag from heavy snow loads."

I've never heard of a case where roof loads caused the plates to sag between the trusses.

You may not spend a lot more on the lumber by going to 2X6 walls, but you can tie up a lot of money in jamb extensions for windows and doors. I think that's often overlooked.

One doesn't discover new lands without consenting to lose sight of the shore for a very long time.

(post #109676, reply #18 of 30)

If you order your doors and windows with the extra depth the cost difference is very small.  I have an unusal layout and have 4 entrance doors, it didn't even cost $20.00 per door. I used vinyl windows so no extra cost there.  In addition the extra depth on the sills looks better.  The buckling problem occurred on lower pitched roof's when we had a huge wet snow fall.  My friend said you could see the drywall buckled between the studs.

(post #109676, reply #29 of 30)

First, I'm not a builder and have no clue on codes for walls, but I was looking in a renovation book the other night and in the section on soundproofing, they showed an example of a wall with 6" footers/headers that used 2x4's for studs that were alternately staggered. The sound insulation material then ran through as a continuous 2" piece so that neither wall was in rigid contact with the other wall. In the case of using this scheme for an exterior wall, you'd only have a continuous wood from the interior to exterior on the horizontal plates and at windows/doors. Using some approximate numbers, lets say you put the 2x4's at 12" centers, and alternated. 1/6th of the wall would have 4" of wood and 2" of insulation for say a minimum R value of 10 (4x1 + 2x3), and 5/6 of the wall would have a maximum R value 18 (6x3). I have no clue if this is an acceptable building practice, but it does seem to address the issue of thermal shunts where the stud goes from wall to wall, ie, R value of 6.

(post #109676, reply #30 of 30)

Wood, it does work. There was a long ago poster here, Gene Leger, who built some energy efficient houses using that sort of wall. I think on 2' centers. I've got an old book somewhere that lays it out, he had almost zero energy costs in the few he built. Joe H