Building Design Questions
Emel
| Posted in General Questions on
After going round and round with many experts in our area (Zone 4, high water table), it’s been decided the best thing to do for our build is a slab on grade. With this knowledge I’m wondering about a few things…
1. It’s been recommended to do a shallow frost wall (around 3’ deep, 12” of it above grade) and wrap the outside with insulation and then extend the insulation out from the foundation. There’s a name for it but I unfortunately don’t know what it’s called. Has anyone heard of this and do you have thoughts? To be extra careful, we would also have a waterproofing membrane painted on the exterior and drain tile around the exterior of the slab draining to daylight.
2. The plumbing and Ductwork: We are lofting the entire ceiling above the main floor into a usable loft space/2nd floor. This would allow all of our ductwork and plumbing to go through the 1st floor ceiling and not the slab. Any thoughts on this or things we would need to do, look out for, or do special to prevent mold or future issues? Mold is our number one concern in preventing due to living in a house in the past with it that caused health issues.
3. For the wall structure, since we are in a more humid and damp environment, we are debating between SIPS for a wall system or standard build with Rockwool or fiberglass batt insulation, then a vapor retarder, then drywall. The benefit of sips seems to be that it goes up fast and would be open to elements for less time and also would allow us to use exterior poly insulation around the exterior. Negative seem to be that it’s not a breathable system so any vapor or water penetration wouldn’t be able to dry out. Is there one system that would be recommended over another? Due to cost, we may have to go with vinyl siding.
Would love to hear thoughts and advice. Thank you!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
I know that houses work as a system, but you might get better responses if you break this into three separate questions.
In terms of the broad question, managing mold is all about managing humidity and temperature. In a modern, properly-built house you keep bulk intrusion of water out (ie rainwater seeping in) and your only two sources of humidity are occupant activity -- cooking, bathing, breathing -- and outside air infiltration. There are two ways to get rid of excess humidity. If the outside air has a lower moisture content, you can get rid of humidity by ventilation. If it doesn't, the only way is mechanical removal -- either air conditioning or a dehumidifier.
So you want a house you can seal up tight and wring out all the moisture from. To the extent you need ventilation air, you want a way of removing humidity from that air.
If the humidity is high enough that condensation occurs, mold will grow anywhere -- I've seen it grow on plastic, stone, concrete, steel, even glass. You definitely want to manage your temperature and humidity to avoid condensation. I'm a bit of a Contrarian in that I believe in humid places that's not enough, wood will support mold when the moisture content gets to 15% or so, which depending on temperature can happen at humidities far lower than the dew point.
In general, in a humid environment you have to be much more vigilant than in a dry climate.
Because we do live in a humid environment, is there a wall system would you recommend? I hear some say SIPS and seal everything super tight. But I’ve read other places that doing stick build, an insulation like rock wool and then a breathable vapor retarder (being sure not to use something like plastic that would act as a vapor barrier), followed by drywall is best.
Also with either of those to do a rain screen on the outside so if something does get wet the moisture would be able to get out and dry. We don’t want to have a mold issue so trying to choose wisely.
Zone 4 I'd imagine you're heating dominant?
You can look up your county here: https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/County%20Level%20Design%20Temperature%20Reference%20Guide%20-%202015-06-24.pdf
One of the columns is "HDD/CDD ratio," which is heating degree-days to cooling degree-days. If that ratio is greater than 1.0 you're heating dominant.
Even in humid places, when you're heating the outside air almost always has lower moisture than the inside air. Moisture tends to flow from warm to cold and from moist to dry, so in heating season all of the vapor drive is from inside to outside. So if you have a wall assembly that blocks vapor from entering the wall on the interior, and can dry to the exterior, it will dry nicely. This is basically the way that walls are normally built in heating climates.
Really the most important thing to keep humidity out outside of the heating season is making sure the house is thoroughly air sealed. The primary source of humidity is going to be air leaking in. This is something that has really changed in the past 20 years or so, the current prevailing wisdom is that houses should be as tight as possible, and modern building codes prescribe a level of tightness that was unthinkable even a few years ago.
It's tough if you live in a humid place and you're the kind of person who likes to have the windows open as often as possible. There's just no good solution.
You don't have a basement, if you had one I'd give it special attention.
Thank you this is helpful. I looked us up and we’re a 3.8, so yes, heating dominant. I think my concern about our climate is in the winter, it’s easy to plan for, however for three months of the summer we heat up to 80’s and 90’s and are humid. Not humid like Florida humid but still too humid to open windows. In this case would it be best to do sips? Or maybe the rockwool so the insulation is breathable and then use a vapor retarder like Certainteed’s Membrain or VaproShield product which would block one way but allow air when needed? I also saw where there are some companies that do a treatment before installing the drywall that aerosolizes a solution that seals every crack and crevice. That was another thing we considered because then you could do a blower door test to make sure it’s satisfactory. I’ve heard there’s a difference between vapor barriers and vapor retarders and a vapor retarder is what you want?
I'm sure there are a lot of great SIP houses out there, this forum mostly sees them when there are issues. The problem is with the S in SIP, the structural part. If the roof or window flashing details and water management are not perfect or air sealing for an SIP roof, the skin can get damaged and there is no simple way to fix it. The way around this problem is to not make SIP structural, at that point you are adding enough wood that you might as well insulate that and skip the SIP.
You say “high water table”
I have to ask will this house be in the 500 year flood plain?
Have you priced flood insurance for this lot?
Do you understand home insurance will not cover flood damage?
Life is too short don’t build on low ground and if you must build on pilings above the 100 year flood line.
Walta
No it is not on a flood plain. The land slopes to where we are at the lowest in our hill at 1136 elevation and the back of our property is at 1120. It would have to rise over 16 feet to flood and is not labeled as a floodplain. The high water table is due to perched water that comes from a granite bed below about 10 feet below the surface. This is why the architect feels we should essentially take off the 2’ of junk soil, replace with sand, compacting as we go, and then build up from there a thin & insulated foundation that we water seal and drain tile to daylight and use dirt from our land to go around the foundation to essentially create a small hill around the house to ensure all surface water drains away.
1. "Frost protected shallow foundations" ... They've been around for several decades and are proven reliable if correctly engineered. Here is a link to a detailed design guide from HUD.
https://www.huduser.gov/publications/pdf/fpsfguide.pdf
Also, the relevant section of the code IRC2021 is R403.3.1.1
https://codes.iccsafe.org/content/IRC2021P1/chapter-4-foundations#IRC2021P1_Pt03_Ch04_SecR403.3
2. Keeping ductwork within the conditioned space is generally a good thing to do, which is one of the arguments in favor of having a conditioned attic.
3. SIPS. Flashing detailing around door and window openings is critical using any construction system. This is where the large majority of water damage occurs in most houses of all construction types. It's no different with SIPS. Do it right the first time. That said, it's also important that SIPS panel joints should be taped. This wasn't always done in the past and any builders who haven't gotten on board with it shouldn't be used. Typically the taping is done on the inside. But I advocate taping the outside as well. Tape is cheap and the labor doesn't amount to more than a day or two for 1 person. Most manufacturers have their own specific details. But it would behoove you to familiarize yourself with the details recommended by the Structural Insulated Panel Association. They've got lots of good resources on their website.
https://www.sips.org/resources/design#
Also, you absolutely want to have rainscreen walls and make sure to check out the vented roof assembly (figure 27) from the SIPs association guide to roof assemblies.
I hope these resources help. Best of luck with your project.
Thank you! Would rainscreen walls be used only is choosing a siding like wood, hardy board or the like? I was thinking you couldn’t do it if you choose vinyl. Or maybe vinyl siding is not a good option?
For a variety of environmental reasons I avoid vinyl. But vinyl siding is easily done as a rainscreen wall. So if that's your preferred material by all means you can do a rainscreen wall with it.
Got it, thank you
Here's a good overview of frost protected shallow foundations: https://www.greenbuildingadvisor.com/article/frost-protected-shallow-foundations.
Like DC wrote, you'll probably get more or better responses if you post separate questions. Though you seem to be off to a good start here.
Thank you, this was my first time posting, so that’s helpful to know.