So have a high efficiency Furnace (pvc combustion air intake and exhaust) with spray foam in the attic. Also see these two exhaust ports, one on the return plenum and one on the supply plenum. It’s my understanding that air from the attic should not be introduced into the home when spray foam is installed because off gases from the spray foam could contaminate indoor air quality. Does anyone know what these exhaust ports are and why they are installed?
If the spray foam has sealed the attic space off from the exterior then it is now inside the building envelope and needs to be conditioned. If there aren’t hvac registers in a foam sealed attic bad things happen. I would be more concerned about mold and rot in a sealed unconditioned space than out gassing of foam that has been there for at least a couple days.
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Gotcha, so one is drawing in return air and the other is supplying conditioned air into the attic. Wouldn’t a better practice be to seal off the return drawing air from the attic space and simply keep the supply open? That is the purpose of installing a High Efficiency Furnace in conjunction with spray foam correct? So the unit draws combustion air from the exterior (above the roof) and not from the attic.
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The combustion air is separate from the return air. The two do not intermix in this scenario.
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Understood. A better question: is the attic return necessary in this scenario since there are two return vents drawing in air from the vents inside the home?
A supply may be good enough. But a supply and a return is typically better. Since this space is considered inside the thermal envelope, it should be treated just like any other room in the home.
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M1602.2Return air openings.
Return air openings for heating, ventilation and air-conditioning systems shall comply with all of the following:
1.1.Openings shall not be located less than 10 feet (3048 mm) measured in any direction from an open combustion chamber or draft hood of another appliance located in the same room or space.
2.2.The amount of return air taken from any room or space shall be not greater than the flow rate of supply air delivered to such room or space.
3.3.Return and transfer openings shall be sized in accordance with the appliance or equipment manufacturer’s installation instructions, Manual D or the design of the registered design professional.
4.4.Return air shall not be taken from a closet, bathroom, toilet room, kitchen, garage, mechanical room, boiler room, furnace room or unconditioned attic.
Exceptions:
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1.Taking return air from a kitchen is not prohibited where such return air openings serve the kitchen only and are located not less than 10 feet (3048 mm) from the cooking appliances.
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2.Dedicated forced-air systems serving only the garage shall not be prohibited from obtaining return air from the garage.
5.5.For other than dedicated HVAC systems, return air shall not be taken from indoor swimming pool enclosures and associated deck areas except where the air in such spaces is dehumidified,
6.6.Taking return air from an unconditioned crawl space shall not be accomplished through a direct connection to the return side of a forced-air furnace. Transfer openings in the crawl space enclosure shall not be prohibited.
7.7.Return air from one dwelling unit shall not be discharged into another dwelling unit.
4.4 Return Air shall not be taken from an unconditioned attic.
It was hot up there…It was my understanding that for an attic to be considered “conditioned” it needs to be roughly the same temperature as the rest of the home as well as be thermally and air sealed.
This “should” be considered a conditioned attic, since the spray foam appears to be applied to the bottom of the roof decking.
There may not be enough supply and/or return air. Was the AC on? Was cold air coming out of the supply duct? Were there any dampers installed that might be closing off the supply and/or return in the attic?
If the HVAC was off due to moderate outdoor temps, it will naturally be warmer in the conditioned attic space. More important than how hot or cold it was, is what the humidity level was like.
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Good points Ryan, I would further offer, there may not be enough supply and return given what looks like rust on one of the openings signaling possible high humidity and condensation as well as the observation of high temps in the attic. The attic should be fully encapsulated so no outdoor air is drawn into the attic and attic should be fairly close to living area temperature and humidity.
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A/C was on and I didn’t measure the humidity but it was fairly humid. Not as humid as an attic without the encapsulating foam but it was more humid than the house for sure. No Dampers on the vents just the vent pipes protruding from the supply and return plenums. The air coming from the supply plenum vent was cool and conditioned. The Supply Vent was the one that was rusted and it appeared that microbial growth wasnon it so there was condensation formation. I just don’t like the idea of drawing in attic air and distributing it into the house, when there is spray foam that can potentially off gas. I know it can off gas VOC’s after installation, especially at a faster rate when it is hot and humid. I don’t believe that one supply vent would be enough to provide enough de-humidification and provide sufficient cool temperatures in the attic, especially in our 100 degrees, feels like 109 with 90% humidity level Summers here in Texas.
Do you think that Return Vent Opening was intended for a Fresh Air Intake that was never installed? I already have it planned to write on the report “HVAC Tech Evaluate” and to consult with the professional. Just trying to sum up my thoughts.
Does the home have an air exchanger?
No Air Exchanger or De-Humidifier
Did you find moisture-related issues in the attic?
Just the black spotting on the spray foam that appears to be microbial growth (3rd picture). That area is directly above where the supply vent is located.
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Could be mold. Might also be dust/dirt accumulation from having the supply being blown directly at it. No way to know without testing.
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That second pictures as me concerned. For a newer house/installation of spray foam why all the rust. It looks like a vent or something else pentrating the duct. Did you have all bathroom, kitchen vents, water heater exhaust accountted for? How old a house/furnace?
That supply pipe is rusted because it is not insulated; it blasts cold air straight off the coil. That appears to be a field modification to satisfy the conditioned space requirement enforced in some states. An insulated duct with a diffusing supply register would likely perform better. Also, if you blast cold air into the room with no return the air will never dry out, likely produce mold and be less efficient.