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

[teddy bear]

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.

Good post but there is some confusion. During high dew point summer weather, extremely air tight homes have less moisture moisture infiltration than the excessively leaky homes. The sources of moisture in all homes is the infiltration/ventilation of outside fresh air and the occupants.
Well insulated air tight homes need mechanical fresh air ventilation when occupied. Yes the a/c will not run enough to remove the mositure from the minimal fresh air ventilation and the occupants. This is usually 2-4 lbs. of dehumidification moisture per hour. Let the a/c run as much as needed to maintain the desire temperature and use a high efficiency dehumidifier to remove the enough of the remaining moisture to maintain <50%RH. When the home is unoccupied, the t-stat can be raised to avoid any cooling. The dehu will maintain <50%RH at a fraction of the cost of trying to cool to <50%RH.
Sorry about picking on a small point, but there is a lot of confusion about these issues. Thanks for the support.
REgards TB

[tipsrfine]

Good post but there is some confusion. During high dew point summer weather, extremely air tight homes have less moisture moisture infiltration than the excessively leaky homes. The sources of moisture in all homes is the infiltration/ventilation of outside fresh air and the occupants.
Well insulated air tight homes need mechanical fresh air ventilation when occupied. Yes the a/c will not run enough to remove the mositure from the minimal fresh air ventilation and the occupants. This is usually 2-4 lbs. of dehumidification moisture per hour. Let the a/c run as much as needed to maintain the desire temperature and use a high efficiency dehumidifier to remove the enough of the remaining moisture to maintain <50%RH. When the home is unoccupied, the t-stat can be raised to avoid any cooling. The dehu will maintain <50%RH at a fraction of the cost of trying to cool to <50%RH.
Sorry about picking on a small point, but there is a lot of confusion about these issues. Thanks for the support.
REgards TB

How does having one of your de-humidifier included in the design of one of these "tight" homes affect the manual j load, and for that matter the manual S calc?

[ChaseAir]

We used the dh90 from Honeywell. Yes ALWAYS have fresh air. The offgassing of that foam has to be toxic as you can smell it years later. On one home we pulled the air out of the attic. I would think the systems would last longer as they hardly run and do not cycle back on rapidly. Great for warranty service. Dh90 is Probly not big enough for these areas but man that 180 is expensive. Is there a better one?

[teddy bear]

How does having one of your de-humidifier included in the design of one of these "tight" homes affect the manual j load, and for that matter the manual S calc?

Good question. The dehumidifier will not have any effect on your manual J calc. The dehumidifier will not run during high cooling loads because the a/c removes more than enough mositure to maintain <50%RH. If the a/c is not maintaining <50%RH during peak cooling load, slow the air flow to get a colder a/c cooling coil. The a/c coil should be 25^F cooler than the desired temp in the home to remove enough moisture.
As the cooling load declines during the evening hours or on a rainy day, the indoor RH will exceed 50%RH. The dehumidifier will be activated by the dehumidistat removing the excess moisture keeping the home at <50%RH.
The Honeywell dehumidifier is a good dehumidifier. A 90 dehu will handle large air home with 70-90 cfm of fresh air ventilation and 4 occupants.
The best dehumidifier is the Ultra-Aire with DEH 3000 controller for scheduled fresh air ventilation. Certianly the Honeywell is ok. There are others that work I am sure.
Regards TB

[rickboggs]

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.

2668 sq ft home
R38 Ceiling
R13 Walls
Total 22506
Sensible 18025
Latent 4479

R25 Foam Encapsulated
R13 Walls
Total 20720
Sensible 17494
Latent 3226

Add duct in the attic

R38 Ceiling
Total 33307
Sensible 26707
Latent 6600

R25 Foam Encapsulated
Total 26620
Sensible 21476
Latent 5144

[ga-hvac-tech]

2668 sq ft home
R38 Ceiling
R13 Walls
Total 22506
Sensible 18025
Latent 4479

R25 Foam Encapsulated
R13 Walls
Total 20720
Sensible 17494
Latent 3226

Add duct in the attic

R38 Ceiling
Total 33307
Sensible 26707
Latent 6600

R25 Foam Encapsulated
Total 26620
Sensible 21476
Latent 5144

It would appear the BIG advantage is if the system (equipment and ducts) is in the attic. Logical for the style of construction down here... probably not that big a deal in areas where equipment and ducts are in conditioned space.

[tedkidd]

It would appear the BIG advantage is if the system (equipment and ducts) is in the attic.

Well, that might be called the "no-brainer" situation.

Sometimes it's not the quantifiable benefits but the unquantifiable quality of life ones. I think really the big advantage is hard to know until you've lived in a foamed house. Not many people who have would go back if they had a choice.

That said, I do like cellulose. But I sell redefine in some circumstances where there is no hvac in the attic. If you have to count on a level of quality in attic air sealing which is not likely to be achieved by regular human beings than redefine is simply the safe approach.

I prefer to avoid recommending work that has a high likelihood of poor or mediocre results. I ask myself is the attic in question particularly likely to be a failure addressed other way?

[beenthere]

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.

A home that needs 17,500 BTU sensible cooling capacity at 95 outside and 70 inside would have a sensible load of 10,769 at 85 outdoor and 70 inside, and a sensible load of 7,368 sensible at 85 outside and 78 inside temp.

Thermostats that require a 2 degree temp rise to bring on the A/c will tend to keep the A/C off too long in a well insulated tight house. And ERV's that are bringing in too much fresh air will cause the humidity to rise.

If there is an oak tree shading the house enough to knock off 3/4 of cooling load, then the tree should be allowed for in the load calc.

[tipsrfine]

2668 sq ft home
R38 Ceiling
R13 Walls
Total 22506
Sensible 18025
Latent 4479

R25 Foam Encapsulated
R13 Walls
Total 20720
Sensible 17494
Latent 3226

Add duct in the attic

R38 Ceiling
Total 33307
Sensible 26707
Latent 6600

R25 Foam Encapsulated
Total 26620
Sensible 21476
Latent 5144

Thanks Rick. How many inches of foam did you use to get R-25, and did you account for the conductive heat gain through the rafters that were not covered in foam? I can't see getting an R25 out of 3 inches of foam and the rafters are left exposed.

[rickboggs]

Thanks Rick. How many inches of foam did you use to get R-25, and did you account for the conductive heat gain through the rafters that were not covered in foam? I can't see getting an R25 out of 3 inches of foam and the rafters are left exposed.

5 1/2" of foam. I think, am not sure, that Manual J assumes 15%..... or is it 10%..... for roof construction.

[rickboggs]

A home that needs 17,500 BTU sensible cooling capacity at 95 outside and 70 inside would have a sensible load of 10,769 at 85 outdoor and 70 inside, and a sensible load of 7,368 sensible at 85 outside and 78 inside temp.

Thermostats that require a 2 degree temp rise to bring on the A/c will tend to keep the A/C off too long in a well insulated tight house. And ERV's that are bringing in too much fresh air will cause the humidity to rise.

If there is an oak tree shading the house enough to knock off 3/4 of cooling load, then the tree should be allowed for in the load calc.

All the above is true true true. Beenthere, man do I have some stories. We've been working with a contractor on some problem foam houses.... data loggers are your friend.

[beenthere]

You could tell us some of them.

[tedkidd]

Thermostats that require a 2 degree temp rise to bring on the A/c will tend to keep the A/C off too long in a well insulated tight house. And ERV's that are bringing in too much fresh air will cause the humidity to rise.

I wish I counted all the lightbulb moments you've provided me. Gonna parse through the meaning of that statement. I think it really supports smaller is better.

We are not likely to get situations where the equipment can always run continuously. When weather conditions make your equipment oversized it is likely you may be uncomfortable either at the beginning or end of a cycle. You will be faced with adjusting the thermostat temp rise (what's the correct term, anticipator?) to a shorter (dead-band?) swing. But this will beat up your equipment.

Wider dead band means longer off times and higher likelihood of being hot and sweaty one movement, then cold and clammy the next.

Needs some polishing. But this explanation can lay off responsibility for discomfort onto the homeowner if you offer 2 stage equipment and they don't bite.

I think many hvac guys sell to homeowner budget and that this is a huge mistake. Consult, recommend the best, then let the homeowner de-select features. Then when their comfort problem is due to a feature they de-selected, it's on them not you.

[beenthere]

On foamed homes. 2 stage should probably be the only thing offered, and be sized as close to the load as possible. And thermostats that don have to let the temp drop 2 degrees to bring on the A/C should probably be the only thermostat choice. Or the home will end up with high humidity from the ERV that will probably be set too high most of the time.

[teddy bear]

On foamed homes. 2 stage should probably be the only thing offered, and be sized as close to the load as possible. And thermostats that don have to let the temp drop 2 degrees to bring on the A/C should probably be the only thermostat choice. Or the home will end up with high humidity from the ERV that will probably be set too high most of the time.

Add in the point that a cooling coil accumulates moisture for the first 15-30 minutes of the cooling cycle before moisture goes down the drain. During the off cycle the moistue on the coil/pan evaporates back to the home.
The occupants and fresh infiltrating/ventilating air contribute the moisture.
During evenings and cool wet days with low/no cooling loads, an undersized a/c or 2 stage a/c will not maintain <50%RH. Reheat or a dehumidifier are solutions to proper fresh air ventilating and occupants moisture to maintain <50%RH. An adequately sized for peak loads a/c and 2-4 lbs, per dehumidifier make a good simple combination for a healthy and comfortable home.
Regards TB

[ChaseAir]

A home that needs 17,500 BTU sensible cooling capacity at 95 outside and 70 inside would have a sensible load of 10,769 at 85 outdoor and 70 inside, and a sensible load of 7,368 sensible at 85 outside and 78 inside temp.

Thermostats that require a 2 degree temp rise to bring on the A/c will tend to keep the A/C off too long in a well insulated tight house. And ERV's that are bringing in too much fresh air will cause the humidity to rise.

If there is an oak tree shading the house enough to knock off 3/4 of cooling load, then the tree should be allowed for in the load calc.

Trees frequently lower the load on a building substantially. An example being when they shade a large area of eastern fenestration. If you include them as a factor, what do you do when the neighbor cuts it down?

The one home we did was designed with no direct sunlight on the east and West fenestration by the use of overhangs and Bahamas shutters. The home was foamed to r-30 all the way around (off grade). As a result we were able to install a three ton on 4400 sq ft. The other guys bid was to install TEN tons. The three ton is a two stage and still does not run enough on typical days to dehumidify adequately so we used a dehumidifier.

[ChaseAir]

Add in the point that a cooling coil accumulates moisture for the first 15-30 minutes of the cooling cycle before moisture goes down the drain. During the off cycle the moistue on the coil/pan evaporates back to the home.
The occupants and fresh infiltrating/ventilating air contribute the moisture.
During evenings and cool wet days with low/no cooling loads, an undersized a/c or 2 stage a/c will not maintain <50%RH. Reheat or a dehumidifier are solutions to proper fresh air ventilating and occupants moisture to maintain <50%RH. An adequately sized for peak loads a/c and 2-4 lbs, per dehumidifier make a good simple combination for a healthy and comfortable home.
Regards TB

You don't like dehumidifiers do you TB?

Hard to believe they used to live in unconditioned homes here in Florida with 90% RH.ow The Waudoni tribe in Ecuador lives in humidity constantly and their only health issues are falling out of trees. I know someone whose family has lived with them for generations.

[tipsrfine]

You don't like dehumidifiers do you TB?

Hard to believe they used to live in unconditioned homes here in Florida with 90% RH.ow The Waudoni tribe in Ecuador lives in humidity constantly and their only health issues are falling out of trees. I know someone whose family has lived with them for generations.

That's a discussion worthy of having. Comfort levels vary in different parts of the world & apparently at different parts in time in the same location. This is partly/mainly why ASHRAE adapted the "adaptive comfort standard".

[beenthere]

Trees frequently lower the load on a building substantially. An example being when they shade a large area of eastern fenestration. If you include them as a factor, what do you do when the neighbor cuts it down?

The one home we did was designed with no direct sunlight on the east and West fenestration by the use of overhangs and Bahamas shutters. The home was foamed to r-30 all the way around (off grade). As a result we were able to install a three ton on 4400 sq ft. The other guys bid was to install TEN tons. The three ton is a two stage and still does not run enough on typical days to dehumidify adequately so we used a dehumidifier.

Does it ever have to run continuous on design days?


If the neighbor cuts the tree down, plant another.

[beenthere]

Chase what brand and model 3 ton was/is it, and what thermostat is being used, also what CFM is the blower set to.