Replies

1. GBA Editor
   Martin Holladay | Jul 10, 2017 08:22am | #1

   Texas Ranger,
   Q. "Would closed-cell foam on the underside of the roof decking + sealing the current ventilation pathways significantly reduce my utility costs?"

   A. Yes. For more information on this issue, see these two articles:

   Keeping Ducts Indoors.

   Creating a Conditioned Attic.

   Q. "I'd also have to extend the bathroom vents through the roof since they now vent into the attic."

   A. You should do that regardless of whether or not you decide to install spray foam on the underside of your roof sheathing. Venting a bathroom fan into an attic is a code violation.

2. 
   Anon3 | Jul 10, 2017 09:11am | #2

   Bad idea, get solar panels instead.

3. 
   STEPHEN SHEEHY | Jul 10, 2017 02:25pm | #3

   Martin: Your response suggests that it's bathroom fans that vent into the attic. But he mentions "bathroom vents." I took that to mean plumbing vent stacks. Either way, they shouldn't vent into the attic and both are code violations.

4. Expert Member
   Dana Dorsett | Jul 10, 2017 02:53pm | #4

   What he said- venting a bath fan into an attic is a code violation almost everywhere, and should be corrected no matter what happens with the insulation scheme.

   Every inch of closed cell foam uses as much polymer as 4" of open cell foam, so the 8-9" of open cell would be the polymer equivalent of just a couple inches of closed cell. Most closed cell foam is also blown with high environmental impact HFC245fa, whereas all open cell foam is blown with low impact water. The bigger the fraction you can do safely with open cell, the nicer it is to the environment (and the pocketbook.)

   In US climate zone 2 &3 (covers all of east TX) you only need R5 of closed cell foam on the underside of the roof deck to protect the roof deck itself, per, R806.5 of the IRC:

   https://up.codes/viewer/utah/int_residential_code_2015/chapter/8/roof-ceiling-construction#R806.5

   The other R33 to bring it up to R38 can be done with (much greener) open cell foam, at half to 2/3 the cost.

   https://up.codes/viewer/general/int_residential_code_2015/chapter/11/re-energy-efficiency#N1102.1.2

   To do R38 all with closed cell takes 6" of foam (in my neighborhood that would run about $6 per square foot.) To do HFC blown closed cell safely and at high quality would require three lifts of no more than 2" per pass, with a cooling period between lifts to avoid a fire risk from heat build as it cures over a few hours.

   Do do it with an inch of closed cell on the roof deck followed by 8-9" of open cell foam would be less than $4 per square foot. Some open cell foam can be safely installed in 9" lifts, but any would be good for at least 6" per pass safely.

   Solar panels would run about $25-35 per square foot (sometimes lower in some parts of TX), but may be a better investment if your electric rates are high and you can be net-metered at retail for 20 years. The shading of the roof provides some direct reduction in cooling load, but the real benefit is the amount of power being offset over the lifecycle. But it's a MUCH bigger investment decision, even if only 1/3 the roof area.

   Whether the roof decks is insulated or not, in your climate a vented attic usually increases the moisture content of the wood inside the attic than an unvented attic. Sealing up the venting without insulating at the roof deck will increase the peak attic temperatures by several degrees, but may lower the actual cooling load by reducing infiltration and the accompanying high latent loads.

   If there's space for it, when the electric water heaters are up for replacement, heat pump water heaters would beat their rated efficiency when located in a hot attic. They tend to be taller than typical electric tanks, so they may not fit. But whether the roof deck is insulated or not, heat pump water heaters will provide both cooling and dehumidification of the attic space, which is all to the good.