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Q&A Spotlight

Questions About HVAC, Insulation, and Ventilation

As he prepares to move into a new home in a more humid climate, a homeowner looks for ways to minimize the risk of mold

Ducts in the attic may not be ideal, but they're apparently in the cards for C. Clark's new house in South Carolina.
Image Credit: Samms Heating and Air / CC BY-SA 2.0 / Flickr

C. Clark is preparing to move from a dry region to Lady’s Island, South Carolina, an area with a warm, humid climate that is the mirror opposite of the climate in Clark’s former home. Clark is highly allergic to mold, and that has him thinking about ventilation, insulation, and his HVAC system.

“I have many questions about HVAC, insulation and ventilation, but will just start with a few,” he writes in a Q&A post at GreenBuildingAdvisor. “If we decide on an exhaust-only system for a new-build home, does also trying to button up the envelope tight work with that type of ventilation?”

He expects to insulate the attic floor with blown-in fiberglass because using spray foam would be too expensive. If he does not insulate at the roof, should he install a radiant barrier to keep attic temperatures down?

“I am very concerned about building in a humid climate [Climate Zone 3], because we are very allergic to mold (bed-ridden allergic for me), so preventing condensation and the mold that comes with it are our primary concern, even over energy efficiency,” he writes. “I read the cellulose insulation can trap moisture, which steered me away from it and to fiberglass. If any condensation build up can happen in attic, I don’t want cellulose to be a mold culture.”

He anticipates that ductwork will be located in the attic because his builder has warned that building chases for them inside the building envelope would be too expensive. As a result, he will be looking for the best way to insulate the ducts.

Those are among the issues for this Q&A Spotlight.

Will exhaust-only ventilation work?

One of Clark’s first questions is whether exhaust-only ventilation can be effective. He asks, “Does trying to button up the…

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  1. Expert Member

    Chris M
    That would probably be a really helpful topic. Understanding where they are coming from might also occasion more sympathy for their approach.

    Unlike the custom builders used by most clients of the houses featured here, tract builders work on a very tight margin. Their profits come from volume, which understandably makes them wary of innovation. When they hear "It only costs 10% more to build more energy efficiently" they now see a loss on the project.

    If you can't get it at the lumberyard they aren't interested. They don't source things from small specialty suppliers and never from the internet. That is due to warranty concerns, scheduling and the potential problems integrating the new products.

    Their experience is that innovation should come from large manufacturers or suppliers who create a demand in their customer base. These innovations concern largely cosmetic changes - styles of kitchen or front facades. They don't see their own attempts at innovation rewarded.

    They are very closely in tune with their potential client base. They share their values and get positive reinforcement for what they build both from their clients and how similar the products of other builders in their subdivisions are. Among their circle there is no perceived problem with the way they build, and they see no reason to change.

    The best path to change the way tract houses are built is the building code. Tract builders bridle at the cost of any new requirements, but as long as there is a level playing field they don't complain too much.

  2. JC72 | | #2

    I have empathy for C Clark as I believe he was working with
    a tract builder. When they don't want to do something they just triple the price.

    IMO an interesting topic to blog about would be suggestions on how to work with tract builders. I think it's important since they build an overwhelming majority of the homes in the U.S.

  3. Andrew_C | | #3

    Energy code compliance loopholes?
    I also empathize with C Clark, and I think you've got a good explanation for why production builders are slow to change. And I agree that better building codes and enforcement are the answer. My frustration in talking with builders even after Michigan adopted 2015 IECC (with modifications) is that the interpretations of the UA Alternative or Simulated Performance energy code compliance paths allow them to continue what they've been doing without any change. My impression is that these alternative paths are loopholes.

  4. Expert Member

    I'm not that familiar with US codes. You are no doubt right.

    My rather random list of production builder's attributes (and prescription for better codes) didn't really offer any useful help to someone like C.Clark who is working with one. Perhaps the best advice would be: if you can avoid it, don't.

  5. GBA Editor
    Martin Holladay | | #5

    Energy code loopholes and advice to homeowners
    Andrew C.,
    You are correct -- as states have begun adopting the 2015 IECC, they have been amending the code to raise the ERI target numbers. The ERI is roughly equivalent to the HERS Index (with low numbers = good and high numbers = bad), so raising the ERI target numbers means that states are making the 2015 code less stringent that the code writers intended. I will be writing an upcoming article on this topic.

    Here's my advice to GBA readers who want to find a local builder who understands energy issues: contact local energy raters (professionals certified by RESNET or BPI -- check their web sites for local listings) and ask them to name local builders who care about these issues. Or contact an energy agency near where you are building and ask for advice. I don't know who to call in South Carolina, but I would start with a phone call to Advanced Energy in North Carolina (919-857-9000). State your frustration with local builders and ask for advice on contacting a builder in South Carolina who understands home performance.

  6. JC72 | | #6

    Energy Star website offers a list of builders/raters
    This might be as helpful since cost to build was a high priority.

  7. morganparis | | #7

    Lowest hanging fruit
    Mr. Clark is planning to build in a relatively mild but humid climate and his overriding concern is avoiding mold conditions. As he has a limited budget and a builder inexperienced in high performance homes I respectfully suggest that the attic distribution ductwork while imperfect is not critical to this issue. The most common and likely source for condensation and mold contamination that we find in the humid southeast is from leaky ductwork in a ventilated crawl space, and by far the most direct route to curing such a deficiency is to encapsulate that crawl space. This is now a straightforward and well documented technology, easily priced, which even tract builders have started to recognize as standard practice. Mr. Clark's attic ductwork may add a little to his power bill but it won't ruin his health. The crawlspace is where he needs to spend his upgrade budget.

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