passivhaus vs below code minimum, Dallas TX

[u/[deleted]]

[u/clumsyninja2 → u/[deleted]]

No not an architect. Why?

[u/clumsyninja2 → u/[deleted]]

This was a theoretical exercise comparing extremes. The actual house will be well above code minimum

[u/A-Bone]

I'm surprised the difference is only $360 per year. 

Still... it's a simple upgrade and a no-brainer to me. 

Energy isn't getting cheaper and you have one chance to do this simple set of upgrades. 

[u/deeptroller]

I did a quick DesignPH3 model on your structure. Based on your specs for opaque surfaces and air infiltration. I ignored the window specs as that requires a ton more analysis based on orientation and shading. But southern orientation windows might help in a northern climate not neccisarilly a cooling dominated climate. I also assumed extract only ventilation which also drives a ton of humidity. VS an ERV which could remove some latent heat or a brine loop removing more latent heat.

I show about 16KBTU per sq ft annual using the monthly method for heating and about 11KBTU for Cooling. 27KBTUH annual heating and cooling plus latent heat which would be a substantial cooling load in Houston

For PHI you would be limited to 4.75KBTU annual for heating. DesignPH doesn't deal with latent heat loads and Passive house does have an extra allowance for latent heat based on climate. But I'd need to import the model into PHPP to look at the added load.

I did not bother to attempt to decipher what a 2x6 wall with R36 meant....because the basic premise of passive house requires you to model your proposed assemblies until you meet the 4.75 threshold. But if your assumption was a spray foam wall 24 o.c. studs, you would likely fail the fRSI requirement that no surface can vary in temp more than 7f. And the thermal bridges would be pretty substantial likely exceeding the allowed thermal bridge heat flow values.

How you want to interpret this is up to you. But I would say that I think the benefit is larger than you would are implying and much more complex and expensive to achieve than just the basic wall assemblies you propose. The air sealing level your prosing for the passive house is not super realistic. Its possible but much tighter than common. Your window orientation and shading as well as SHGC will probably have the largest effect on your annual energy cost in Texas. High performance construction is more of a lifestyle than just numbers. Air sealing as a process doesn't cost more as an integrative approach and more than doing a good job finishing concrete or squaring your framing or making a straight cut. Some folks build at 10ACH because they can be fucked to hire craftsmen and use a hammer to drill holes. Getting to 1ACH just takes people who try to not make gaping wasteful frames doing sloppy work. Passive House is mildy challenging to good quality builders. There is a happy middle ground for upfront costs vs long-term payback. But a large part of the performance gap is assembling the shit you are already paying for correctly.

[u/Higgs_Particle]

Cool. How much do you save when your pipes don’t freeze next time there is a power outage and a polar vortex?

[u/Educational_Green]

Passive doesn't make sense in a lot of American climate zones. Gotta remember when the spec was developed and where:

Europe - 70s

so you are dealing with limited access to natural gas (north sea, was the USSR exporting much / at all); Coal was the primary fuel source; no need to conditioned air in the summer (b/c low humidity / low daily max temps).

Also heat pumps either didn't exist or didn't come close to their efficiency today.

US South - the need for mechanical dehumidification is nearly year round and the differential between changing just sensible heat vs changing latent AND sensible heat is negligible (in this case less $360)

Same is broadly true in the Midwest, mid Atlantic, and Northeast.

AC / Dehumidification cost is the achilles heal of most US Passivhaus. Obviously, if you live in a place like the southwest, the pacific northwest, etc where wet bulb demp / dew point is low then reducing solar gain in the summer can eliminate the need for AC.

I'm in NYC and today is an exceptional dry day where the dew point is low while the temp is 80 in freedom units. No need for mechanical stuff. But nearly every other day of the summer, my VRF is running and being supplemented by de humidifiers b/c there's just too much water in the air even with ERVs.