Exterior foam is getting costly: Any options at this stage?
I had previously discussed the merits on here of spray foam vs exterior rigid foam + blown insulation for an unvented 2400 sq.ft roof (enclosed attic) in zone 2 on a pretty good house and the general consensus was exterior foam was preferable avoiding the health risks with spray foam, GHG impact of closed cell and moisture risks of open cell. At the time the difference in cost between the two seemed negligible. However my contractor has now told me that due to a reluctance in crews to work with exterior rigid foam (it’s just not done in Houston) and material cost increases, the difference for the rigid foam is now about $6000 more than open cell spray foam (and that’s with cheap recycled 2” rigid foam). I’m still reluctant to go down the spray foam path (5 min in my friends spray foamed attic gave me a headache) despite its insulation qualities, and have been thinking hard but struggling to see cost effective options. However at this premium, the payback time vs insulating the attic floor even with HVAC in the attic would be about 20+ years if I look at the bills of my current house.
A ventilated roof with interior rigid insulation seems out as we have two valleys in the roof preventing ventilation of every roof bay.
Also the framing is already up (2x 8) so not much scope for modifying the structure.
Ive struggled with identifying a cost effective approach which is a weakness of the options presented here since cost is rarely discussed enough.
The logic seems to go:
1. HVAC in attic in south – go enclosed
2. Have a non perfectly simple roof – go unvented.
3. Unvented roof – spray foam – bad GHG or moisture control, health risks, installer risks etc, go exterior foam.
4. Exterior foam – two layers of decking, lots of install labour – have a really expensive roof….
Any ideas from southern builders?
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Replies
Markus,
For an overview of the issues you are talking about, see this article: "Solutions to the Attic Duct Problem."
Of all of the possible solutions to the attic duct problem, my favorite is to install ductless minisplits, eliminating ductwork. This is an excellent approach -- but it may be too late for you to adopt it.
Your analysis is correct: At this point, you have three options:
1. Install exterior rigid foam.
2. Install closed-cell spray foam on the underside of the roof sheathing.
3. Install open-cell spray foam on the underside of the roof sheathing.
All three options may also include a second type of insulation on the interior side of the foam insulation.
For more information on this issue, see "Creating a Conditioned Attic."
With option #3 above, you may be able to use a diffusion vent.
A diffusion vent requires a vapor permeance greater than 20 perms to meet code. Open cell foam at code-min R values would be substantially lower permeance than that. (Even at R20 half pound foam is under 20 perms.) You might get there with fiber insulation though.
See page 5:
https://www.energy.gov/sites/prod/files/2017/05/f34/BA-Webinar-unvented-attics-2017-may-18_0.pdf
Read the full text of section R806.5 subsection 5.2 carefully:
https://codes.iccsafe.org/public/document/IRC2018/chapter-8-roof-ceiling-construction
Thank you all for your responses. After much thought I decided to go for the open cell approach based on where we are already. In hindsight a vented attic with mini splits might have been wiser.
I will have a heat pump water heater in the attic so that should contribute to humidity control and I will install humidity monitoring to ensure whether further dehumidification is necessary.
I will also have an independent ERV system. Has anyone considered installing just an extract vent in the attic to ensure slight negative pressurization and draw air from the house rather than other way round? Since it would vent to outside it would presumably avoid fire code issues with return AC central air ducts? I see this as beneficial in ensuring any offgassing is vented out and drawing drier air into the attic.
I say stick to you plan. Ask for more bids until you find someone willing to work for a fair price. The current bid has a I do not want this job price tag. The problem is the siding guys don’t see it as part of there job not want to install the foam same for the framer that install the sheeting.
HVAC in the attic is stupid. Say we do not do stupid in my house.
An unvented attic is an act of desperation reserved for when all the good plans fail.
Walta
Markus,
If you install an exhaust fan in your attic, you will, of course, incur an energy penalty (since you will be removing conditioned air from your house). I'm not saying you shouldn't do it -- just pointing that out.
If you have any doubts about whether this plan violates any fire safety codes, you should check with your local code enforcement official.
For an overview of issues in conditioned attics insulated with open-cell spray foam, see "High Humidity in Unvented Conditioned Attics."
Martin,
Thanks for your response, I read through your article and comments and it looked like Joe Listburek proposed an extract vent as one potential option for his Kansas City code ammendments a couple years ago. Not sure what the outcome on that was but I will explore it with the city if the HPHW heater is not enough for humidity control.