It is easy to look at the R value rating of a material and assume that our entire wall assembly matches it uniformly. But when you factor in the thermal bridges caused by framing materials and any discontinuities in the wall structure, you can also quickly deduce that this is not the case.
It doesn't take expensive equipment or an engineer to identify a thermal bridge in a home, run your hand along your walls on a really cold day and they will find you.
This isn't only an issue of heat loss and the higher bills that come with it, it is about comfort and quality of life. On really cold days it can make being too close to walls quite unpleasant, rendering part of your home somewhat unusable at times.
Wood, metal and concrete in exterior walls act as thermal bridges, conducting far more heat than the insulation on either side of them. But with most types of wall assemblies we have little choice but to use some kind of heat conducting material as wall frames; foam studs wouldn't carry a Canadian snow load.
What you can do is factor that in during the design phase so you achieve the actual wall performance you want. Installing R19 batts of insulation with R5 wood studs on either side results in a wall that performs closer to R13 or even lower, depending on the amount of studs. The true or 'effective' R value of a wall is further impacted by thequality and amount of windowsyou install.
If you ran an energy simulation model to find out how well a wall would perform but you neglected to calculate thermal bridges, you would be trying to solve a math equation without using all the variables and your conclusions would simply be wrong.
Thermal bridge-free construction:
1- Be conscious of the use and type of exterior framing members. Some of the wood used in exterior framing can be safely eliminated - headers on non-load bearing walls, unnecessary cripples, redundant studs in corners etc.
2- Avoid metal fasteners of any kind that span the entire wall assembly.
3- Design your basement wall so it isbetter protected against moisture and water damage. This can allow you to use wood instead of switching to metal as a durability precaution.
4- Leave metal stud cavities on exterior walls empty, and instead use the money you would have spent on batts to thicken up a seamless layer of board insulation. You may lose a couple of inches of interior space, but that space would be more comfortable and you will get a much better return on your insulation investment.
Insulation between metal studs:
金属钉在住宅建筑中越来越常见,但效果并不总是很好。一些建筑商喜欢它们,对于内墙来说,这是没问题的,但如果我们忽视热桥,外墙用木材代替金属的建筑的能源性能可能会受到严重的影响。
Metal conducts heat so easily that there is little point in putting insulation on either side of it. It is estimated that metal studs with batts in the cavities can reduces the overall performance of that insulation by 60 to 70%, or more.
According to Building Science Corporation,"The R-value of 6 inch deep steel studs installed at 16 inch centers with cavity insulation of R-21 is reduced to R-7.4, a value only 35% of the nominal".
If putting heat conduits on either side of batt insulation reduces their performance even by half, that means you essentially paid double the price for the R value you think you purchased. So your money may be better invested by employing alternatives.
Building codes and thermal bridges:
这不仅是明智的解决热桥建设,它正在成为法律。最近建筑规范的修订增加了许多地区建筑围护结构的热要求,包括需要热中断。
Contrary to common assumptions, the insulation values in building codes are not about promoting the most efficient and cost effective wall systems, they exist to protect home buyers by ensuring there is a bottom rung of performance we don't fall below.
So if breaking thermal bridges has now made it into code as a 'must do' practice, then you know the effects are significant. Thermal bridges affect a home's energy performance but they also have health and durability implications by causing cold spots that will increase the risk of condensation. So beyond heat loss and comfort, air quality and structural integrity can be a factor as well in terms of mold, mildew and rot.
Insulating exterior wall stud bays is not something we should stop doing, but we should not ignore the overall effect of thermal bridges when we add up all that wood, and we definitely shouldn't replace insulated wood frames with metal if it can be avoided.
Using metal studs as a means of holding insulation was called an 'abomination' by Joe Lstiburek of Building Science Corporation who eloquently explained it like this:
''Putting insulation between metal studs is like eating a sweater to try and stay warm.''
Thermal bridges that will cause significant heat loss are not limited to just studs in a wall. They include discontinuities in the insulation, corner junctions, badly installed insulation (convective thermal bridges), concrete slab junctions at walls and balconies, service openings (structural thermal bridges), and 'penetration' thermal bridges such as metal I-beams that pass through wall assemblies.
计算建筑围护结构的热流比计算单个材料的热流要复杂得多,特别是当像钢铁或混凝土这样的高导电性部件在起作用时。所以写在绝缘材料上的R值表明了它的性能,它的实际性能如何取决于设计师和建造者。
图片说明是“辐射冷”。可惜没有这种东西。
Busted! For the sake of getting a point across in that short sentence we were a bit loose in explaining the 2nd law of thermodynamics so as not to lose the attention of readers by heading down a rabbit hole. So you are absolutely correct (as I'm sure you know) in saying cold does not radiate, heat always moves from warm to cold. But it can feel that way and we thought it best to stay on topic about the importance of thermal bridging. Come to think of it, I think that can be slightly modified with an addition of 'gives the sensation' of cold radiating.
Best regards.
我看到标题就忍不住了。喜欢这篇文章。再接再厉。欢呼。
Thanks, and keep keeping us on our toes! Mike.
Hello. We burn a pellet stove, wood fireplace, and have an industrial cooking system. We live in Montana, with very cold weather. Our home is almost 20 years old. My window trim, ceiling, and windows are black. Is there anything I can do to keep burning and not live with black?
你好,蒂娜,我不确定你说的工业烹饪系统是什么意思,但至少对于传统的木材或颗粒炉来说,如果你的家里有煤烟,那么是的,有一些解决方案,你可以而且真的应该做。
我更关心的是空气质量,而不是变色,听起来有点健康风险。一个正确安装和运行的木材或颗粒炉不应该在家里留下残留物。听起来你哪里漏水了;也许是炉子,门上的垫圈,也许是烟囱,或者是你的房子有负压,正在吸入烟雾,无论哪种情况,我们都强烈建议你去找专业人士进行清洁和检查。
My renovator says the costs of thermal breaks are more than the costs of thermal bridging. True?
Case 1: Wall
-- he says: R-24 batts, 6" wood studs, vapor barrier (polyethylene sheet with blue tuck tape), drywall.
-- I say: before the vapor barrier, install either rigid foam board over the whole wall (marking the location of just-hidden studs on the foam board as we go), vapor barrier if it's not included on the rigid foam, then drywall
or
不是在整面墙上的刚性泡沫板,只是在螺柱帽上——我曾经看到过一种产品的名字,现在尽管搜索了十几张谷歌的图片,也找不到——而是围绕着螺柱的u型泡沫板。
(How do you hang shelves etc if you can't screw into the studs? (Very lightweight things can be supported just by drywall, but heavier things?))
Case 2: Cathedral Ceiling
——他在屋顶覆盖物下面喷了3英寸的泡沫(让我惊讶的是,没有通风通道)(从多伦多往北走一小时);he thinks just install drywall
-- I say add rigid foam board or [stud] caps under the trusses, then drywall -- or, since the shingles need to be replaced anyway, add rigid foam board on top of the roof sheathing, then a membrane, then shingles. Raising the roof would require new fascia - $700 installed.
I searched the internet looking for a calculator / spreadsheet / process for calculating the benefits vs costs of thermal breaks. Found nothing.
I did find how to calculate U-value or R-value of assemblies of different materials.
But:
我曾经读过一篇文章关于隔热阁楼地板的热焊接。我现在找不到,但大致的想法是:如果100平方英尺。阁楼有1平方英尺,与其他地方相比,只有50%的隔热,你不能用99% x R-50 + 1% x R-25来计算阁楼的隔热值,因为天花板下面的热空气会~冲到那个地方。那么如何计算阁楼地板的r值呢?or instead:
Since you can't calculate the insulation value of a wall-assembly just by adding the value of each component in a line from inside to outside, how do you calculate the insulation value of a wall where thermal bridging means heat will flow in a way similar to the attic floor example?
Thanks
Hi Jonathon,
What is your climate zone? The lack of a U in the word Vapor (instead of Vapour) indicates the US, but if your renovator is building an hour north of Toronto, Canada, well…there’s only more Canada up there, so where are you located? As for the advice you’ve received from your builder, I wouldn’t put much faith in it since using only 3 inches of spray foam for roof insulation isn’t even half of building code, and building code is about half of what actually makes sense for durability, comfort and a sensible return on investment.
I’m not sure I follow all of this, and I’m not trying to dodge your questions but I’m going to be pretty general
“My renovator says the costs of thermal breaks are more than the costs of thermal bridging. True?”
No, not true, that is laid out pretty well in the article. It is not just a matter of ‘saving money’ but also durability – cold spots on a wall can lead to condensation, condensation leads to mold.
your questions a bit tough to address as they are, and we would be happy to work through them with you in order, could you separate them and pop them into our advice column? But as for the advice you received, here is another page that may be worth a peek aboutbalancing insulation in a home, it may help you work through a few of your questions, and this page as well written by our engineer aboutwindows for high performance homeswhich clearly addresses the issue of well insulated portions of a home compared to poorly insulated sections.
很好的讨论,尽管实验室的测量通常不被现实所知。在典型的墙壁部分,16英寸中有1.5英寸是木钉,14英寸。5英寸的空腔要绝缘。木材约占总数的10%。任何安装得当的隔热装置都需要解决空气流动问题,因为这个问题对抵抗热量流动的有效性有重大影响。1/4英寸的辐射屏障可以提供更多的好处,如果安装最小的空气渗透,比3.5英寸的不合适的玻璃纤维棒。这就引出了热桥接,它应该被定义为可以通过给定的墙壁或地板系统的热量的测量,而不是像许多隔热产品上标示的那样,通过特定材料的阻力。
Which brings us to the cavity/stud assembly. If that stud has been properly assimilated into that wall system, (with carefull application of sealants), and it has an R value of R1, versus the 90% of the wall at R7, a scientific analysis would yield a wall system value of 6.43. It then becomes clear that if in fact, this bridging effect is so disastrous on the insulative value, it must be because of things other than R value. It is faulty construction. So carefully wrap and seal your homes. Seal sheathing to studs (spray foam is a great way to do this. To prove my point, the increased effectiveness of the spray foam method to insulate homes is not easily accounted for by the inch R value difference in the materials, it is its sealing characteristics.
This holds true throughout the plates, windows, doors, floors and roof assemblies throughoiut a structure, so is very challenging. But chasing R Value is not always a solution.