Any HVAC or foam insulation experts here? Contractor issue

[Neilk]

Hello,

We are building a new house in Dallas and I am not liking what the HVAC sub is proposing so I am looking for second opinions. We are going to use foam insulation, but the sub is using R values of 13 for the walls and 21 for the roof joists. It seems as if those values are barely any better than regular insulation.

The reason I ask is that the HVAC guy is proposing 8 to 9 tons of capacity for what I was hoping to be a very well insulated 5,000 sq ft house. I was expecting to be around 6.

I am sure we have some experts here that can point me in the right direction. Should I tell him to up the R Values? Our builder is concerned that if we tell the HVAC guy to change his proposal, that we won't have any recourse should the design end up not working on our house. This part of the build has been a PITA.

Any HVAC guys here in Dallas?

Thanks for any advice!

[VincentVega]

Not a pro but I'm going through a reno so I've been reading code a lot lately. Fun stuff.

Your #'s dont add up, check this https://energycode.pnl.gov/EnergyCodeReqs/?state=Texas

Contractor should calculate load with a manual j. Not exact but I think this is a decent estimator, HVAC Load Calculation - Maunualj - Whole House Loadcalc

[look 171]

Originally Posted by Neilk

Hello,

We are building a new house in Dallas and I am not liking what the HVAC sub is proposing so I am looking for second opinions. We are going to use foam insulation, but the sub is using R values of 13 for the walls and 21 for the roof joists. It seems as if those values are barely any better than regular insulation.

The reason I ask is that the HVAC guy is proposing 8 to 9 tons of capacity for what I was hoping to be a very well insulated 5,000 sq ft house. I was expecting to be around 6.

I am sure we have some experts here that can point me in the right direction. Should I tell him to up the R Values? Our builder is concerned that if we tell the HVAC guy to change his proposal, that we won't have any recourse should the design end up not working on our house. This part of the build has been a PITA.

Any HVAC guys here in Dallas?

Thanks for any advice!

You want a 6 ton unit to cool or heat a 5000sq' hosue in Texas or are you asking if you increase the R value in the insulation is necessary?

[porsche4life]

8 or 9 still sounds good for that big... We are around 3k sqft and have two 5 ton units, one for each floor.... But this is Phoenix and they get a workout...

[URY914]

Your contractor should have a size/load calculator which is simple to use. You add in the square footage, r-value, window type, etc, etc and it calculates the tonnage required. You can change any one of the variables if you want to play "what-if".

There has to be an easy one on-line somewhere.

[billybek]

He should be considering what the actual insulating value of the walls and ceiling space is going to be.
Most manufacturers have a system calculator that takes this into consideration as well as window size/type and insulating values and ventilation requirements/infiltration.
A new home with tight construction and great insulation is much easier to cool and keep cool. Curtain and blinds can also make a huge difference in the required tonnage.

[Scuba Steve]

They need to perform a true calculation, which takes into account windows, building orientation, and a number of other things. If you're going with foam insulation in the walls and ceiling that probably changes things up a bit too since there will be a lot less air infiltration than if you had traditional insulation.

[javadog]

The R-value of a wall assembly takes into account a multiitude of things.

Thickness
Type of studs
Spacing of studs
Type of insulation
Type/thickness of exterior sheathing
Exterior veneer, or finish
Number, size and type of penetrations
Etc.

It's not as simple as the R-value of a certain thickness of insulation.

You don't give us any details, so who knows what your contractor should use. The numbers sound low, unless you have a bunch of windows, etc.

The actual calculations for tonnage needed are pretty involved and there's a whole bunch of factors to consider. It's more complex than it sounds and maybe your guy is being conservative. On the other hand, I have a total of 12 tons (3 units) in a two story house slightly smaller than yours, on a heavily wooded lot, in a slightly milder climate. 6 tons sounds a little short, to me.

JR

[rbp]

Oddly enough I am an expert. Not familiar with your climate but there are 2 important things you should consider and without getting into a long technical babble here it is.

First off (if done right) the AC system needs to be sized to maintain a 12 - 15 deg. F difference between the outside and inside and should run for long periods and not cycle on and off. The intent here is not to hang meat in the house but take the edge off and remove the humidity. If the unit is oversized then it will reduce the temperature too fast and leave the humidity high resulting in a damp cave like feeling.

In my opinion, if your house is well constructed and sealed properly then you should be looking at somewhere around 1 TR per 1,000 sqft. If I were you I would buy a 2 circuit coil and 1 - 2 TR unit and 1 - 4 TR unit. Obtain a smarter programmable type controller and you will have 3 stages of cooling, stage 1 is the 2 TR on and the 4 TR off, stage 2 is the 2 TR off and the 4 TR on and the 3rd is both units on for a total of 6TR.

When space temperature is above 74 deg. then on stage 1 cooling. If stage 1 cooling has been running for more than 8 hours and space temperature is above 76 deg. then off stage 1 and on stage 2, if after an additional 4 hours of operation the temperature continues to rise above 78 deg. then on cooling stage 1 and run on stage 3. Continue to run on both units till temperature drops below 77 deg. then off stage 1 and continue operation on stage 2. When temperature drops below 75 deg. then off stage 2 and on stage 1. When temperature drops below 72 deg. then off stage 1.

A humidity sensor should be added such that your sequence would be, when space humidity is above 50% and space temperature is below 72 deg. then on stage 1 cooling. When space humidity is below 45% then off stage 1 cooling.

Our summers are very hot and humid. We get 90% and temperatures of 95 deg. F. in months of July and August. Today will be about mid 80's to 90's and still high humidity. My lake house is well constructed and is 2,500 sqft and I have a 1 TR and a 2 TR unit running the sequence as per above. I have never seen the 2 TR run. The house doesn't need more that the 1 TR. Most people way oversize the AC capacity. Since we do have cold weather in January and February the house is well insulated with R-20 in the walls and R-40 in the ceiling space. Oversizing is the worst thing you can do.

[Porsche-O-Phile]

It's hard to know exactly what your question is. If you're asking, "is the HVAC system sized appropriately?" then you need to consider code requirements, best practices and perform a full envelope thermal performance calculation (as has been mentioned).

Based on your description there's not enough information to adequately understand exactly what's happening or what the scope of work is. Are you using open or closed cell foam? Is your intention to have the foam function as an integral vapor barrier or not? If so, did you define your and goals and performance requirements? In your climate you'll need a vapor barrier on the outside face of the wall (closed cell foam will usually suffice for this but you need to ensure the particular product being used is rated for this and if so, what its performance / permeability is). You don't want moisture from a humid, hot exterior getting into the wall, cooling to its dew point and precipitating out rotting stuff.

Lastly - don't EVER let an HVAC guy size your system - hire a mechanical engineer. You're begging to get ripped off otherwise. The calculations will show what's required and the engineer will own the compliance with the code if code requires something else. Then you can price competitively on that basis. Hiring the mechanical contractor to design the system is like asking a car salesman, "what car should I buy?"

[javadog]

Originally Posted by rbp

First of (if done right) the AC system needs to be sized to maintain a 12 - 15 deg. F difference between the outside and inside

That might work in your neck of the woods but it gets a lot hotter down here. We saw highs of around 115F a couple summers ago and Texas can get just as hot, or hotter.

Did you mean a 12-15 degree delta between supply and return?

JR

[dad911]

I believe texas has adopted the IECC Energy code, If it is like NJ, the architect has done a fairly thorough calculation of doors/windows/wall assemblies, submitted with building permit. We also submit Furnace & A/C specs and calculations. I'd ask your contractor for a copy.... and make sure he follows it.

As to A/C, not enough info here, but I agree with Builder, if you reduce size and it doesn't maintain, you will have no recourse. How many zones? We're doing a new 4800sf home now, 3 zone.

For a point of reference, 3" sprayfoam, closed cell, we were quoted as r-19, 2" r-13.6 .

[DerkPerk]

Originally Posted by rbp

Oddly enough I am an expert.

First off (if done right) the AC system needs to be sized to maintain a 12 - 15 deg. F difference between the outside and inside

...

This is the dumbest thing I have ever heard.
In your heat calc you specify what your design outdoor temp is along with what the desired indoor temp is. At my place in MN, I figured OA design of 90 and indoor desired of 70. On those "design" days my ac will not shut off providing perfect, dry indoor conditions.
Oh, and your delta from SA to RA should be right around 20°f for comfort cooling.
To the OP, you definetly don't want to be oversized in cooling capacity so be sure to calc the heat load for whatever insulation you do install, not what the mechanical contractor thinks is going to be there.

[javadog]

Originally Posted by rbp

Our summers are very hot and humid. We get 90% and temperatures of 95 deg. F. in months of July and August.... Oversizing is the worst thing you can do.

So, when you're at 95F outside, you have an indoor temp of 80-83F? That seems way too hot for me.

I agree with oversizing and short cycling, but we have to size things for a worst-case day. It's not uncommon to go a month at a time with daily temps over 100. Peaks can be much higher.

JR

[MBAtarga]

Originally Posted by rbp

When space temperature is above 74 deg. then on stage 1 cooling. If stage 1 cooling has been running for more than 8 hours and space temperature is above 76 deg. then off stage 1 and on stage 2...

8 HOURS? why would I want to wait for my AC unit to run for 8 HOURS and then add cooling capacity if it hasn't cooled enough?

[look 171]

Originally Posted by Scuba Steve

They need to perform a true calculation, which takes into account windows, building orientation, and a number of other things.

This

[stomachmonkey]

I have no clue but I do recommend ridge vents.

I'm up the road from you and have the same sq. footage that you are building.

House is now 7 years old and was originally not built with ridge vents which I found odd.

Had to redo the 3 year old roof after that hail storm a few years back and had them install ridge vents.

Made a huge difference.

[Rikao4]

I did what ST did when that storm came thru..
& pretty much everyone else in our hood did as well..

Rika

[red-beard]

My next air conditioner will be VFD

[TimT]

There has to be an easy one on-line somewhere.

Here is a good one from SlantFin.

Also insulation required is dictated by code...

Its simple enough to find out what R value is required for you geographical area...