Should I add an Attic fan with humidstat?
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| Posted in Energy Efficiency and Durability on
I am building a tight house on a wooded lot without attic access upon completion (r38 batt insulation between STANDARD scissor trusses, ridge and eve venting, metal roof) http://www.flickr.com/photos/48876863@N08/. My concern is that with the standard trusses, I may be inviting moisture problems without additional ventilation.
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What makes you say this? Moisture must have a source. Often, the moisture is brought up from the home. If this is the case, creating negative pressure in the attic could be counter-productive. You are building a "tight" home and might not have this problem. Alternatively, moisture could come from outside. If the temperature is higher in the attic space than outdoors then you should experience lower relative humidity in the attic and be unlikely to cross the dew point. Generally, I reccomend passive ventilation. Active vents are just one more thing that can break down. You've already planned for an Energy Star roof (I hope) and should be managing things quite nicely. Send specific questions and concerns to: [email protected]
- Tony Olaivar: Energy Specialist and Licensed roofer (Illinois).
Frank,
There are a few problems with your insulation plan, but high attic humidity is not one of them.
1. It looks from your Flickr photos that your scissors trusses don't provide enough room near the eaves for full insulation thickness. If my guess is correct, you will need to install closed-cell spray foam in this area to achieve the necessary R-value. It's a vulnerable spot, and if you leak heat there, you may have ice dam problems or ceiling mold.
2. Fiberglass batts are a poor insulation choice. You may want to talk to a cellulose insulation contractor.
3. As long as you make your ceiling as airtight as possible -- which means that all partitions need to have the drywall caulked to the top plate, and all electrical penetrations and plumbing vents need careful air sealing -- there's no reason to believe you will have any humidity problems in your attic. Needless to say, you don't want any recessed can lights.
Martin
Your comment above, #1, is consistent with another opinion and is what prompted my posting. You mention spraying closed-cell spray foam in this vulnerable spot...could you elaborate on the application?
Also, if I adjust to blown in cellulose (I was advised I could get 7" by the time the cellulose has reached the conditioned space starting from the bird vent insulation baffle), would that be enough to alleviate potential problems (e.g., ice dams, ceiling mold)? thanks
Martin
I understand your comment #1 to mean ... that unless I achieve enough R value in the vulnerable spot (the area above the ceiling drywall closest to the exterior wall, where the standard scissor truss will restrict application of a R38 batt), I risk condensation on top of the drywall, in the attic space. This could eventually lead to mold, and rare ice damming (Seattle climate).
Digging a bit further, I found I could achieve R 24 with PROPER application of loose fill cellulose in that vulnerable area. (ref http://bct.nrc.umass.edu/index.php/publications/by-title/cellulose-insulation-a-smart-choice/).
Does R24 get your vote of confidence?
What is the code requirement for your area?
Frank,
I'm not sure of the code requirements in your area, but it makes sense to have R-38 at the most vulnerable part of your roof, if that's what you're aiming for elsewhere. If your insulation thins out to R-24 at the eaves, more heat will leak out in that region than elsewhere.
Closed-cell polyurethane foam will give you about R-6.5 per inch, so to get R-38 you need about 6 inches of closed-cell foam. I'm not sure I can "elaborate" on my recommendation; you'll have to call a local contractor who installs closed-cell spray foam to find out more details, I guess.
Be sure that your insulation covers the top plate of your wall.
For readers wondering what Frank might have done to avoid this dilemma: he could have ordered raised-heel trusses. Too late for that now.
Frank,
You may have a misconception about where condensation occurs. You seem to think that you will have high moisture or condensation in your attic. But as both Tony and I have explained, attics are usually dry. As long as you have a good ceiling air barrier, the moist interior air should never reach your attic.
Here's why mold sometimes grows on bathroom ceilings, especially near exterior walls: that's where the ceiling is coldest. This problem is typical in homes with standard trusses like yours, where there is insufficient room for insulation.
Note that the source of the moisture is the interior of the home, and that the mold grows on the cold bathroom ceiling. The mold doesn't usually grow in the attic (as long as you have a decent ceiling air barrier).
Martin
....looks like we're showering downstairs!. Thanks for the clarification. If I understand correctly, because of inadequate insulation near the eaves with a standard truss, the dew pt is on the interior of the wallboard. When the humidity is high in the conditioned space (read bathroom), moisture occurs...which could lead to mold.
Can't I control this moisture content with spot ventilation and a HRV? thank you
Frank,
The lower your interior humidity, the less chance of condensation or mold. And you're right — mechanical ventilation during the winter months tends to lower indoor humidity.
However, ventilation doesn't solve the energy-leak problem or the cold-ceiling problem. If anything, increasing the ventilation rate simply increases your energy bill.
At your eaves:
1. Closed-cell spray polyurethane foam is better than stacks of rigid foam board.
2. Stacks of rigid foam board (sealed at the perimeter with canned spray foam) are better than cellulose.
3. Cellulose is better than fiberglass batts -- especially fiberglass batts subject to wind-washing.
Frank,
Probably the most cost effective would have been the use of healed truss & cellulose; however, at this point I would think of open-cell or closed cell foam under the roof deck and 2"-3" rigid insulation above the roof deck. It allows you to have ducts in a semi-conditioned space plus all the can lights you want!!! The rigid insulation serve as a second moisture barrier to the roof and helps with thermo-bridging and condensation in the attic.
If you have concern about internal moisture use a dehumidifier, plain an simple. Nowdays is trendy for too many people to use ERVs, yet a dehumidifier attached to the air handler is usually a better fit.
If it's just in the bathrooms, use a humidity controled fan or a timer wired to the bathroom fan.
Not knowing your climate zone or location it could be another option...
Frank,
If you're building in the humid Seattle climate, then you may be right to worry about in-attic condensation caused by the venting of outside high-humidity air and nighttime cooling of the roof deck.
I almost never advise against roof ventilation, but in your case - with inadequate insulation depth at the eaves - I will suggest a hot roof with a couple inches of foam above the roof deck, strapping and then metal roofing.
Eliminating the eave and ridge vents will not only eliminate the entry of humid outside air but also allow more insulation depth at the eaves.
Frank - back to your original question - looks like the volume of your attic is very small - odds are any Powered Attic Fan is going to be too large and your exhaust (PAV) is going to be way larger than your intake (soffit vents) so to make up for the imbalance it will be pulling conditioned air from your second floor into your attic thus wasting energy. During the winter you would more than likely be pulling more humid air into the cold attic creating more of the problems mentioned above.