Incorporating Passive House Principles for an Addition
dvogelsang
| Posted in General Questions on
Hi,
I’m planning a ~675 SF single-story addition to my 1500 SF single-story house. I would like to implement some of the passive house design principles specifically an air-tight envelope and conditioned attic. The existing house was traditionally built. I live in zone 4a. I understand my remodel will not function at the level of a passive house given it is connected a leaky, poorly insulated house, but I was hoping at least the addition would be a little more comfortable to live in and energy efficient. Is there any reason why I should not Frankenstein a passive-ish addition to my traditionally built house?
Thanks,
David
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
This question comes up sometimes on my projects. I see no reason to skimp on performance in an addition, regardless of how the existing house is built. You an always upgrade the existing house at a later date, and in the meantime you will have a comfortable, efficient new space.
The argument I usually hear against this approach is that the new space will make the old space feel uncomfortable. There is actually some science that says it's good for us to have different temperatures and conditions in different parts of our homes, so it's not an argument that I agree with.
On the other hand, maybe that will provide motivation to improve the performance of the old space!
Building science theory says that every surface in a building performs independently of every other surface. So a well-insulated wall gives the same benefit regardless of how the rest of the building is insulated.
The exception is when the old space is so leaky that air infiltration overwhelms the new space. As an extreme example, imagine a three-walled structure that had one side open to the elements. No amount of insulation on the other three walls is going to make a difference.
David,
As others have said, there is no downside to building the addition to a higher standard. Passive house has associations with the accretion program of the same name. A more forgiving approach would be what's called the Pretty Good House: https://www.greenbuildingadvisor.com/article/martins-pretty-good-house-manifesto
All very helpful! Now I just need to figure out how crazy I can go without blowing up the budget :).
Tell us why you want to conditioned the attic.
A flat well insulated ceiling will preform better and cost less.
When you write passive house, I read unlimited budget try setting your goals in terms of R values and air tightness.
Do I think spending an extra 5k on the foundation to make it an R20 passive house foundation is a wise choice I do not. Do I think you likely to still be alive and own this house by the time the foundation insulation saves enough heat to recover its costs, I do not.
In most cases passive houses fail to make economic sense.
Walta
What is your heating fuel? This is important in estimating an ROI. If you are using $4.00 propane or $5.00 heating oil, superinsulation is the minimum.
This is the book that gave me a clear understanding of the principles involved in energy efficient building and the energy needs thereof.
https://www.amazon.com/Solar-Decision-Book-Homes-Remodeling/dp/0471082805
"Now I just need to figure out how crazy I can go without blowing up the budget :)."
Even though it's imperfect, I think the best way to answer that question is with energy modeling and careful cost-benefit analysis.
Walta's post above makes perfect sense—you need to determine a reasonable ROI (based on sustainable energy costs) before throwing arbitrary R values at your assemblies.
I like the tactic of considering at what point the cost-to-energy-savings ratio of adding more insulation equals the cost-to-energy-production ratio of adding more PV panels (you don't actually need onsite pv for this to be valid: the cost of small-scale onsite PV is sort of a proxy for the maximum cost of sustainable energy).
Looking at things that way, combined with good energy modeling software, helped me to decide that for my specific project, resistance heat and very good double pane windows made more sense than a mini-split heat pump and triple pane windows, but that a heat pump hot water heater and an ERV were far better investments than conventional hot water and exhaust-only ventilation.
Does my build meet passive house standards? Almost certainly not. But is it net zero (with on site PV) and pennies on the dollar energy cost versus conventional construction? Absolutely, according to the software. Plus increased comfort and minimal electric bills from day one versus conventional construction, and ROI well under the warranty period for the PV and other systems.