We are building a new home that will be insulated with spray foam. The builder has come back with a 2.5 ton unit and a 3.5 ton unit. 3200 square foot.

My father in law used to be in the construction business 20 years ago and swears that we are not putting enough tonnage into the house. He says we need a 3 and a 4 ton unit.

Is the old rule of 1 ton for 500 square foot still applicable with the new spray foam technology? Thanks in advance.

Where are you located? I was told by many people to be wary of foam insulation in Texas.

We are in Lubbock. What are the reasons to be wary of foam? Tia.

Your father in law is not familiar with foam insulation...

Over 4000 sq-ft in Houston. We like to keep it very cool inside.

3T Upstairs, 4T Downstairs is plenty for our house. Hell, that even keeps our entire attic area in the low 80s in August.

There are summer days I go upstairs and it is 72 at 11am and the AC hasn't even turned on yet.

I had a very experienced HVAC engineer think I was crazy for putting all my media equipment in the attic area due to the heat produced...hahahahaha... my attic is air conditioned.

He did not have experience with foam.

The only reason to be wary of foam is contractors who don't know what they are doing. Absolutely no soffit or any other kind of attic vents. Need fresh air intakes and dehumidifiers to help change air in house.

We run the furnace about 1 day per year if that. Our summer electric bills are $250 max. 76 in most areas of the house and 66 at night in the master.

Foam cost us a little under 10K compared to about 5K for fiberglass. HVAC for the house is complex with variable blower and 2 stage compressors with 8 zones... It cost just over $20K. Absolutely two of the best things we did with our house.

Aggie_Eric98 said:

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Have someone go do a j load calculation.

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Absolutely this! Rules of thumb don't take into account insulation, ceiling height, windows, trees and orientation of the house.

The Fife said:

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Aggie_Eric98 said:

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Have someone go do a j load calculation.

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Absolutely this! Rules of thumb don't take into account insulation, ceiling height, windows, trees and orientation of the house.

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More importantly, be sure to use a reputable HVAC company with experience in calculations involving foam. R value is R value to the calculation whether it is foam or fiberglass.

What has to be accounted for is the cold air stays in the house because the foam kind of forms an igloo. Also, the hot air does not get in thru the foam in the walls and ceiling.

Put another vote down for manual J calculation. First, your HVAC contrator seems iffy. He came with a 2.5T and a 3.5T offering? What if you need a 3T? Your house requires one or the other....not either/or and YOU shouldn't be deciding that. Sound's like he's guessing too. You just need a contractor that knows how to do Man J. Some charge extra. Pay it. Thhey don't need to know about how to build a house, or anything about spray foam insulation except that you have it and how to incorprate that information to the load calculation. There IS NO RULE OF THUMB for HVAC equipment. If I ever get a contractor that gives me that, he is sent on his way down the road because he is either old and stubborn and has "30 years experience" or just has no clue about his job.

Manual J for load calculations. Manual S for sizing.

Get the reports and the information. This way, when your system works really great, or works really poor, you can whip that out and know that the sizing of the equipment is NOT the problem.

Your father in law might be right, or he might be wrong. He, I, you, this thread....we are all guessing until the heat load calculation is complete.

I also would spring for the best HVAC technology you can afford. I have a single 5 ton unit that services about 3500 square feet. My home is not spray foamed. The system has 3 zones, 3 FULLY COMMUNICATING thermostats with ECW zone control, VS blower, 2 stage cool, 3 stage heat. I like my house at 78 in the day and 75 a night...its nearly too cold for me, but the system keeps my humidity at 40-45%. Moving it to the low 70s is just uncomfortably cold.

The good news is that in Lubbock, you don't have to worry about the dehumidification problems if you oversize the units.

Old school mech. eng Bill Kellett TAMU prof.
475sf/ton or 525sf/ton if an insulated floor above = 6.72 tons (at least two units since they max out @ 5)
R13 walls R30 ceiling, one story
Ironically remember after all this attention to R-values, your windows (R-2?) compare as openings in the wall!
Note also how much glass is exposed to due West summer setting sun, just high enough to penetrate house before it finally sets a bit North of that.

Foam can be overpriced Open Cell, almost same as old school fiberglass/mineral wool or
high priced Closed Cell like they use in refrigerators ... you really NEED such???
Spider insulation has been the smart choice of those "in the know" ... but I digress.

There is a way YOU can figure all this out:
http://www.hvaccomputer.com/hvacsoft.asp?gclid=CLDsw_mps9MCFdi6wAodXggHPQ
OR
demand the HVAC design/build/installer's Manual J calculations, checking for accurate SF, R-values, etc.

MousepadMarauder said:

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We are in Lubbock. What are the reasons to be wary of foam? Tia.

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I considered foam until I saw it burn. I was at a friends house under construction that was insulated with foam. Took a piece of the foam they had cut off after it expanded. Burn about as quickly as a piece of clothe soaked in gasoline.

Ironically we have similar size homes, he has foam in walls and attic and I have cellulose we both have variable speed AC units and our electric bill has been the same or less over the last year. Now I guess you could argue that he technically has more sq ft because his attic is only a couple degrees warmer than the rest of his house.

I was completely shocked at how quickly that foam burned. Cellulose burns about as quickly as dirt.

Just built a 3300 sq foot home in Central west Texas. We have 1 ton per 800 sq ft. HVAC did manual J. Average bill since middle of last summer is $80 for all electric home.
There only "rule of thumb" with foam is get a manual J. I've been around a lot of foam and never seen "burning foam".

MousepadMarauder said:

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We are building a new home that will be insulated with spray foam. The builder has come back with a 2.5 ton unit and a 3.5 ton unit. 3200 square foot.

My father in law used to be in the construction business 20 years ago and swears that we are not putting enough tonnage into the house. He says we need a 3 and a 4 ton unit.

Is the old rule of 1 ton for 500 square foot still applicable with the new spray foam technology? Thanks in advance.

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1 or 2 story ?

What are the ceiling heights ?



How many sq ft of windows and doors face west ?

Any large trees around the house providing shade ?

Metal or shingle roof?
What color roof?
Tech shield decking or standard decking ?

Make sure you use certainteed shingles. Most shingle manufacturers won't warrant their shingles if you use foam because they believe it doesn't allow enough air flow to keep the shingle from burning up. I think they are still used to closed cell foam that doesn't allow anything to penetrate through it. Odds are the foam you will use open cell foam. That's what's current.

Bad Azz nailed it. Built similar home with competent hvac man. With spray foam and multi-stage ac unit downstairs we never had an electric bill over 200. This was in houston and keeping the ac set to 72 24/7.

Oh, I forgot to mention that a conditioned attic is pretty cool.

1 story.
Ceilings are all at least 10, some 12, living room vaulted to 24.
No large trees providing any shade.
13 windows.
Dark shingle roof.
Don't know about decking.

The Builder had the system "engineered" by an HVAC engineer, so I assume he did the J calculation? Thanks for all of the replies.

Ovalo said:

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Just built a 3300 sq foot home in Central west Texas. We have 1 ton per 800 sq ft. HVAC did manual J. Average bill since middle of last summer is $80 for all electric home.
There only "rule of thumb" with foam is get a manual J. I've been around a lot of foam and never seen "burning foam".

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I was surprised to see how quickly it burned and the amount of black smoke that was produced. Now when I pulled it out of the fire, the fire went out fairly quickly. So I guess as long as the foam doesn't stay exposed to the fire source then it should be okay.

I recently sold a 14 year old 3600 sq foot all electric home, we had already moved out so in Feb, I turned the HVAC off, to see how low the electric bill would be, it was $59. I completely forgot about the hot water heaters. 3 electric hot water heaters still generated almost $60 electric bill.

I am impressed by your average $80 a month electric bill, with people living in the home, I would assume 2 hot water heaters, and regular use of standard appliances. What other energy efficiencies did you incorporate into your house.

MousepadMarauder said:

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1 story.
Ceilings are all at least 10, some 12, living room vaulted to 24.
No large trees providing any shade.
13 windows.
Dark shingle roof.
Don't know about decking.

The Builder had the system "engineered" by an HVAC engineer, so I assume he did the J calculation? Thanks for all of the replies.

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Engineered means one thing in the state of Texas... that means stamped by a licensed, professional engineer.

Since home HVAC plans do not require professional design or stamping by state law, this is where you really need to actually start asking questions.

My builder brought me two of his HVAC subs and it was obvious that they had no clue about foam insulation with respect to HVAC.

I went and found an HVAC contractor with a lot of commercial experience who employed competent HVAC engineers and they did an excellent job.

I am an engineer. I have specified and had direct oversight over dozens of HVAC systems the past 25 years.

A manual J is an absolute requirement, it will ensure your HVAC system is not undersized.

The concerns are the following:

Residential HVAC contractors do not typically employ engineers. I would ask a lot of questions... It isn't hard to tell when they start bs'ing.

Manual J software is only as good as the data input. The key input has to do with ventilation / infiltration. Air cannot get into and out of a foamed house the way it can a traditionally insulated house. Ventilation and dehumidification become the most important factors.

As mentioned, no vents in soffits. Vents in soffit allow hot humid air into your attic to ventilated it in traditional systems. For foam systems, the attic is part of the air conditioned space. Allowing the free exchange of hot humid air into the attic is a disaster.

There is a fresh air source required at the unit in the attic.

A dehumidification system is very useful if not a requirement for houses with foam insulation. It does in ahead of the return air.

Foam, just like other building materials is tested to ensure it does not propagate a fire. It is not fireproof, but it does not propagate a fire. Walls with insulation and gypsum board on the inside form the required fire barrier. Since foam is sprayed to the underside of the roof instead of ceiling joists, there is no barrier. Reputable installers use a different fire resistant foam in this area.

Choosing foam insulation directly impacts home construction and HVAC design. Do not treat them separately as they are intertwined.

Had I trusted my builder, our HVAC would have likely been a disaster.

We did several things to reduce energy consumption.

1) We only have 1 window (4x4) exposed directly to the west sun. The rest are under a large overhang.
2) We used high E windows.
3) We used 3 1/2" open cell foam in walls and 6" on roof deck. We foamed the garage walls and roof and used insulated garage doors. House is zoned and we foamed interior walls between zones.
4) Bought a 80 gallon GE heat pump water heater and installed in garage. Because we foamed garage, coldest temp in winter was 55 degrees. This allowed WH to work at max savings during winter. We will lose performance in summer with insulated garage but higher incoming water temp makes a big difference.
5) Bought high performance ZC fireplace with outside combustion air.
6) Using 5 stage AC with VS blower.
7) LED lighting throughout.
8) On demand circulating hot water
9) Caulked all the plates, cripples, and headers. Tripled checked that windows and door were foamed completely.
10) Using induction cooktop.

This is our empty nest home. Wife works nights as RN so we rarely set thermostat to change. However, wife is skinny and I'm on blood thinner so we rarely set thermostat below 77 in summer. Seem it's more expensive to heat than cool.

Ovalo said:

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We did several things to reduce energy consumption.

1) We only have 1 window (4x4) exposed directly to the west sun. The rest are under a large overhang.
2) We used high E windows.
3) We used 3 1/2" open cell foam in walls and 6" on roof deck. We foamed the garage walls and roof and used insulated garage doors. House is zoned and we foamed interior walls between zones.
4) Bought a 80 gallon GE heat pump water heater and installed in garage. Because we foamed garage, coldest temp in winter was 55 degrees. This allowed WH to work at max savings during winter. We will lose performance in summer with insulated garage but higher incoming water temp makes a big difference.
5) Bought high performance ZC fireplace with outside combustion air.
6) Using 5 stage AC with VS blower.
7) LED lighting throughout.
8) On demand circulating hot water
9) Caulked all the plates, cripples, and headers. Tripled checked that windows and door were foamed completely.
10) Using induction cooktop.

This is our empty nest home. Wife works nights as RN so we rarely set thermostat to change. However, wife is skinny and I'm on blood thinner so we rarely set thermostat below 77 in summer. Seem it's more expensive to heat than cool.

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Thanks, I will have to add a few of those items to my list for the next house, if that day comes. We built last year and I went back and fourth over foam vs cellulose and after going over budget for several other items we opted for cellulose. The fire issue was not a deal breaker as I would have considered a couple options for an ignition barrier in the actic.

We do have gas so heater, cooktop and on demand water heaters help cause gas is fairly cheap but for a 2800 sq ft we average $152 per month over the last 12 months. We did add extra cellulose to attic, variable speed HVAC (which I think makes a huge difference of single stage), LED lighting through out and upgraded windows.

MousepadMarauder said:

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1 story.
Ceilings are all at least 10, some 12, living room vaulted to 24.
No large trees providing any shade.
13 windows.
Dark shingle roof.
Don't know about decking.

The Builder had the system "engineered" by an HVAC engineer, so I assume he did the J calculation? Thanks for all of the replies.

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What are the dimensions of the vaulted living room?
With that tall ceiling, dark roof and little shade I would personally lean towards the bigger units. Regardless of what others say, a 1/4 or 1/2 ton oversized is not going to cause a humidity problem in Texas. I've seen an 1100 sq ft house that ran off a 4 ton for years and it kept humidity around 45-50%. Same house is now ~2050 sq ft after additions running the same unit, and humidity is still in that same range. I can tell you builders tend to cut corners on hvac sizing. I've seen this numerous times in the field.

damn there is a lot of misinformation in this thread. it hurts my head so much I can't even respond.

The only answer is a Manuel J calculation. 49 years building experience.

Gaaaahhhhhhh......

Why are we guessing!?

shalackin said:

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damn there is a lot of misinformation in this thread. it hurts my head so much I can't even respond.

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I can tell you this... I have a real life, working HVAC system in my home with foam insulation. There were multiple manual J calcs done. They were done by licensed mechanical engineers with decades of experience (most without foam experience) including myself.

Experience with foam as it relates to ventilation and air changes isn't the same as fiberglass. My system is 20-30% smaller by tonnage than typical manual J calculations would yield for a non-foam house.

The misinformation is for someone to throw out the term manual J and act like that is the end of the story. A properly done manual J with a properly designed hvac system is the answer.

Gold star!

It seems like 99% of the HVAC contractors out there have no idea what they are doing in a foam house. And the problem is 99% of the foam guys have no idea what to do with ventilation in a foam house.

Foam houses do not require as much tonnage as a standard batt and blow house. But they do require correct turnover of air with fresh air. Make sure you find that 1% of HVAC companies to consult with or hire if you foam your house.

Overall, the qualities are worth it. It just takes a lot of research and discussing.

shalackin said:

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Gold star!

It seems like 99% of the HVAC contractors out there have no idea what they are doing in a foam house. And the problem is 99% of the foam guys have no idea what to do with ventilation in a foam house.

Foam houses do not require as much tonnage as a standard batt and blow house. But they do require correct turnover of air with fresh air. Make sure you find that 1% of HVAC companies to consult with or hire if you foam your house.

Overall, the qualities are worth it. It just takes a lot of research and discussing.

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This was my exact experience.

I ran thru 10 or more foam and HVAC contractors before I found one of each that understood the issues.

we foam the majority of our custom homes. and I am still not sure we get it 100% correct every time. but we don't just manual j it and throw in the system it says. I would bet the majority of foam homes are over sized.

shalackin said:

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Gold star!

It seems like 99% of the HVAC contractors out there have no idea what they are doing in a foam house. And the problem is 99% of the foam guys have no idea what to do with ventilation in a foam house.

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A large majority of HVAC contractors do not know how to properly size a system, how to size line sets, or how to properly size ductwork.

Companies with larger commercial sides of the business are usually better, but that doesn't mean the knowledge makes it to the residential size. This is especially true if they do a large amount of residential new construction.

I would like to bump this thread, because I am dealing with an issue from too much air in a spray foam house. It's better to be a little bit undersized in a spray foam house than oversized when it comes to HVAC equipment. Why? The number one problem with moisture issues from humidity is that a spray foam house is so tight, that it can cool too quickly. This does not give the unit enough run time to dehumidify your house and moisture issues often arise. Anyone else have similar issues?

Bumping this old thread. I am building a 3200 sqft home spray foamed and did one 4 ton two stage 3 zone system which is what the manual J said. Anxious to see how it does this summer.

Anyone recommending a single speed system on a foam house is really just saying they have no idea what to do A manual J is a must and true variable speed equipment is perfect foe foam houses...

College Station rule of thumb
via Prof Bill Kellett Mech.Eng + Arch
475sf/ton, attic above
525sf/ton insul. floor above & what one would employ if rafters are foamed, no air infiltration

GENERAL Bottom line = 6 tons
BUT

Obviously higher ceilings, lots of electrical loads
more windows/doors than normal& those exposed the setting sun invasion
+ 1-ton/20 guests
all add more load
but would show up in your HVAC guy's calculations.

Always opt for the higher tonnage if between standard unit tonnage packages.

What about a single speed in combination with a ventilating dehumidifier?