Misc. Questions on New Build
Hi there, I’m Sean. I’m building a new home in Climate Zone 6B, North Idaho. Single level with bonus over garage, 2850sf finished with a 468sf unfinished basement under part, a vented crawl under the majority, and a vented attic. Aiming for a “pretty good” house.
1. Aside from minimizing air leakage, it seems exterior rigid foam is the next best bang for our buck. The wall assembly I’m planning is drywall > 2×6 w/ R-23 batt > plywood sheathing 19/32 or 15/32 I’m not sure > 2″ polyiso > air gap/furring > Hardie siding.
1a) I’m looking at R-max Pro Select R-Matte 3 polyiso which has a stated R-value of 13.1 for 2″ thickness, but being in Zone 6, I need to discount the 13.1? Do I use R-4.5, so I only end up with R-9 total in winter? Am I correct in concluding then that this won’t work since that’s less than the 11.25 required to keep the sheathing warm? Does the R-value of the fiber-cement (Hardie lap) siding help insulate the sheathing at all even though it’s on the other side of an air gap?
1b) The Sika R-max I found includes a foil facer, the product data says: “While one side of the board has a reflective surface, the other side has been coated to have a non-reflective white matte surface. Boards can be installed with either the silver or white side out. For additional radiant barrier properties, install silver side against air space if one exists.” Is the radiant barrier they’re referring to meant for hot climates? I am going to use vertical furring strips so there’s an air gap, but in cold climates the foil should be aimed in toward the house to reflect heat and bounce it back in, correct?
1c) I was hoping to be able to tape the seams of the foam and use it as a moisture barrier, but the product data also states “Per the IBC, a WRB is required behind the exterior wall veneer.” Well, my AHJ doesn’t lock me into any building code, let alone IBC, I can kind of do whatever I want. Either way, to keep things simple for the builder should I plan on house wrap like Tyvek over the sheathing?
2. I’ve read on here that recessed can lights can be a major source of air leakage. Like most pretty good, newer homes, we are planning on quite a few of them. But we currently use LED faux can lights that are just a fixture and no bulb, like this:
Does anyone have experience with a gasket of some kind for these to minimize air leak?
3. I’ve been thinking/angsting a lot about the HVAC system with our house layout. I’m not sure the mechanical sub is going to be the best person to design the system in the way I want. We had originally planned on a Daikin unitary air source heat pump, but with the layout being very split – one side has kids rooms, a big great room in the middle, and the master suite on the other side – I’m curious if one or more ducted/ductless splits would be more efficient than one big system for everything. I’m wondering if there might be a Daikin sales rep or someone who would be willing to focus more on my goals than a rural Idaho HVAC guy or just a straight up mechanical engineer?
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Replies
1. Not quite. The biggest energy savings would be by sealing and insulating your crawlspace. It will also make the house significantly more comfortable as the floor would be much warmer. It is also a great place for HVAC equipment and ducting.
After that, exterior rigid is a pretty good R value bump provided your local builders are used to working with it. If not, you can also look at 2x8 24" OC walls with R30 batts or 9.5" I-joist as studs dense packed without any exterior rigid.
1a. I wouldn't worry about the R value drop of the polyiso, even in Zone 6 this will only happen for part of the winter and mostly the outermost bit of insulation. For design you can use the LTTR value, which would be R11.4.
1b. The shiny surface should face towards the rain screen cavity. This way it would give you an R1 bump, so not a whole lot. Not something I would worry too much about.
1c. House wrap is the simplest. It makes it easier for the builder as it is closer to a standard assembly. I like to put the house wrap over the foam and build small window bucks to bring the windows out in plane with the foam surface. This puts everything in plane, make window trim easier and gives you much bigger window sill. This is not a standard assembly though.
Important item is always tape the seams of your sheathing as your main air barrier.
2. You want lights that install into a standard ceiling junction box such as a Halo SMD 5. These have a gasket on the back to help it seal. Also use a vapor tight device box, these have a gasketed flange that seals against the drywall.
3. A well sealed and insulated house has very even temperature. The only time zoning is worth it is if you have something like a sunroom with a highly variable heating or cooling load.
A single properly sized cold climate heat pump with a multi position air handler is your best bet. These can supply very long duct runs. As long as the ducting is sized properly, reasonably sealed and kept inside the conditioned envelope, it will work just fine.
Bonus rooms are very hard to detail right. Even done properly it will never be comfortable. With a single story house, I would skip it and increase the footprint to make up for the extra space.
Thank you! I don't think my contractor has much experience with rigid insulation on walls. I might look for a specialty insulation sub to do that, along with air sealing, blower door, and the crawl space.
You've now sent me down the rabbit hole of crawlspace sealing/conditioning. You mentioned it would make the house more comfortable, but I still can't find any information/data on the energy benefit. I guess it would probably depend on the house and other variables.
The property appears to be within a high radon area, so aside from testing I like the idea of putting in a passive system - a little more digging, some pipe and gravel seems like cheap insurance. Should probably do that either way, right?
Very good to know on the HVAC, I was racking my brain trying to figure out how to get more efficiency out of that element. I think if we seal the crawl space, putting the AHU in the basement and running duct in the crawl makes the most sense.
I hear you on the bonus above garage, we have one now and it is never the right temperature in there. Unfortunately we're too far with the plans to remove it. I'm getting pretty deep into the air barrier/insulation details for that space. Do you think a duct run in a chase from the basement up to there would be better, or does a separate mini-split make sense?
An insulated and sealed crawl will require much less insulation overall than insulating the floor, so for simple build costs that is the way to go.
To figure out the difference in heat loss, you can ask your designer to run a man J for both options. My guess the vented crawl with the associated duct losses will add about 3/4 ton of heat loss.
Radon is something that is relatively straight forward and inexpensive to deal with on a new build. One additional measure I would recommend is to install a stale air pickup in the crawlspace and basement for your HRV/ERV. This will vent any radon that does make it in plus at the same time provide the necessary conditioned air flow.
For the bonus room, I would go for a wall/floor mount mini split on its own outdoor unit. This is gives the best efficiency and full control without needing the long runs from the main air handler.