I have designed a 4920 sf house in Dallas with a sealed crawl space. The crawl space walls are concrete and a have 2x10 floor joist system on top of it. The crawl space walls are insulated with 1” Polyiso and the rim joist is insulated with 5.5 open cell foam. The walls above are R23 BIBs in cavity + R5 exterior rigid foam over sheathing, and 5.5 open cell foam on the floor truss rim. The attic is ventilated with R50 BIF. The builder in previous houses has gotten air leakage of ≤1ACH50, so we know he does a good job there. The HVAC contractor doing the MJ says that he needs to do a “SEMI-TIGHT” construction because of the crawl space AND semi-tight fireplace installation when we are installing 2 sealed combustion fireplaces. I say the construction and fireplace should be label tight. It all boils down to add an additional 1 ton of AC form 5 to 6. I know is not much of an issue for most folks, but when I’m trying to design maximum efficiency (HERS 55), 1 ton extra is a lot, specially when we know all MJ are 15%-25% oversized from the get go. Any thoughts here? Thank you.


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Maybe it's just me, but I hate when a program has a built in adjustment (oversizing) that isn't documented, in other words so we can remove it.  A 15% to 25% adjustment for an old leaky style building wasn't that much.  But the same fudge factor on a very tight home makes it an unnecessary overkill.   Basically, all of the slop has been removed during construction and the Manual "J" needs to eliminate the oversizing.

A couple of comments from the cheap seats, I don't like blown in fiberglass and I hate acronyms (BIF).  Sorry, but people other than energy geeks read these threads.  Did you install baffles to protect the fiberglass from wind washing?  How much of that AC system is in the attic?

Have you run your own heat loss to compare with the manual J?  Have you run a blower door and duct leakage test?  I agree with you that the smaller size would be more acceptable, but I would also want some numbers to justify it. 

Also, if you spray foamed the crawl space rim joist, how does he justify a semi-tight designation due to the crawl?

One more question and I'll leave.  What else did your contractor do to make your home efficient?

Sealed and insulated electrical boxes; air tight IC can lights; glued wallboard and sheathing; glued bottom plate; extreme insulation on any ducts in attic;  vapor barrier where?; any radon provisions; Are the provisions for external combustion air for the fireplaces already in place?






Thanks for your reply. This home just started framing but the complete Thermal Enclosure will be performed. More specs are: Tile roof, ventilated attic (Baffles and Vents), Air-tight Drywall Approach in walls and ceilings, all HVAC and ducts are in the conditioned space, 2-16 SEER-2 stage AC units, 2-95% AFUE furnaces, IAQ thermostats & make-up air. Blown fiberglass was chosen by builder, but I do like cellulose better, and open cell foam on rims. Sheathing is taped with WRB over it, then 1” rigid foam (sealed & taped), 2-coat stucco. Windows are Marvin, U 0.28, and USHC 0.2-0.3. 2 sealed combustion fireplaces.

My manual J calcs are 1.8 & 2.3 ton systems, which I would have to install a 2&3 ton systems since those are the smallest units they sell, but is already oversized by 25-35% and 45%-50% (my best case scenario). The HVAC Company’s manual J is 2.3 & 2.8 tons, but they have semi-tight construction which accounts for the additional 20% oversized. Their best case scenario is oversized by 50% and 75%; and that is unacceptable to me. Old leaky houses were always oversized, but high-performing homes should not be. Again, it has a designed HERS 55 and we expect to have a ≤1ACH50 when done (or closed to it).

I do agree that Manual J software needs to get with the program (no pun intended) and find a way to not oversized loads. However, the HVAC industry feel they “need” to have that as safety buffer and they are not running fast to get it changed.

I’m hopping that someone here with good experience in running MJs for high performing homes can share their opinion.


Thanks for your reply. I really wish Clint Eastwood would take care of them HVAC varmints and we start getting good numbers to do the job right; same with the MJ software companies, as they help perpetuate the problem.

It should be a worthy reminder that MJ is a program used for ESTIMATING heat loads. Anyone that "estimates" anything gets better with experience....MJ users are no different. Once the estimate is known and equipment is selected and installed it should also be tested. Equipment can be tested and measured for BTU output and energy consumption to know if its properly sized. Moreover, you can use a third party (like me) to validate your contractors work. This is called PBC or performance based contracting, a growing trend that certainly adds value to the job and removes all the smoke and mirrors. As third party validations become mandated by municipalities ( and some already require them) we will have more integrity in our green efforts.

MJ7 is tried and true in my opinion while MJ8 is great for teaching.

Thanks Armando, much better description.  One thing to add, if not there, would be a raised cat-walk in the attic as eventually someone will need to go up there and BFG (blown fiberglass) does not hold up well to traffic.

The stucco siding doesn't excite me, but totally because I know nothing about it.

If you ask the HVAC contractor to run the numbers with "Tight" then you would have what should be their "high" numbers.  With "semi-tight" you are actually seeing two factors pushing the numbers up, the effects of "semi" and their normal oversizing.

Here's another thought, with two systems there are issues with separating them for load calculations ie, there always seems to be some sharing of output.  In other words, if the whole house needs 5 tons, can you be sure 2.5 plus 2.5 would be a solution.  In some cases, mostly up and down, one system tends to dominate while the other runs less frequently.  Even with an extended ranch, one would rarely seal and insulate between zones so the sharing here needs to be considered as well.  The results are that one unit may need to be larger than calculated while the other is smaller. 

We have been talking AC and you will have 2 furnaces.  From where I'm standing, about as far away as one can get in north America, wouldn't those be oversized as well.  Even in the north country we have problems finding heating systems small enough to match the reduced load of an energy efficient home and you have two.  And, to get that 95% AFUE they need to be condensing units that need long steady runs.

As for contributing to your original question, "experience with MJ's and new efficient buildings", I can't help, my building days were long ago.


Bud, modulating furnaces are the solution to oversized furnaces. The modulating feather is like an accelerator pedal, it only feeds as much gas as is needed to satisfy the load.

But unfortunately in my territory, modulating systems are rare, oil doesn't make that easy.  Propane is very expensive and natural gas has just started to be introduced.  However, oversizing by any name (IMO) is still not acceptable.  A new heating system installed today should still be around 15 to 20 years from now and there is little chance that home owners will be making changes that increase their heating loads.



Hi Armando! The AC contractor can do a manual S and not need to use a "fudge factor" in regards to having enough capacity.

Thanks Dale, I ran off to learn a bit about "S" and found a nice study which may be relevant for this home.  Manual S and J oversizing issues are discussed along with hot dry climates.



Humidity is a key component in equipment selection as equipment has both sensible and latent capacity. When you're building in the desert you can apply half of the latent capacity of the equipment to the sensible capacity as the desert does not have humidity concerns. If the equipment does not have to remove humidity this capacity can be used elsewhere allowing a smaller unit to get the job done. 


The load on the home should be performed as Tight construction.  If your contractor is up to date and has Manual J eighth edition, you can even input an air change rate rather than selecting from the three general categories. I recently wrote a White paper addressing HVAC issues in very thermally efficient, tightly constructed homes.  It is titled,"Proper Design of HVAC Systems for Spray Foam Homes and it is available at:


Proper Design of HVAC Systems for Spray Foam Homes

To request a copy, please email sprayfoam@huntsman.com

The physics of spray foam insulated homes and your home are identical from an HVAC perspective.  They have greatly reduced sensible heat loads, but the latent loads remain unchanged, they are very tightly constructed so ventilation must be provided, draft and combustion safety issues must be addressed, a/c sizing is critical as is correct duct design and register selection to ensure effective air distribution with the reduced ratio of CFM's of airflow to square footage of floor area. 

First is ventilation with fresh outside air.  That needs to be sized according to ASHRAE Std. 62.2.  I would recommend doing this by using a ThermaStor Ultra Air 150H unit to handle the ventilation and as a way to address the high latent load that is present in Dallas.  For those of you who think Texas is a dry desert I ask you to take note of the IECC climate zone map showing that much of Texas in inside of the ASHRAE " Hot and Humid" zone of the country. 

Next, I suggest following ACCA Manual S methodology to select the equipment, especially the evaporator coil and blowers.  This is again to ensure sufficient latent control and occupant comfort.  The coil must yield a low Sensible Heat Ratio (0.75 or less) to do the job in a humid town like Dallas.  The velocity of the air in the ducts should be at the high end of what ACCA Manual D allows and the registers need to have a low pressure drop and sufficient throw to provide good mixing and distribution.  Curved blade reisgters are a good idea here. You already have sealed combustion appliances so you are good to go there.  Thermostat/humidistat controls with variable speed ECM type blowers are a good idea.  A two speed a/c system would also be a good idea.  I lay odds the unit would operate most of the time in the lower capacity mode. 

I built our new home last year (2,500sf, ten foot ceilings, too many 4x8 windows) and installed a 1.75/3.0 ton two speed unit.  On our 112 degree all time high temperature day this summer here in Austin it stayed in the low speed mode all day and yes it held the 75 degree set point with no float.  I have only heard it go into high speed when I change the thermostat by several degrees after a set back for a trip. 

Anyway, get the white paper, give it to your a/c guy and see if that helps. 


Doug Garrett, CEM

Nice replies.


Go with multi-stage (see Doug Garrett's response) or better yet inverter driven units and the oversize issue is abated. 


Install a Carrier Infinity Greenspeed and it'll recognize and manage latent as well. 


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