Can I run supply or returns in conditioned attics? What about ERVs?
So I’m building a new house in Houston, TX (5200 sqft – two story, with a conditioned attic). My goal is to air seal the house to passive standards (0.6 ACH@50). I’ve been trying to work with the recommended HVAC guy (25 years in business) and we seem to be at a huge disagreement. Can someone here help me clarify?
I initially asked him about running an ERV supply and return in the attic to keep air circulating (and prevent heat/humidity from accumulating). He then went into an entire spiel about how this is against code and that it’s a fire hazard since there are exposed electric wires running through the attic. His reasoning is that, since the wires aren’t in conduit or covered by gypsum, there’s a bigger risk of fire. If a fire occurs, a return would transport the fire to other parts of the house putting my family at risk.
A little surprised, I read through the IBC codes to try and find what he was referring it. Not locating it, I then emailed him to ask which codes he’s referring to. His reply is pasted below:
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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:
1. 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.
2. 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.
Also, it’s not referred to here, but the attic does not have plenum rated cables which is my main concern with having the open directly connected return air. Either way it is clear that you can not pull return from the unconditioned, mechanical room attic space.
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I pushed back, telling him that we could close up the part of the attic with the air handler creating it’s own “mechanical room” and that this would free up the rest of the attic to do the supply and return. However, he’s still saying no, as the attic is actually a “semi-conditioned” space (as opposed to a living space) and that it falls into the same class as the “unconditioned attic“. Needless to say, he’s still refusing to do it. He’s cited multiple “building science” people whom he says agrees with him and has referred me to them.
Is he correct? I’ve completely sealed off the attic and it shares the same air as the rest of the house. I feel this makes it a conditioned space and that I am not violating code. Thanks.
P.S. I’ve already read Martin’s article on humid attics (https://www.greenbuildingadvisor.com/article/high-humidity-in-unvented-conditioned-attics) along with Joe Lstiburek’s response. I’m willing to install rockwool (instead of spray foam), but am not sure if that still presents an issue.
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Replies
Once you have a thermal barrier over the foam you can treat it as interior space. This might be worth it as now you can use the attic as storage space.
Your HVAC tech is correct. You can't have a return but you can have a supply duct which is needed for conditioning the attic. A sloppy job on the return ducting in the attic is also fine with the code.
You are also in warm enough climate that you can do a conditioned attic with batts only with a diffusion vent at the ridge. This is way cheaper than SPF without any of the potential issues. You might still need a bit of SPF by the soffit area as this is a hard transition to air seal properly if you have overhangs.
If you must have spray foam, closed cell spray foam flash and batt will also work. Beside reducing the amount of spray foam needed, this also gets you a thermal barrier over the foam.
Thanks. The HVAC guy says he doesn’t feel comfortable installing supply without a return. And as you’ve said, return is against code. At least it’s against code if I don’t wall off the air handler.
I’m willing to do rockwool with a diffusion port. However, TPO is being installed on the roof. The diffusion port is no good if the vapor can’t diffuse past the TPO.
TPO means low slope. Vapor diffusion is only allowed for pitched roofs, so that won't work for you.
I would go for a flash batt install with closed cell spray foam. In your mild climate you need very little SPF and the rest of the R value can be cheap batts. Since you are using cheap batts for the bulk of the insulation, you can also bump up the R value a bit which never hurts in the Texan sun.
Conditioned attic needs to be, well, conditioned. So the code min CFM supply register is a must. Your ceiling will never be tight enough for supply only to be an issue.
The ERV stale air pickup is not a return but an exhaust. This means you can put a stale air pickup in the attic. If you want better, you can size this stale pickup branch to match the flow for the conditioned supply. The ERV pickup should be installed near the peak of the roof for best results. This has the benefit of venting your attic air to the outdoors which is better for indoor air quality. The remainder of the ERV stale air flow can be routed to a couple of bathrooms and near the kitchen area.
Thanks for the detailed response!
Would you mind sharing the code you’re referring to for supplying air to the conditioned attic? Also, do you have any reference material for your closed cell + batt recommendation? I’m trying to understand the reasoning. Thanks again!
Conditioning the attic:
https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
Flash and batt roof (Table 1):
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
Thanks for the article! I have Zip R6 installed on the roof (on top of zip 5/8). Joe seems to believe I should be fine then just installing batt in the rafters.
Back to the ERV, I’m convinced that I should be fine supplying air to the attic combined with an ERV exhaust. thank you very much for your help!
Are you asking about exhaust and fresh air connected to an ERV or about supply and return connected to an air conditioner? Seems odd those return codes would apply to exhaust for an ERV since the air goes out of the house.
I am pretty sure you can have ERV exhaust inputs in bathrooms by code. That is how our mechanical engineer designed our ERV system. That would suggest the return section of the code cited doesn't apply to ERV exhaust.
I’m asking about both. He seems comfortable installing return ervs in bathrooms. But he’s explicitly said he doesn’t want to do it in the attic.
Consider the possibility of not putting your HVAC equipment in your attic and finding the space to keep it below the ceiling.
Consider that your attic floor is likely to have about half the number of square feet to insulate as the roof.
Consider how much less fluffy insulation cost than spray foam you could have R60 on the attic floor for 1/3 the price of R24 sprayed on the roof.
Walta
Thanks. At this point the city has already approved me for conditioning the attic by insulating the roof. You’re asking me to go back to the city to ask for a variance, which in itself is full of headaches. Id like to be able to resolve this sticking to my initial decision of a conditioned attic with minimal detours.