Attic Ventilation option?

[TedHouston]

Hello guys,

I've been asking around for the best solution to ventilate my sealed attic space. The way my townhome is built, I have no eaves(soffit) for intake air. Sides of my townhome is built up past roof line on all 4 sides in order to hide a conventional asphalt shingle roof.

I put additional batt insulation between joist to reduce heat transfer but it made almost no difference. The underside of the roof has "Tech Shields" and I have 22 inches of blown-in insulation on attic floor. I am planning on adding radiant barrier (foil) when the weather is cooler but I want to add some sort of passive ventilation to help it cool down.

What are my choices? I've called several contractors but their suggestions are to
1. Install powered fans. I read this is not a good choice since my attic gets really hot, the motor can prematurely fail and for it to work, it needs air intake, which I don't.
2. Spray foam. I DO NOT want any chemical in my attic so this option is out.
3. Ridge vent. For the ridge vent system to work properly, I read soffit is a must.
4. Whirly Turbines. Again, without intakes, not sure if they will work properly. I live in Houston, TX and experiencing two hurricanes, I think I want to avoid such devices from rain driven damage and a possibility that it can blow away.

This is my roof.

 

[Dana]

What is the goal here- are you trying to improve the cooling efficiency, comfort levels, or... ??

Are there ducts & HVAC air handlers in the attic, above (or embedded in) the insulation?

Have you measured the ceiling temperatures and compared them to wall temperatures? (A pistol-grip infra-red thermometer is a good tool for debugging this stuff.)

There is no venting solution to be had here, and even if there were it would reduce the hurricane resilience. A vented attic will see substantially more uplift forces on the roof decking in a high wind than a tight attic. With the parapet surrounding the pitched roofs the aerodynamics are even more complicated, but it's best to leave well enough alone.

Attic ventilation is far more about purging moisture than it is about cooling the attic, but in your humid climate venting the attic will ADD moisture to the attic of an air conditioned building. Keeping it relatively air tight between attic & outdoors the dew point of the trapped air in the attic tracks that of the conditioned space below (more or less- sometimes less due to buoyancy effects). But the attic will run a bit hotter, and the radiant barrier roof decking limits the rate at which it cools off at night when the roofing is cooler than the attic.

Fans or turbine vents will depressurize the attic, drawing conditioned space air through any leaks in the ceiling (electrical & plumbing vent, or duct boot penetrations, etc), which will bring the attic temp down, but only at the expenses of pulling outdoor air into your living space, which isn't exactly what you're looking for. Without ample free-air area intake attic venting for the cfm volume of the fan or turbine this effect can be pretty severe.

Fiberglass batts are somewhat translucent to infra-red radiating down from the underside of the roof deck. Blown cellulose is IR-opaque, and does measurably better, but it's probably not worth yanking your recently added batts to make room for the cellulose. A 3" overblow (assuming you have 3" left) is enough to block the IR convection loops between the attic air and the entrained air in the batts but if you're at 22" it's not going to matter much. (What is that, a double-layering of low density R38s?)

So what you're left with is improving the radiant barrier. With the pre-existing radiant barrier roof decking you already have some relief, but the rafters also radiate. Perforated aluminized fabric radiant barrier stapled to the underside of the rafters will cut the direct-radiated heat transfer by quite a bit, since it the top side of the RB reflects the heat back to the roof deck, and the under side has emissivity as low as the shiny roof decking, but is also blocking the heat radiated from the rafters. Peak temperatures in the space between the RB and roof deck will be higher than the attic temperature, and that higher temp keeps the shingles hotter, radiating more of the heat away even during the day. Using perforated fabric RB avoids the potential moisture trap that sometimes happens with foil or bubble-pack type RB.

 

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[TedHouston]

Thanks for your reply Dana. I really appreciate your thorough advice As you probably guessed, I just wanted to be more efficient and make my top floor more comfortable as my top floor HVAC is having a hard to keeping up a comfortable temp (2 HVAC+ducts are in my attic). This fall, I'll go ahead and staple radiant barrier (perforated foil) between joists. Since RB foil is relatively cheap, I'm thinking about putting a layer of RB on top of insulation on attic floor (not sure if this will have additional benefit). Hope next summer will be more comfortable.

 

[Dana]

Measure the ceiling temperatures with an infra-red thermometer. It's likely that your performance & comfort problems are primarily due to the HVAC being in the attic, not the lack of insulation between the attic and conditioned space. Duct insulation may be the bigger culprit (but maybe not the biggest.)

With the ducts and air handler in the attic tight ducts/air handlers and duct insulation becomes critical. Putting the RB on the underside of the rafters will kill some of the parasitic directly radiated heat uptake, but won't do a thing for HVAC air leakage losses or conducted heat uptake from hot attic air. RB between between the radiating elements (roof deck & rafters) and the air handler/ducts can help that if there is little or no duct/air handler insulation. The RB should NOT go between joists, it should be stapled to the under side of (and fully covering) the rafter edges. Adding RB to the attic floor would do almost nothing for you if you already have RB roof decking + RB under the rafters.

Air sealing every duct & air handler seam with duct mastic, (use foil tape for service panels to the air handlers) and every duct boot to the wallboard, and all penetrations of plumbing/ducts of the ceiling is usually a worthwhile (and low-cost) endeavor. "Windows & Doors" type can foam can seal even fairly large gaps pretty well, but for sealing the boots to gypsum board sometimes calls for an additional wrap of housewrap tape from the finished side of the board lapped over to the interior side of the boot to get good long term air tightness.

Flex duct that isn't stretched fully tight can be a bigger impedance to air flow than you might imagine, undercutting the system performance. If your duct "system" looks like a ball of snakes mating it could easily be the primary factor.

You have TWO separate HVAC systems? In a TOWNHOUSE? The odds that those systems are optimally sized for their loads is almost vanishingly small, unless it's a very large townhouse with a lot of unshaded west facing glass. Gross oversizing can be a comfort factor in locations as humid as Houston, due to poorer latent load handling. A right-sized system would run almost constantly during hotter summer afternoons into early evening, but would still be able to keep the indoor temperatures under 80F. Is it possible that the upper floor system is right sized, but the other system is grossly oversized? An oversized system downstairs would keep up with the temperatures, but won't remove as much humidity, and it could be that the upper floor system is doing almost all of the latent cooling for the house.

 

[TedHouston]

I did find some leaks between duct joints and I did what I could to seal some areas with aluminum foil HVAC tape but I couldn't stay up there too long, it got too hot. I'm pretty sure there are more so I will seal them this time with mastic (will order "Red Devil 0841DX F Seal 181" on Amazon) as you suggested, once the weather is more manageable. Like you suggested, I will staple underside of rafters to cover entire ceiling with RB and skip another lay on the attic floor.

I didn't know that Flex duct needs to be fully stretched out. Many of my flex ducts are not stretched and some have sharp angles so I will correct this as well. Will be hard to dig through 22" of insulation but I will seal any air gap on the attic floor as well. Since my house will be tight, now I'm wondering if I should install ERV for my house but the only place I can put it is in my attic so I don't know if it's a good idea.

I have 2 HVAC systems. 1 HVAC for the first and second floor (zoned). Another one for 3rd floor only. I believe 1st/2nd floor HVAC is 3 ton and 3rd floor is 2 ton. Both are Lennox EL16XC1 series 17SEER. My house is around 2400 sq ft with lots of windows but only few facing South. I suppose I will get the desired temp if I let it run all day but my electricity bill will be $300+/mo cooling an empty house. I'm also trying to be more environmentally friendly and efficient for my house. According to Nest Thermostat, my 1st and 2nd-floor humidity level is around 45(AC)~48(away). Third-floor humidity is around 50(AC)~54 (away). So I don't think I have humidity issue. Away temp is set to 83, at home 79, & sleep 77. At this schedule, Second floor HVAC runs around 6 hours/day while third floor HVAC runs 12hrs/day when 95+ outside. Just comparing run time, 1st and 2nd floor are well insulated as they can maintain the desired temp for a long time but 3rd floor gets all the heat from the attic so AC runs constantly. I can really feel the temp difference as I walk up the stairs from 2nd to 3rd while there is negligible difference between 1st and 2nd.

I really appreciate all your advises Dana. Keep them coming so I can plan for them this fall

 

[Dana]

Five tons of compressor for 2400' of space is a ton per 480'. Most houses have ratios between a ton per 1000' and a ton per 2000'. See this graphic compiled by an HVAC consulting company in GA on dozens of Manual Js they have done for clients (most of them in the Gulf Coast states.)

Judging by that chart you'd be looking at less than a ton of load per floor for actual load, maybe a ton or bit more for the top floor, but not 2 - 3 tons. A bigger air conditioner might cool it down faster, but not necessarily better, and usually somewhat less efficiently than a right-sized air conditioner.

Kinked-up tight-radius not fully stretched flex has moved up in the suspect list- you're probably not getting the design flow. A primer on how it should be done lives here. More here. Checking all of the static pressure drops through the system with a dual-port hand-held manometer (you can get somewhat decent versions for under $150, crummy ones as low as $50) as a diagnostic before & after stretching and sealing the duct & air handler can be a useful exercise. They can be used to find the worst-offending balance problems by measuring room to room pressure differences with doors closed/open etc., to see if you need to improve return path flows. The room-to-room pressure differences should be no more than 0.02 water inches, which is at the finest resolution end of most duct testing manometers, but if you see a room popping out as 3 it's worth addressing. A fully commissioned Energy Star duct system would test at no more than 0.012"room pressure difference under all conditions, using more sensitive manometers.