Monday, April 25, 2022

Recovering from a Bad Idea Whole House Fan


I am preparing to replace six dirty and badly-placed roof high vents in a 1982 home disserved much of its life so far, by a whole house fan. Shutters of the whole house fan were quite ineffective in blocking unwanted movement of moist heated air to the attic. Walking a plank to toss on pieces of kraft-faced insulation, rarely happened. Now in 2022 the attic is stinky. Roof sheathing is widely blackened.

The fan motor is dead, but otherwise the fan is as-new. The fan is remarkably simple and cheap, with cardboard as part of the enclosure.

The fan shutters were leaky all-around. 

From below, shutters of the dead fan were tolerated as interesting in a grand stairwell ceiling.

This is what an attic, now  durably insulated >R49,. Extensive (useful) flooring upon raised supports, is not a forests-consuming indulgence. By my Perpetuiity Math, see that tight,durable insulation has present value many times greater than usually-poor added insulation soon to be trampled.

All six roof high vents are misaligned with sheathing cuts and are fouled with sticky matter not removable by my strong vacuum.

This is a 1600 sq ft attic. Prescribed net free area, NFA, for static venting is 1 sq ft for every 300 sq ft of attic area, 5.3 sq ft., Distribute this 60% at inlets, and 40% at outlets. The high vent NFA should be 5.3 sq ft * 0.4 = 2.1 sq ft, 307 sq in.

I chose these roof caps, manufactured to order in Idaho, at Famco. Louvered Roof Vent, 50 sq in NFA

50 sq in is the area of an 8" pipe. The Famco throat is 9" dia., and is not screened. I intended to build screened aperture boxes centered on each vent, and now serviceable by vacuum and easily replaced..

With my spun-aluminum 8" roof penetration adapters, I could fix the cap misalignment, in a rebuild with new shingles and with strong repair of the sheathing hacks. I intended to make a nibbled 10" dia. cut in each Famco cap, removing the 9" pipe. With a penetration adapter, a cap may be simply a rain cover. This plan failed where the Famco caps are very badly designed.  The 9" pipe I would scrap, is a problem for anyone else. It is far too tall, reducing cap NFA to 38 sq in. It should have been evident that outlet louvers are upside down, catching rain with inward momentum, sometimes being carried into the 1 1/2" tall throat of the penetration adapter. 

While waiting for the Famco caps, I did what I could to add some vent inlet area. Along one accessible wall I jockeyed this Little Giant ladder to reach four of six plugged soffit vents, increasing area 

At hand, screens of enlarged cuts for soffit venting, Construction Metals.FOV1686, 40 sq in NFA. 

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