Manual J:Encapsulated foamed attic vs. traditionally vented/insulated

[tipsrfine]

Could one of the pro's do a quick comparison test with their manual j calcs to answer a question I have? Take any house that you have a manual j done on and leave everything the same except for the attic (with no ducts or air handler in attic) and see what happens to the load when you change from a vented attic with traditional insulation levels of fiberglass or cellulose on the attic floor(whatever the R-level is for your area) to an attic spray foamed at the roof with 3 inches of foam (closed cell and/or open cell). Which method results in a higher cooling load? Remember, no ducts in attic. Thanks to anyone willing to do this.

[ChaseAir]

There is a button on the wrightsoft software called encapsulated. It always reduces the load in Florida heat to eliminate duct gain completely. R-8 flex reduces substantially the load on a conventionally insulated home.

[lynn comstock]

It also depends on the sealing level of the air ducts and equipment.
Another factor is the climate.

The Manual J v.7 which I use for retrofits, assumes a 15% duct heat gain for ducts in the attic. That served me well for the past 30 years. I did get bit one time on a flat roof house that I downsized 1/2 ton. The problem was excessive duct leakage and the ducts were not accessible. I had to replace the unit with the original size. (before Aeroseal.)

[Southern Mech]

ummm, he said nothing about ducts, well the op did. there are no ducts nor equipment in attic.

[tipsrfine]

Could one of the pro's do a quick comparison test with their manual j calcs to answer a question I have? Take any house that you have a manual j done on and leave everything the same except for the attic (with no ducts or air handler in attic) and see what happens to the load when you change from a vented attic with traditional insulation levels of fiberglass or cellulose on the attic floor(whatever the R-level is for your area) to an attic spray foamed at the roof with 3 inches of foam (closed cell and/or open cell). Which method results in a higher cooling load? Remember, no ducts in attic. Thanks to anyone willing to do this.

?

[Southern Mech]

?

that's what I was screaming, you said no ducts or equipment in attic, and have 2 answers that factor duct leakage in the attic. People are quick to answer yet fail to read and comprehend.

[tlj000]

1300 sf attic
R38 in the ceiling S=12705 btuh L= 2916
R35 encapsulated S=12172 L= 2916

Climate zone 6A

[tipsrfine]

Could one of the pro's do a quick comparison test with their manual j calcs to answer a question I have? Take any house that you have a manual j done on and leave everything the same except for the attic (with no ducts or air handler in attic) and see what happens to the load when you change from a vented attic with traditional insulation levels of fiberglass or cellulose on the attic floor(whatever the R-level is for your area) to an attic spray foamed at the roof with 3 inches of foam (closed cell and/or open cell). Which method results in a higher cooling load? Remember, no ducts in attic. Thanks to anyone willing to do this.

R-35 with 3 inches of foam sprayed at the roof???????

[tlj000]

1300 sf attic
R38 in the ceiling S=12705 btuh L= 2916
R18 encapsulated S=12637 L= 2916 (3" of R6 foam)

Climate zone 6A

[tipsrfine]

1300 sf attic
R38 in the ceiling S=12705 btuh L= 2916
R18 encapsulated S=12637 L= 2916 (3" of R6 foam)

Climate zone 6A

Thanks, I appreciate it. It really makes me think I need to buy the software and learn it well. It seems one could use it as a sort of learning tool simply by making small changes such as you did with this, and then investigate WHY the program made the changes to the loads.
I wonder how the program interpreted "3 inches of foam". Does it consider that the raftors are completely encased in 3 inches also, and not factor in the conductive gains through them? I mean, take all the raftors and put them together and you have a large area that only has an R-1.
I can see why this tool, like all other tools, is only as good as the person using it. I certainly can't see relying on a calc prepared by someone who has little experience using it. Thanks again.

[ga-hvac-tech]

Interesting thread... I have done a few 'foam' homes.

The homes I did had either 2x10 or 2x12 roof rafters, filled. The difference was amazing!

One thing to be careful with foam homes: MUA! Is there enough air to allow the fireplace to draft? Is there enough air to allow the kitchen vent to exhaust? Is there enough air to allow the bath fans to exhaust? When one opens or closes a door, will it fight you because the house is too tight?

We had to use ERV's (one for each system) to make up for an over tight house... worked well.

[teddy bear]

Interesting thread... I have done a few 'foam' homes.

The homes I did had either 2x10 or 2x12 roof rafters, filled. The difference was amazing!

One thing to be careful with foam homes: MUA! Is there enough air to allow the fireplace to draft? Is there enough air to allow the kitchen vent to exhaust? Is there enough air to allow the bath fans to exhaust? When one opens or closes a door, will it fight you because the house is too tight?

We had to use ERV's (one for each system) to make up for an over tight house... worked well.

When dealing with very air tight homes, I would suggest using make-up air ventilation instead of balanced ventilation to give the clothes drier, kitchen hood, and bath fans a chance of getting some air to function. Balanced flow ERV exhaust as much as they supply. This does nothing to help exhaust devices.
Most occupants do not realize the importance of continuous fresh air ventilation for purging the pollutants and renewing oxygen. They tend not operate the ventilating during the warm months when it is needed the most. Properly ventilated homes will be damp during the shoulder seasons unless you have a whole house dehumidifier to maintain <50%RH during low/no cooling loads.
Regards TB

[Timber]

These foam houses really do not impress me so far. Especially if u need an ERV or even worse a dehumidifier. Seems u just need more equipment to deal with the tightness of the home. Then u got to maintain / repair this additional equipment.

If I built again dont think I would use foam. JMO

[ga-hvac-tech]

These foam houses really do not impress me so far. Especially if u need an ERV or even worse a dehumidifier. Seems u just need more equipment to deal with the tightness of the home. Then u got to maintain / repair this additional equipment.

If I built again dont think I would use foam. JMO

Here in the Atlanta area... I know a couple of spec builders.

They do the roof (under the roof) with foam, then use conventional insulation in the walls. The result seems to work out as a good compromise.

[Timber]

Here in the Atlanta area... I know a couple of spec builders.

They do the roof (under the roof) with foam, then use conventional insulation in the walls. The result seems to work out as a good compromise.

Do you still need the ERV's / vetalation / or dehumidifiers ? If not, then I would consider that option.

[ga-hvac-tech]

Do you still need the ERV's / vetalation / or dehumidifiers ? If not, then I would consider that option.

Mostly no. Depends on how tight one builds the house... but generally no.

The main thing about a foamed attic (under the rafters) is not having that 'oven' just above the ceiling of the upper floor.

Hope this helps.

[teddy bear]

These foam houses really do not impress me so far. Especially if u need an ERV or even worse a dehumidifier. Seems u just need more equipment to deal with the tightness of the home. Then u got to maintain / repair this additional equipment.

If I built again dont think I would use foam. JMO

Any well built home needs fresh air ventilation to pruge indoor pollutants and renew oxygen during the calm warm weather. Even worse, supplemental dehumidication is needed in green grass climates if you want the best in comfort and avoid growing mold/dust mites.
ASHRAE, American, Lung ASS, EPA and most modern building codes suggest mechanical fresh air ventilation for the above reasons.
The posters on this site represent traditional opinion not the building research community. Check out the above IAQ experts or researchers like Building Science.com for the state of the art specifications.
It hard to part with tradition when dealing a changing industry.
I have been involved with IAQ for 25 years and find is hard to believe how difficult is has been to get the concept of controlled fresh air ventilation and real humidity control in a modern well built home.
Good luck on getting a quality home to live in.
Regards TB

[tedkidd]

TIPS, I've yet to see load, modeling or any cost/benefit software that can accurately reflect the energy or load impact of foam.

As you may be seeing in these threads, guys talk about going to foam houses on 100f days and the equipment doesn't run once the whole time they are there. That indicates gross oversizing to me, how about you?

Good luck on getting a quality home to live in.
Regards TB

Since you were considerate enough to respond I guess I'll drop in behind you.

I think when people don't do or ask what the numbers are, they're defining their ability to make smart decisions.

Some would rather pay $2 per sf/year for energy than $0.25 per sf/year for energy and $0.50 per sf/year for the increased mortgage for the better home. To me that's living in a crappy home and throwing away $1.25 per sf a year.

The difference between ignorance and stupidity is ignorance doesn't yet know, stupidity doesn't have the capacity to know.

[ga-hvac-tech]

TIPS, I've yet to see load, modeling or any cost/benefit software that can accurately reflect the energy or load impact of foam.

As you may be seeing in these threads, guys talk about going to foam houses on 100f days and the equipment doesn't run once the whole time they are there. That indicates gross oversizing to me, how about you?



Since you were considerate enough to respond I guess I'll drop in behind you.

I think when people don't do or ask what the numbers are, they're defining their ability to make smart decisions.

Some would rather pay $2 per sf/year for energy than $0.25 per sf/year for energy and $0.50 per sf/year for the increased mortgage for the better home. To me that's living in a crappy home and throwing away $1.25 per sf a year.

The difference between ignorance and stupidity is ignorance doesn't yet know, stupidity doesn't have the capacity to know.

Sometimes stupidity knows and does not care... I hate to see that... but it is out there.

I cannot tell you the number of folks that say: Gee, it will not pay back in a year... why bother. Amazing IMO.

[ChaseAir]

The homes we have done were so tight they really needed a dehumidifier to remove moisture. An 85 degree day was simply not enough to cause the units to run with the stat set at 78. Gross oversizing?

Well even 2 stage units are "grossly oversized" on a light load day with a homeowner who doesn't have thermostat set as low as the "design conditions" were determined in the load calc.

Not to mention that big oak tree on the west side of the house that reduces the load by 3/4 ton in some circumstances. Needless to add, the load calc is a great tool, but it does not answer all things.