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