Replies

1. Expert Member
   BILL WICHERS | Dec 22, 2019 09:07am | #1

   Have you had a blower door test done yet? If your house is already really tight, additional sealing with aerobarrier is less important. You need to know what you’re starting with.

   Bill

2. 
   shermpack | Dec 22, 2019 09:41am | #2

   Hi Bill. No blower door test yet. Just framed in.

3. Expert Member
   Dana Dorsett | Dec 22, 2019 10:26am | #3

   >"... 2,447 sq.ft... "
   -------
   >" Lennox 96 AFUE furnace with 16 seer 5 ton A/C"

   Unless this house has HUMONGOUS amounts of "sunset view" window area that AC is likely to be at least 2x oversized for your peak cooling loads, even with derating for higher peak outdoor temperatures.

   Even the WORST performing houses in the 2000-4000' size range in Allison Bailes' compiled graphic of square feet per ton against Manual-J cooling load calculations on real houses would call for only 3.5 tons, and the average would be 1.5-2 tons:

   https://beta.greenbuildingadvisor.com/app/uploads/sites/default/files/images/Bailes_graph_for_Manual_J_blog.preview.png

   https://www.greenbuildingadvisor.com/article/manual-j-load-calculations-vs-rules-of-thumb

   Most of those houses were in the Gulf Coast states, with perhaps lower sensible cooling loads, and only slightly lower 1% outside design temps than Sedona's 98F, but MUCH higher latent loads than Sedona's negative latent loads:

   https://higherlogicdownload.s3.amazonaws.com/ACCA/c6b38bda-2e04-4f93-bd51-7a80525ad936/UploadedImages/Outdoor-Design-Conditions-1.pdf

   The furnace/air handler that would come with a cooling coil that big is likely to be 4-5x oversized for your heating loads.

   Large oversized factors are the opposite of comfort, and I STRONGLY advise you to get an engineer (not an HVAC contractor) to run the load numbers before picking equipment. This house would likely be most comfortable with a very high SEER 2 ton modulating heat pump (an save the cost of hooking up to the gas grid.) But to know for sure requires running the numbers.

   A proper Manual-J is more valuable than AeroSeal taking it from 5.9ACH/50 down to 3ACH/50. It doesn't take rocket science or aerosol sealants to hit below 3ACH/50- just some level of consciousness among the trades that air tightness is important. Many homes in the upper mid-west built prior to code-maximums will test that tight.

4. 
   shermpack | Dec 22, 2019 01:31pm | #4

   Great advice Dana. It’s been a struggle getting any of the trades onboard with building science. This includes HVAC. I have a meeting tomorrow with the HVAC contractor and will drill him hard for his “facts”.

1. Expert Member
   Dana Dorsett | Dec 23, 2019 06:57pm | #5

   So, any updates on the HVAC front?

   It's worth taking a peek at Nate Adams' take on the effects of oversizing on comfort. Although he is usually dealing with retrofits, physics has a habit of working the same in new construction:

   http://www.natethehousewhisperer.com/home-comfort-101.html

   http://www.natethehousewhisperer.com/hvac-101.html

   http://www.natethehousewhisperer.com/hvac-102.html

   The short videos are a quicker "read" than downloading the book chapters, and captures the gist of the problem pretty well.

   Don't expect good load calculations from HVAC contractors- the industry average is pretty abyssmal even among contractors to run a pro-forma Manual-J.

   A couple of years ago I reviewed an HVAC contractor's Manual-J on a superinsulated high performance house where all of the inputs on the windows & R values were all code-min, with code-max air leakage. The heating load estimate was about 3x what an engineer came up with after carefully calculating all of the non-standard and WAY better than code U-factyor on the wall & attic, and correctly entered the published U-factors of the better-than code high performance windows. The HVAC contractor had been given the full details of the wall & roof construction, as well as a list of the window U-factors, and apparently couldn't deal with it.

   Putting the contractor on the spot and making them run a Manual-J usually ends up with a lot of input errors as a fat thumb on the scale to back up the original proposal with only modest changes, which isn't a great outcome.

   Your house isn't as non-standard as that high-performance house, but you're still shooting for better than code-min air tightness, and have slightly better than IRC code wall (though you're somewhat sub-code on the slab edge insulation, maybe the attic too unless it's a 2x4 truss). Compare your R-values & window U-factors to the zone 4 prescriptive values in Tables N1102.1.2 and N1102.1.4 in the IRC:

   https://codes.iccsafe.org/content/IRC2018/chapter-11-re-energy-efficiency

   The R38 roof deck attic is often done when going for code compliance on a whole house performance basis, but you're probably no worse than a typical code-min house, and an IRC 2018 code min house is pretty decent- would usually have a cooling load only a fraction of what 5 tons of AC compressor would deliver.

5. 
   burninate | Dec 24, 2019 04:31am | #6

   > Sedona, AZ
   Keep in mind that a large portion of the benefit of airsealing is protecting the heavily insulated (and thus, extremely vulnerable to rot) walls from moisture problems. In Arizona, this is not a problem you have. You're looking at it purely for the thermal benefits, and I find it questionable that on these grounds we'd be able to justify the cost of Aerobarrier in Sedona before it was mandatory in the Northeast.