Once the drying is complete, the boards are then planed to a standard size. These numbers can vary slightly, but modern lumber manufacturers are pretty adept at delivering consistently-sized supplies of dimensional lumber. Keep in mind that dimensional softwood lumber that is sold at your home center or hardware store is expected to be used primarily for construction projects rather than fine woodworking pieces. Again, keep in mind that hardwoods and plywoods use entirely different sizing conventions.
See this one, for example:. Harry Applin. Yes indeed! I'll get to some of those in the future. So your numbers are off by quite a bit. I would recommend a 4. I know of homes in South Texas - and Columbia SC - built this way on a protected concrete slab that is being passively cooled by the earth. Charles, those aren't just the cavity R-values. I'm not sure why you read it that way. And yes, I know there are other types of walls. This article was already plenty long with just the narrow focus I chose. Bob Hastings. I had a similar thought when I was reading it, but came to the conclusion that you're listing nominal cavity R-values for simplicity and allowing your manual J calculator to factor in the entire assembly.
Is that correct? Interesting read. If yes, I'd guess this would significantly reduce those percentages. Are there studies that try to quantify the actual real-world air transfer typical of homes varying significantly with build style and build date, no doubt? Leaky windows, doors, drywall, floors, etc.. Opening the door 10 or maybe it's closer to ? Thomas Dugan. Allison, I always love you blogs because they are always thorough and well thought out. What I tell my customers is that of the three ways heat transfers conduction, convection, radiation , convection rules. I can have all of the insulation possible and if air is still moving its effectiveness plummets.
I support the move to exterior insulation as long as it is properly sealed. Attempting to seal up a stick framed house by caulking the inside of the stud cavities is like caulking the inside of a boat for leaks. Just plain silly to me. Building codes on air infiltration are woefully behind and only mildly effective. NC only calls for a blower door test of below 5 ACH. You might as well leave a window open.
We always address convection first and then add insulation and radiant barriers as appropriate. As you may remember, we don't build with stick framing either. Paul Brown. Great overview - thank you for sharing. I've seen similar studies on different framed assemblies and they all come to a similar conclusion on the limited advantage of more insulation in warmer climates. However, the engineer that did one study pointed out that this is only true for similar framed walls and holding all other variables the same.
Thermal mass and air infiltration are significantly important to overall energy efficiency in these climates. Much higher results can be achieved, jut not with framed walls. We also have to be thinking of thermal comfort which is largely driven by radiant conditions MRT.
James Howison. Is there any reason to adjust for walls being beaten with sun? Seems to me that while the air temperature might be deg, the surface temp of a wall hit with sun is much higher. You mention "solar gain problems" with windows, but don't they also apply to walls albeit less so. I'm guessing that the models do adjust for solar gain on the roof do they? James, as a compromise, due to complexity of modeling shading from adjacent walls, nearby structures and vegetation, Manual J does not account for differences between different wall exposures.
I believe this is the case with all modeling tools commonly used for residential. MJ cooling loads are known to be somewhat conservative so the only practical impact of this compromise is on air balancing, which is based on relative room-by-room loads. And given how poorly most homes are balanced relative to design loads, I would argue that any differences derived from an accounting of solar gains by exposure, assuming that's even possible, would be lost in the noise.
Garry Whelan. It would seem that external insulation wrap would still be optimal as it 'works' well for the heat, cold and humid elements. Would you be inclined to agree? Is thermal mass inside insulation a topic for modern American building methods? I've some sustainable building experience and rammed earth, stone etc inside insulation has some research pointing to benefits.
Bob Ellenberg. Good article. Most articles on beefing up wall insulation are pretty expensive measures and this is looking at often-built cost effective walls. Here are several things you might address in a future article": Whether 2x4 or 2x6, 16"oc vs 24"oc and aligned studs with single plates, all makes a big difference in how much cavity insulation there is and heat transfer through the wood. Fasteners for the siding have to penetrate the studs how much depends on the type of fasteners and the wind load zone you are in but at a minimum you are over 3" if you leave a proper drainage plane.
There are ways to do it but I think a better alternative to the 2x4 with 2" of foam is a 2x6 with 1" of foam. Though it is a slightly lower performing wall it doesn't present as much of an installation problem. Lastly, when you address foundations, mention sealed insulated crawl spaces. They have much less air leakage around the perimeter than through the insulated floor of conventional vented crawl spaces; it gives you a conditioned space to run your ducts and pipes; no duct insulation is required and you get heated floors in the winter without the expense of radiant tubes.
Have you seen the "production builder friendly" insulated wall? The idea is that offsetting the 2x4 enables the rigid foam layer to remain co-planer to the slab edge all the way up to the roof. Gary Stockton. I'm most likely the central Texas client that you referred to at the end of your article. Termites ARE the reason that I didn't insulate the slab perimeter.
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They are rampant here, building dirt tunnels inches high in the grass. I wish that I could have insulated the slab, but it's just not worth the risk to me. That was the same amount we paid in our sqft rental while we were building. I'm a very satisfied and happy client. Nice to hear from you, Gary. Although I guess I could have been referring to your home, the one I was talking about in the article was actually a job we're working on now in the Hill Country near New Braunfels.
Termites are definitely an issue, but that's not why they didn't do slab-edge insulation. The recommendation came to late for them as they already had their walls up. Glad to hear about your low cooling bills! I look at this differently When you do optimal-value engineering, it's clear that 2x6 oc should be the baseline for all other assemblies. Then the question becomes how much exterior insulation?
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In cooling dominated climates, anything beyond 1" is usually not justified depends on several factors. The challenge is getting builders on board with oc and advanced framing techniques in general. The vast majority of homes built in my area over the past 12 to 15 years have 2x6 frame walls spaced oc.
That makes no sense! John Proctor. I'm with you David. I hope we can get this done on the replacement homes from the fires in northern and southern CA. I encourage you to quantify the MRT effect. The wall surface temperature change is quite small and is easily compensated for with a much smaller change in air temperature. Brad Marshall. Great topic and analysis, Allison.
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I love reading your blogs for the sly humor and detailed analysis. I like that you started with the standard 2 x 4 stick frame wall and show it can perform just fine in a warm climate without a lot of fancy engineering. I think too often we seek answers in exotic materials or new technology and not in well planned and executed installation procedures. I didn't include the effects of infiltration in these results.
That would be a bit beyond the simplistic calculations I did here. That's a job for a more robust hygrothermal modeling tool, like WUFI. Does that translate into the 2x4 wall 16" oc and the 2x6 wall 16" oc? Since 2x6 walls are usually framed at 24" oc, what framing factor does that translate into? Is the framing factor just the percentage of wood in the assembly, or does the placement of the wood affect it? What would the framing factor be on a 2x4 wall 12" oc but offset every other stud with 2x6 sill and top plate?
Real-world framing factors vary several percentage points depending on number of corners, wall and window heights, and framing details notably advanced framing. IIRC, the framing factor for a 2x6 wall oc ranges from about 10 to 15 percent. A 2x6 wall oc has the same FF as 2x4, although the u-value would obviously be different. The 2x4 studs would increase the assembly u-value. Not sure what purpose they serve? High-density batts for 2x6 framing are rated at R Flash-and-batt or flash-and-blow with a layer of closed cell foam would be a bit higher than that.
Here's the part of the table from their standard showing wall framing factors:. For example, in a 10' section of wall, 8' high, there would be 6 studs and 3 plates. Those are 'default' values based on conservative assumptions within each category. Following your example Also, keep in mind that with advanced framing, windows framed without jack studs reduce the overall framing ratio, especially when aligned on a 24 inch module with studs, and headers sandwiched with rigid insulation.
I can't find my reference at present. Yes, actual framing factors can be all over the place. In homes designed without any thought given to the enclosure, they can be significantly higher than the defaults in the table above. Some of the designs I've seen have corners all over the place, too many windows, and headers where they aren't needed.
Then the framers show up without a detailed framing plan to work from, and the result is a thermal disaster. I'd agree with that statement if you put the word "can" right before "reduce. But if someone is doing advanced framing and eliminating jack studs, yes, in general windows should reduce the framing factor. Steven Clark. Great article! I've been really impressed with the quality of articles on your blog. Keep up the great work. Dave Barnes. I would love to see the comparison in cost and performance between the 2x4 wall at 16" o.
Adding your choice and quantity of exterior foam on top of either would cost the same from there. How about coming up with the cost of that 10' section of wall talked about, one 2x4, one 2x6 OVE? Great discussion, I always learn from these great articles! Skip to main content. Going Beyond 2x4 Walls in a Warm Climate. Posted by Allison Bailes on December 12, Total R-value A wall assembly is a combination of materials that allows heat to flow in both series and parallel.
Nominal vs. Actual Lumber Dimensions
Here's a summary of them assuming grade 1 installation : The first one is your standard 2x4 wall with plywood or OSB sheathing. HVAC design loads To see what effect these different R-values have on the size of the heating and cooling systems in a home, we have to put in actual numbers for a house. Energy savings Well, if you're not going to save much on the first cost of your heating and air conditioning equipment, what about the energy you'll save each year by reducing the heat transfer across your walls?
The formula for annual heating load is: The HDD is heating degree days. Is it worth it? Other reasons to go beyond the 2x4 wall For some people, the cost-effectiveness based on current energy costs determines whether an improved wall makes the cut. Where you may find more savings If I were building a house for myself here in the mild climate of Georgia and were choosing from the wall assemblies above, I'd go with the 2x4 wall with 2" of continuous exterior insulation. Here are the top three areas where the most stupid things happen: Windows - A good window is still a poor wall.
A window that meets code here in Georgia has an R-value of about 3. That's a heck of a lot less than the R wall I just said I'd build, so I'd do two things. I'd install windows a good bit better than R And more important, I'd make sure I didn't have silly solar gain problems.
Windows can kill you on your cooling loads, so you've got to pay attention to placement, overhangs, and sizes. Ducts - First of all, I'd have no ducts in the attic. Then I'd have no ducts in any other unconditioned spaces. Then I might even have no ducts at all , at least for heating and cooling. And whatever ducts I do have would be designed properly , installed well, and commissioned completely. Foundation - A lot of comfort and moisture problems originate with the foundation, whether it's slab-on-grade, crawl space, or basement.
The details matter. Even in a hot climate like central Texas, slab-edge insulation can save you a lot on heating. We've got a client there building a Mediterranean style house with a lot of exposed slab perimeter. And believe it or not, their heating load is higher than their cooling load. Comments Bill Swanson. Why is the continuous R-value not added directly? If you are adding a "skin effect", why is it not identical in each wall?
Allison Bailes Allison. So maybe not a rabbit hole, but a termite tunnel? And yeah, that's why I called it a termite hole. It would have been nice if you had included data from Minot North Dakota. Cause North Dakota is a well known location in a warm climate How would this apply to a northern location? Matthew, see my comment to Gary Reecher just above yours.