Pressure difference between attic and home, pulling in fiberglass particles? (2024)

Hello, new member here!

I am not yet permitted to post pictures, so I'll describe, hopefully in under 1000 words.

In my 2300 s.f. single-story home in the San Francisco Bay Area I recently had my HVAC overhauled: all-new ducts, single-zone heat pump and fan coil replacing 20-year-old furnace and A/C, new bathroom exhaust fans; removed "whole-house fan" in attic that was set to draw air from living space to attic when attic temperatures exceeded 90F. I also had insulation replaced and air sealing added between living space and both attic and crawl space. All work was done by professionals. Though we are not yet in the hottest part of summer, I'm very pleased with how the temperature is now maintained, in an energy-efficient way. Below I'll give details about the system, but first I want to give the minimal background to get to my main issue and request for advice.

I believe the living space is now sometimes or always underpressured relative to the attic, leading to air being pulled in from the attic. The main entry points seem to be the ~60 can lights (20 years old): years ago I replaced the lights in these cans with LED retrofit trims, and since the recent HVAC work some of those trims have popped off the ceiling by about half an inch.

Before you groan, I did consider air leaks through the cans in the planning stage. The insulation/air sealing contractor commonly places a rock wool cover over each can, to reduce such leaks, and they did that on my job. Note: I had thought the cans were non-IC-rated, making the covers also important to allow safely placing insulation on top. Though I've now seen they are actually IC-rated (model H7ICT), they're certainly not air-tight. The rock wool covers are from Tenmat, and they're sealed with spray foam to the attic floor. I knew rock wool is somewhat air-permeable, but I thought adding these covers would dramatically reduce air flow, improving the efficiency of insulation. If you go to Tenmat's website you can find a link to air permeability measurements for the covers -- I'm not yet allowed to post links.

I'm trying to change this situation of pulling air from the attic to the living space, mainly because since the work was done I've been having breathing problems whenever I'm in the house, but not outdoors. In some of the light cans, and in ceiling skylight wells, I can see apparent deposition of fiberglass particles. I hadn't noticed that before the recent work, though it's possible I missed it. The attic insulation was removed and replaced as part of the work, but it's been unfaced fiberglass batts both before and after. My breathing is better when I have standalone HEPA filters running -- I have a bunch of these that I originally got to catch smoke from wildfires, and to help prevent COVID transmission, but before the new HVAC work I had not been running them routinely recently.

I'll give more details below, but first let me say what I would appreciate advice on:

1. Diagnosing/verifying what's going on.
2. Possible solutions.
3. How I might brainstorm with the separate insulation/sealing and HVAC contractors, in language that makes sense to them. The HVAC contractor is much more technically sophisticated than the insulation/sealing contractor, but both seem very competent in their realm.
4. Of course, any other comments and insights are welcome.

Fan coil and heat pump models are 4 ton, and are top of Bryant's Evolution line. Integrated with Bryant thermostat. One zone, but many supply registers and many return registers across the home, so each bedroom has both supply and return. Bryant Heat recovery ventilator set at 100 cfm, fed into the main duct system. I think the fresh air supply and exhaust on this should be pretty well-balanced. This is intended to provide fresh air since the living space is now more tightly sealed (I don’t have an up-to-date blower door test, but I’m open to getting one done.) Air from outside is filtered, MERV 11 I think. There’s also a high-flow MERV 13 filter box in the main duct circuit. And a UV lamp — I’ve read mixed reports on how useful this is for preventing bacterial or mold growth on the fan coil, but decided to go for it.

Following guidance from the HVAC contractor I've set temperature to be maintained by heating and cooling within a relatively narrow range around the clock: presently 66F to 72F. Fan steadily on high, corresponding to around 1600 cfm flow. For permitting, the duct leakage was measured to be a very low 1.5% of this flow.

Under normal operation of just this ducted system, it may be that pressure is nearly equal between attic and living space. However, the nice new bathroom fans installed by the same contractor turn on based on humidity, and stay on for 20 minutes. Their 100 cfm exhaust (for each of 3 bathrooms) is not presently balanced by corresponding fresh air supply. My guess is that this should cause under pressure in the living space. I have not tried to measure pressure difference between attic and living space, and I’m not sure how to do that accurately. I’d welcome advice on tools. Perhaps there’s a relatively low-priced product that has good enough precision so I could measure pressure separately in the two locations and take the difference?

With my past system I only ran A/C when temps got very hot, and ran the furnace in the winter. I did not have a fan circulating air steadily. And the air sealing of the living space from attic and crawl space was much less careful. I did not have these respiratory issues.

Many thanks!

Pressure difference between attic and home, pulling in fiberglass particles? (2024)


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