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How to decide on effeciency targets in additions for older homes?

mn_johnb | Posted in Energy Efficiency and Durability on

I’m in the process of designing assemblies for a modest (400) sq ft addition to my 1937 cape in CZ6A. The old house is a work-in-progress with regards to energy efficiency. The entire second floor has been spray foamed at the rafters all the way to eaves (i.e. no unconditioned space under the roof). This has cut down on stack effect but the band/rim joists still need attention as well as penetration sealing at doors, windows etc. More improvements are planned, mainly further air-sealing and dense-pack cellulose on the first floor, an adhered air/water barrier and drainage plane when we reside, and window and door replacement/refurbishment. I’ve pretty much ruled out deep energy retrofit-syle approaches meaning no thick thermal insulation added to the outside of the old house.

So….. I know this will never be a truly high-performance structure and I’m not sure how to decide “how good is enough?” when designing new walls to connect to the older, compromised structure. Specifically, it seems like folly to include large amounts of thermal insulation to the new addition (especially wall assemblies) when ten feet away the conditioned space is enclosed by what is an inherently leaky R9 (maybe?) wall from the late 30s.

So how do you extend the “pretty good” concept into additions/remodeling?

I have some assembly ideas I’m leaning towards but would love to hear other’s thoughts on this first.

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  1. user-6623302 | | #1

    I would aim for code compliance plus any low hanging fruit. For example, if 2X4 walls will meet code, but 2X6 walls at little additional cost, may give you a better result. Air sealing is probibly more important than complicated high R value walls. I built an bedroom suite addition using 4" thick SIPs. Very tight and easily heated and cooled. I could have gone thicker but I am totally satisfied. This is on a 1830 farmhouse. Have a heating plan worked out, It shouldn't be an afterthought.

  2. mgensler | | #2

    I second going for code minimum. Make sure to pay special attention to how the addition connects to the existing structure. The previous owners put an addition on to our 1960 ranch. Structurally it is built extreme well. Air sealing and insulation was an afterthought and has given us all kinds of issues.

  3. Expert Member

    I agree with Jonathan. A well executed, slightly above code addition will be comfortable and proportionate to the older inefficient main structure. Unless the addition was much larger, there isn't any point building it to a standard much higher than you wil be able to bring the rest of the house up to.

  4. mn_johnb | | #4

    Thanks everyone. It feels like we're all on roughly the same page: Code minimum or a little above, but getting the details right, especially air sealing. This was my gut feeling too.

  5. charlie_sullivan | | #5

    A lot of this is a question of your priorities, which can't be answered by building science, but there is an implied building science question embedded in there: Is it a waste to have a high-performance wall as part of your envelope, when there's lots of low performance wall as part of the same envelope? In other words, if you would build to R-30 building new, but your old walls as R-10, is it a waste to build the addition to that R-30 standard, and you should back off to perhaps R-20.

    The answer is that the upgrade from R-20 to R-30 has just as much benefit as part of a weak envelope as it does when the whole envelope is R-30. The heat loss through that section of wall is Q = ΔT*A/R-value, regardless of what is going on with your other walls, unless they are so bad that you aren't keeping the interior warm and ΔT drops.

    If you were asking which is better, to upgrade the new contruction from R-20 to R-30, or to leave it at R-20 and upgrade the same area of old walls from R-10 to R-20, making everything R-20 is better, because each R-10 increment has diminishing returns: going from R-10 to R-20 cuts the heat loss in half, but going from R-20 to R-30 cuts the (new, lower) loss by a third. So if you have another R-10 to spend, the best place to spend it is improving the old part.

    But if upgrading the old part is off the table, and the question is just what's worthwhile in the new part, the tradeoff is no different from if you were building new.

    1. mn_johnb | | #7

      Thanks for bringing this perspective. The phrase "if you have another R-10 to spend" will likely stick with me. Budgeting for future tightening of the old house is definitely a factor in designing the enclosure of the addition.

    2. maine_tyler | | #8

      This is well worded Charlie.

      The challenge of course is that upgrading an older section of house from R-10 to R-20 is typically MUCH more invasive (expensive) than bumping a new wall design from R-10 to R-30. The financial calc becomes quite complicated and project specific.

      Justifying the new R-30 wall alone is difficult in terms of total percentage of heat loss: i.e. an R-30 wall is an R-30 wall in terms of heat flow impedance no matter, but as a percentage of total building heat loss, you won't be taking a big bite if the rest of the walls remain R-10.

      One could perhaps rationalize still building the new walls to R-30, however, if there is a possibility that the old walls will be retrofitted in the future. Someday the siding will have to come off, or the interior will be gutted. Building the new wall to R-30 may save future costs.

  6. plumb_bob | | #6

    I am also planning an addition onto my older house, and will be aiming for code minimum plus a little. I foresee the future heating of my house being done by zone, meaning the old is one zone and the new is another. I currently heat the entire house with wood heat, but with the addition the wood stove may be undersized. If this is the case i will add a heat pump for the new area of the house.

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