Scot, congrats on the good research. You are asking all the right questions. Curious what state youre in.

I do not think 4.5 is good enough personally but it probably will be difficult to get it lower in your situation. International energy code is 3 ACH@50 for zone 4 and I think this is still not close to a point of diminishing returns. Its certainly worth a shot though as the material costs are low and DIY is cheap. I would try to caulk any bottom plates to subfloor and especially masonry transitions. Tape the window jambs to the rough framing (or a meticulous caulk job) from the interior before sheetrock.

If the bottom flange of the window is taped from exterior, I would do my best to remove it. If windows are not installed yet, I would slope the rough sills to the exterior and flash it with a continuous piece of tape 6" up the sides and pulled over the ZIP to the exterior. This window flashing detail is especially important on gables or exposed areas.

If its not too late, eliminate all recess cans on the attic level. Otherwise, look into building airtight boxes around them. You could always do that after move in but you will degrade the insulation around them so plan on having more on hand. I would also question what insulation is being used for R38. If its blown-in as it should be, consider upgrading to R50 which is international code minimum for zone 4.

Great plan on attic access. Its a huge weakness of course so do your best on airsealing this detail.

The windows are a.. point of contention. They are fully sealed by the builder via tape and won't let me touch them. I've already talked to Huber and the window manufacturer, both say it is "not the current recommendation, but acceptable" to tape down all four sides... Everyone agrees three sides and a sill is better, but it isn't the way the builder does it. VERY frustrating.

As for the recessed cans, that's a no can do for pure function. Floor lamps are not acceptable to the family, so recessed cans it is. I have replaced IC cans in the attic with ICAT before in another house, so I think that should be simple enough to do again, if they are IC. I agree that they are a bad solution, but there are very few inexpensive and good looking options for recessed or on ceiling lighting today.

I could build boxes, but that seems like a hell of a lot more work than pulling the housing, replacing it and caulking. Then pushing the blown in cellulose back on-top. Is there a reason to box it in that I am not aware of?

As for the state, it is VA. So we are still under ICC2009, and not 2012. So r38 in the attic. Using the BEopt application it looks like any benefits I can make in the ACH value should pay back quickly, 4.5 going to 3.0 would pay back about 150 a year, where 38-50 would only be about 50-75 per year.

Going to 3.0 might be difficult though, with only caulking and basic sealing at my fingertips.

So-called ICAT cans are anything but 'AT'. I used 20 of them in our build, and when you go up in the attic, you can see light shining around them. I built boxes of foamboard to cover them, and carefully foamed them to the ceiling drywall. My LED inserts do seem to be air tight, and I may glue them in place with caulking to be sure they are sealed to the ceiling. As long as they are supposed to last, dealing with the caulk when I eventually change one is something I'm willing to deal with.

Depending upon the bulb used, cans can cause other problems, even if they are air tight. If the can itself gets warm, the warm air will rise, and can cause ice damming on the roof by melting snow in a small area, which has nowhere to go on the cold roof. I have found that LED cans don't get very warm, and we rarely get snow here in SC.

3.1 ach50 is the minimum requirement in my jurisdiction.

windows are foamed in and taped on the outside

What more would you do? (Other than use windows with lower U values.)

No basement insulation? The first few feet, which includes the abovegrade portion, are important to seal and well insulate with moisture tolerant materials.

it isn't the ICAT housing that leaks, it is the penetration
in the sheetrock around the ICAT.

I use mastic tape, drop the trim piece & tape
sheetrock into housing.

recently I was on a job...and always look at recessed
lights. these IC cans had orange ICAT stickers ...
but were IC cans. so always look at the housing of
the can lights for openings..those are IC, no openings
are ICAT. I emailed mfg..but they could care less...
oh well...

'air tight' trim kits..LOL! they don't make the hole
in the ceiling air tight at all.

if you opt for building boxes...which wouldn't be necessary
if sheetrock penetrations are sealed and housing is air tight..
there has to be 4" clearance on all sides and top of box
to light. sheetrock works well, but sealing to attic floor is
difficult with caulk. again mastic tape works well and is
a long lasting seal.

you'll also want to seal the oversized cut hidden behind
bath fan covers and at hvac supply boxes, behind supply
grills. hardcast brand 1402 is the stuff to use.

sounds like you are on track OP. maybe test after
you do your final air sealing?

best of luck.

Do you recommend the mastic tape sealing the housing to drywall on the attic side or on the lip of the housing to the drywall from the finished side?

BTW, the basement has r13 on all walls, including unfinished.

One thing that confuses me a bit, the outside surface of the house is zip sheathing and zip taped on all seams, including the bottom lip of the sheathing to the concrete. That seems to be a way to be very tight in and of itself. Do you think the majority of remaining air leakage is as a result of the cans, doors and other similar internal penetrations like electrical or interior walls to the attic spaces?

I can take some pictures of other houses that are under construction to show comparison if that might help.

Posting as husband of runnergirl79.

Your thread is very timely as we prepare a custom build (slightly different scenario than yours in that respect). However, local customs in Eastern NC don't place emphasis on air sealing (code of 5ACH is considered good), insulation (batts), atrium windows, etc. What concerns me about your builder is he's only getting 4.5ACH with zip system and air sealing as you indicate. 3-3.5 should be easily achievable using that system, routine bottom and top plate gaskets, sealing plumbing/wiring/etc. Can anyone else provide insight?

I think pictures would greatly help in this situation. Recently walked through a house with foam at all openings, sealed bottom and top plates, sealed ICATs, and sealed flex duct supplies at ceiling. However, there was one duct not sealed and it left a 2cm diameter hole around the entirety of the duct, which would create a huge hole in an otherwise tight house.

Here's a link to a recent JLC article on air sealing. Basically, it comes down to someone paying attention before the drywall goes up. If that someone is you, and you foot the cost of materials, the builder might be ok with that.

Here is a link that might be useful: A Drywaller's Approach to Air Sealing Homes

this may help, see attached pic.

I use hardcast brand 1402 mastic tape,
and use it in many places. this is a good strong
tape that has a long life. even at $30 per roll
(3" wide 100') it is a good deal.
personally I hate to have to go back & re-do...
so over the years, this is my only tape to use.
invest in this tape and you'll only seal once.

as you can see from the pic, the sealing is done
from inside the house, much easier than working
from attic side. the pic is from an existing home...
but as long as sheetrock is up...you'll want to
do this for recessed light cuts in sheetrock &
bath fans. stove vents..anything that penetrates
into attic space.

if all zip seams are sealed, sole plate is sealed to
slab/subfloor, windows & doors are properly flashed
& sealed...then look to ceiling for leakage.
thermal bypasses like fireplace openings in attic
that are not sealed at attic floor. dropped ceilings,
and any holes cut into ceilings.
builders do good with making walls air tight, but
trades people cut holes in ceiling, this is the air
barrier to the attic.
holes in ceiling will suck attic air into living space,
so sealing these holes & creating as perfect as
possible air barrier is the goal.

and don't sweat it if you can't get it all done
during construction. I can pretty much walk
you through sealing methods during or after
home is completed.

I'l have to check out JLC article later.
what I tell my clients is to tell sheetrock crew
that no ceiling moldings are being installed.
this way they tape & seal the wall/ceiling joints.

air tight drywall approach...even just to google
it will show you lots of ways to seal.
here in the south...we don't gasket much or
caulk sheetrock to studs...but in cold climates
these methods are more of an issue.

kudos to you for putting ductwork inside cond
space...this is such a great thing to do...I wish
arch/designers would incorporate this in every
new home. my analysis show I have a 25% energy
savings by putting my ducts in furdowns in the
cond space.

I meant to ask earlier...where are you located??
best of luck.

I"d have liked to read the article, but no longer
a subscriber..

Thanks for the picture! I was definitely planning on doing something like that, the hint for the mastic tape is very appreciated. I used regular foil tape before to do that to some of my vents, and it "worked" but mastic tape would probably be vastly better, especially over time.

Location is Virginia.

I think its good the direction of the thread is focusing on the air-sealing details of the attic floor/ceiling plane of vented attics which is the most common type of roof for most production builds. Most custom construction in our area is using spray foam under unvented roof deck so those details arent quite as important. Some high performance projects seem to be using a flash of closed cell spray foam over the attic floor in vented attic situations followed with thick levels of a more typical loose fill insulation. I would highly recommend people explore this approach for upgrading their vented attics. Its also a good idea to have the sheetrock crew install and tape the ceiling prior to interior walls. Ive read that some passivhaus projects are just having the carpenters install ZIP sheathing on the ceiling instead of the drywallers making an extra trip.

When I weatherized my existing home I was surprised to find evidence of significant air leakage through typical ceiling mounted lights and ceiling fans, mainly through the gap between sheetrock and junction boxes. Ceilings have very high vapor and air pressures during windy or cold weather and strong stack effects.

I wanted to do my ceilings first, before I built the interior walls, but that is not allowed here. The inspectors are not willing to go up in the attic to inspect. It sure would be easier to drywall one big room, rather than working around all of those interior partitions.

I foamed all of the ceiling electrical boxes. both from underneath and on top. The boxes themselves are rather leaky, plus the gap around them.