Reworking an Existing Building Envelope for a Hot-Humid Climate
Long time lurker, first time question asker
Background: I am renovating an old Sears and Robuck’s house- my second over hundred year renovation- and it needs to be fully reworked so I am spending a lot of time thinking about the envelope since I will be rebuilding it anyway. I live in the Southeast (45 NE of Atlanta in climate zone 3A) with “hot humid summers” and “mild winters” but my family’s usage pattern in our current home and likely in this house do not match most other households. We do not use our air conditioning. Our bills are high in late December, January, February, and early March due to heating but beyond that we don’t really use our HVAC system. My plan for this house is to make sure that I have lots of cross ventilation and fans for open windows in summer and to plant some shade trees- we’ve spent a lot of time up there and even without those changes it doesn’t really get crazy hot. For winter we are planning insulated shutters inside and out to reduce heat loss from the windows and are very excited about having a wall of south facing glass under our porch. However, I think I will need lots of insulation and at least a ductless mini-split for heating those 3 months. Currently the house has basically no insulation, standard ventilated attic and crawlspace.
This leads me to my question: Should building envelope design change based on behavior? How should I think about those modifications?
My primary concern is how I should think about/approach the building envelope, especially considering that several months of the year the indoor air (if you can even call it that since the entire house will likely be open). I want my house sealed up in winter to keep the warm air in, but am concerned about trapping moisture inside, spending too much money insulating the wrong thing (there is a lot of information on closed roof space in my climate zone to reduce heat but given my usage I don’t think its beneficial) or relying on lots of electricity powered equipment for minimal results. GBA has a lot of guides for my area that center around optimizing for a high heat climate and maximizing cooling which isn’t right for us, but for sure treating my home like a house in Ontario isn’t the answer either. Any good resources for figuring out the right for us building envelope?
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Replies
As always air sealing is the first priority.
If you are trying to understand how much insulation will pay for itself over time. The best way is to build a computer model of your home with your thermostat settings and local weather. Be sure to find the training videos on YouTube.
https://www.nrel.gov/buildings/beopt.html
If you want to live without AC you may want to plan on a whole house fan that can ventilate the house at night and cool the place off. I would want to build a door that could seal the fan for the winter.
Walta
Thank you Walter! I am unfamiliar with that tool, but it looks very interesting! I will need to see if I can borrow a windows computer to play around with it! The whole house fan is something I had considered, but was incredibly concerned about moisture (I was under the impression they are not recommended in the south for this reason) and thought a ducted HVAC system was needed in order for them to operate? I am worried my open air lifestyle combines with my super green over insulating tendencies will lead to mold!
I'm interested to see the replies because this is not a topic that building science talks about much. In particular, in an unconditioned house with the windows open, what are the moisture drives?
Some observations:
You'll want hefty roof insulation. With the windows open you want the roof providing shade from the sun and not letting heat in.
In the winter when you close up and turn on the heat, you want to be really able to close up. Air tight and well insulated. Even if the economics don't justify it you will be much more comfortable in a snug house. And in the summer you may find that on the hottest days you're better off closing the house during the hottest part of the day and opening it up at night.
In the winter your moisture drive will be from interior to exterior, and this will be the time of year when you have the most moisture drive. So design the house to dry to the exterior and with interior vapor retarders -- which is probably the opposite of normal in your climate.
I love whole house fans, on a summer evening it's great to have cooler air flowing through the house. They're just problematic in a tight house. This might be a good application for one.
Moisture control in your basement is going to be an issue, specifically condensation in the summer. The only solution I can think of is air sealing it and running a dehumidifier.
You hit the nail on the head for a lot of the things I am worried about! Moisture control is my biggest one- I really like a humid house- ~60-70% is comfortable for me (best guess because I don't have a moisture meter). I am particularly concerned about mold and condensation in a tight house but also think that having the house tight for winter is pretty critical. I am torn between if its not broke don't fix it (vented attic and crawl) vs wanting everything tight as possible for that winter weather and hot summer afternoons.
Ventilation without conditioning doesn't help with condensation and mold, it just brings in more moisture to replace what condenses out of the air.
DCContrarian brings up several building science points. On a somewhat different track, I would suggest that occupant behavior should not drive a significantly different design. Houses last for a long time, and most people don’t stay in a house for very long these days. What happens when the next occupant takes over? It’s likely that once a house is built, or once a major renovation (once in a hundred years?) completed, it’s going to be difficult to go back and change some things, so doing them “right” is important. Appliances can be easily changed, building control layers, not so much. Generally speaking.
Green houses are supposed to be durable and resilient. If it can’t serve occupants with different behaviors, it’s not very resilient, and durability may be questionable as well.
Food for thought?
Andrew,
I agree. It's much the same reservations I have with the idea of building a p0orly performing house and using the m0ney you saved on some other carbon offset scheme. It conflates two things: One is our individual lifestyle and choices, and the other is the common housing stock we all move between over time.
I'm thinking that whatever modifications are made for behavior should be additive and not subtractive. IE, instead of saying "I'm going to skip this feature because I don't think I'll ever use it," say "I'm going to add features that reflect the way I live."
I agree in general that when designing a basic home for the general market that is the case, but we just bought our dream property and I doubt we are moving (if we don't move and stay reasonable on renovation we will be financially independent within 5/6 years). My concern is that if I live in this house for an extended period my behavior will cause problems. It is pretty common for modern homes that are unoccupied to get moldy because they don't have air circulating and there is no place for the humidity to go. And part of what I like about the heat is the humidity I wouldn't want to run a dehumidifier to get too much of the humidity out. I am having trouble fully understanding how a control layer is chosen so that my behaviors won't cause problems for the structure long term. I understand I will need to eliminate moisture in terms of water infiltration (that is conceptually very simple) but thinking about air sealing and potential vapor barriers get more complicated with my lifestyle.
If you install a mini-split, the next owner might decide to turn it on in cool mode in the summer. You make a good case for not precluding that.
The question: is there a way to design the house so that neither behavior is precluded? I think so. Make it tight, well-insulated, but with doors and windows that open wide and a whole-house fan.
I should have read your comment before replying above! This is sort of what I have informally settled on, though I would need ducting for a whole house fan! However, getting granular can be tricky, especially when choosing insulation because I want to make sure it doesn't exacerbate the humidity problem- I am considering wool of all things in the stud bays since it is supposed to be better at this, but have reservations about it.
I'm assuming you mean 45 mins NE of Atlanta (Lake Hartwell?)
Is this a new primary residence or a second home? I'm going to assume a gut-rehab of a primary residence in order to justify the financial investment.
Living in Atlanta myself I have to ask how you intend on managing pollen season while leaving the house wide open? Do you leave the house open because you like the fresh air or are you just accustomed to not using air conditioning? I'm asking because if the issue is wanting fresh air then you could look at adding an ERV with a dedicated duct system?
As far as sealing the house IMO that's going to depend on the depth of the exterior stud cavities. My knee jerk reaction is going to be cut-n-cobble approach (https://www.greenbuildingadvisor.com/article/cut-and-cobble-insulation) but boy it's not something I would want to do and I doubt the local labor is going to want to do it either. In addition local labor isn't going to be familiar with applying an interior vapor retarder outside of latex painted onto sheetrock.
So money no object suggestion is the following:
Carefully remove existing siding, throw up ZIP-R (R3), insulate the stud-bays with whatever insulation you want other than spray foam (imo, installers just can't get it right).
Add a rainscreen and re-attach the siding.
Look at a whole house dehumidifier and consider an fully ducted ERV
Seal up penetrations in the second floor ceiling and add more insulation above it (ie. attic space)..
New doors/windows.
Mini-split for heating/cooling (depending on the configuration and assuming no existing HVAC)..
Seal the crawlspace
Excellent suggestions. I am in Canton, right near lake Altoona. This will be a primary residence and it is currently is pretty bad shape. I will be doing the construction myself over the next two years and mostly with salvaged materials so money is an object but based on our last renovation if you really understand something you can make a good design with what you can access and spend lavishly on the stuff that is essential (ie building envelope, roof, general structural improvements). The stud bays are shallow and I plan on doing wool if I can afford it (fairs better in humidity) or cellulose. I have single layer shiplap siding in excellent condition which I will need to take off to get a good envelope so I plan to do rockwool sheets as exterior insulation, rain screen, then reattach (so VERY similar to your plan). As for why I have open doors and windows- I do like fresh air and I am out in it a lot. I like the breeze and I like to feel outside. Perhaps it's because I farm? I do also like the humidity in summer air which can't really be replicated by a ERV. I am curious about the crawlspace since if I seal it I want to do it right- its pretty common for crawl sealing to fail and I am wondering what the benefit is of the insulated space if there are no ducts down there and pipes are insulated. Is it just that it is very hard to insulate the floor above like one would do with an attic?
To piggyback on something DCContrarian said: "You'll want hefty roof insulation. With the windows open you want the roof providing shade from the sun and not letting heat in . . . " I can't help but think of Peter Pfeiffer's roof design as applicable for your situation/climate (he is all about passive cooling to mitigate reliance on mechanical equipment). Check out his radiant-barrier roof system: The Evolution of an Energy-Smart Roof.
Interesting article though I think I design like that would require *flawless* execution to avoid issues. I am wondering if its better to insulate the roof or the attic in a situation like this though it seems like the most recent building science recommendation for longevity is unvented insulated crawl and vented attic with insulated floor (however I have been wrong about these things before!)
To gain control of heat and moisture flows through the house starts with robust air sealing (just as Walter said in the first response.) Without proper air sealing much of it is out of your control.
Only when the building envelope is sealed can it really be controlled. But the more air tight house also REQUIRES that there be controlled ventilation & conditioning.
For newbies it's useful to download Nate Adams' freebie chapters & short videos on Home Comfort 101, HVAC 101, and HVAC 102:
http://www.natethehousewhisperer.com/home-comfort-101.html
http://www.natethehousewhisperer.com/hvac-101.html
http://www.natethehousewhisperer.com/hvac-102.html
Cellulose actually does quite well with humidity, but doesn't do well with bulk water incursions. On a 100 year old house before ANY insulation is installed the flashing and other bulk water management details need to be verified and beefed up if necessary. (My house is only 98 years old, so I can just ignore that stuff. :-) )