What is the best approach to insulating an enclosed attic re-model?
I have read many of the posts about creating an enclosed attic space but they have mostly spoken about new construction. We are considering spray foam and have been quoted 5.5 inches of open cell to an R-20. Most of our HVAC equipment and ducting is in the attic spaces hence one of the reasons for enclosing. With a 1950’s block and stucco wall and windows home, what should I try to achieve in terms of attic insulation for an enclosed attic space? Still want to be able to use the space for storage. Want an attic space that will improve performance but not over-perform given the rest of our low tech house. Doesn’t seem to make good economic sense. This house is in Zone 4 (Northern Virginia). This home has a standing seam metal roof. Thanks
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Replies
Woody,
According to the 2009 International Residential Code, the minimum R-value for ceiling or roof insulation in Climate Zone 4 is R-38. As you probably know, many green builders try to install more insulation than minimum code requirements.
So the right answer to your question is "at least R-38."
That said, many homeowners are constrained by a budget that is less than optimum. If you can't afford to install R-38 insulation, then you should still go ahead and install R-20 insulation. It's better than what you've got.
Under IRC 2012 that R38 is getting bumped up to R49 for zone 4.
There is also a potentially serious issue with applying open cell foam to the underside of the roof deck without venting the roof deck in climate zone 4. Open cell foam is fairly vapor open, and unless the roof deck can dry toward the exterior you may be setting yourself up for moisture problems at the roof deck. If the standing seam is mounted on purlins and the roofing felt is #15 not #30 goods you'd be fine, but otherwise it's prudent to do something a bit different on the interior. See Tables 3 & 4 in this document:
http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems
One approach that works would be to use 2" of closed cell foam on the roof deck with R15 rock-wool batts or R15 UNfaced high-density fiberglass on the interior, with NO interior side vapor barrier. (Kraft facers and mid density batts would be too vapor tight and too air-leaky for this stackup.) With the thermal bridging of the rafters factored in you're looking at a whole-assembly R of about R13-R14. From a budgetary reckoning point of view, closed cell foam is about a buck a square foot per inch of depth, so at 2" you'd be $2/foot. R15 rock wool (preferred) or HDR15 fiberglass at box-store pricing runs about $0.65 a square foot so with a sheet rock air-barrier on the interior you'd be at about $3/foot with this solution if you did the fiber as a DIY.
Another approach would be to go ahead with the open cell foam as planned, trim it flush with the rafters, and attach 3" of unfaced Type-II EPS (R12.5, 1.5lbs per cubic foot nominal density) rigid foam to the underside of the rafters held in place with horizontal 1x furring/strapping 24" o.c. through -screwed to the rafters with pancake head timber screws, sealing the edges & seams with can-foam. With any foam insulation for the attic to be code compliant as a storage area requires a thermal barrier equivalent to half-inch gypsum which can be attached to the furring, with minimal thermal bridging The combination would give you nearly 2x the performance of the open cell foam alone, and provides a ~1 perm vapor retardent layer (a minimal class-II vapor retarder) to limit the rate of moisture adsorption at the roof deck over the winter, but sufficiently vapor open to dry seasonally. This combination would also perform as well or better than R38 between rafters or joists despite being only ~R32, due to the fact that the R12.5 EPS insulates over the thermally-bridging rafters. The cost-adder for R12.5 of Type-II EPS is about $1.25 per square foot, I'm guessing the open cell foam is probably about $2/foot (correct me if this is wrong- I assume you have quotes) so this solution will end up costing about $3.50/foot, a 50 cent/foot cost adder over the other solution, but it would have nearly 2x the performance.
Thank you both. The code is based I presume off of greater insulating value than I will even have in my walls. I guess the real question I have is because of the inefficiency of the rest of the home, what makes sense from both an efficiency, practical and cost perspective and less from a code standpoint.
It's worth doing it all right eventually, but it doesn't all have to fall on the same schedule. Unless you're planning to demolish the thing and start over it's worth doing the best you can on the attic R. Since you have a 50+ year roof with the standing seam, you'll never be adding foam OVER the roof deck, but that doesn't mean you should just give up. Putting the rigid foam under the rafters is best done now, since you'd have to re-do the gypsum thermal barrier to add it later.
Code R values are based on a long-term energy-cost but way too conservative on a lifecycle cost basis. For new construction (which is cheaper to get there than during retrofits) much higher whole-assembly R values can be rationalized on a lifecycle cost basis. See table 2, p10 of this document:
http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones. (Note the recommended compact roof = R45 for zone 4, where you're located.)
Code regarding stackups using foam have more to do with moisture resilience and fire issues.
Without knowing the state of the rest of the house it's hard to say if the roof is the most cost-effective place to start, but it might be given that the ducts and air handler etc are in a completely unconditioned attic. If you're heating with propane or resistance-electricity it's definitely cost effective to do the under-foam.
And as I pointed out, in the prior post, from a practical point of view open cell foam alone has moisture and rot-risks- you either have to use some amount of closed cell foam or rigid foam to get the vapor retardency between the attic dialed to 0.6-1.2 perms to mitigate that risk. A "smart" vapor retarder like Intello Plus or MemBrain might get you there, but by the time it's all said and done applying that money toward rigid EPS as the interior vapor retarder has more value.
Doing it on a the cheap, if the current roof has soffit-to-ridge venting you can probably do OK with R15 batts in the cavities and 2" of foil-faced rigid polyiso on the interior side, which would have a whole-assembly R of about R20- which is higher R and probably cheaper than the open-cell only solution, with a low-risk fully vented roof deck that has ~2" of vented air space. That's substantially higher performance than any cavity-fill-only solution, but not as high as the o.c. foam + EPS solution. The 2" iso is about R13, at about $1.30/square foot, the high-density batts about 65 cents, so with gypsum you'd be at about $2.50/foot.
Martin. So this is my question. Code requirements aside, is the ROI there with the rest of the 1950's house as it sits to justify going from an R-20 to an R-38?
Thank you Dana
Woody,
Q. "Is the ROI there with the rest of the 1950's house as it sits to justify going from an R-20 to an R-38?"
A. You haven't provided enough information for me to make the calculation. And even if you did provide all the information, the answer depends on a variety of assumptions that can't be firmly pinned down. For a deeper discussion of these issues, see Payback Calculations for Energy-Efficiency Improvements.
ROI never.
I build and renovate. Not one project I have done has had a monetary gain. Still value is added, like comfort, style, convenience, etc.
So if this year you are desirous to add insulation to your roof, I vote go with your proposed plan. all others discussed are interesting but too complicated and too expensive compared to the actual negligible gain. You will gain value though not green value.
I have built a project with your amount of roof foam. Results have been fantastic. Not a fan of spray foam (it's not very a green product) but done well by the best in your area it will perform very well IMO and from personal experience.
Dana. Thank you for the information. I have read and been told that the open cell foam was preferred under a roof deck to allow for the detection of leaks and the prevention of rot. This seems opposite to your comments in Post #4. I have noticed on This Old House they use closed cell under roof decks almost exclusively. As for the rest of the house. Just gable venting on these attic spaces. There are 4... 1960's single pane double hung wood windows and wood doors. My next question is going to revolve around how to insulate the partial basement and crawl space.
Dana. Your first suggested approach looks to be the most promising. What was the effective R for this option? I am finding R15 rockwool (48x36x11) for $37.95 a roll at HD. Doesn't seem to add up to $.65 a sqft. Maybe my math is wrong...
Martin. Does the R 12-13 value Dana provides include the 2 inches of CC foam, 3 inches of Rock-wool and sheet rock air barrier?
"One approach that works would be to use 2" of closed cell foam on the roof deck with R15 rock-wool batts or R15 UNfaced high-density fiberglass on the interior, with NO interior side vapor barrier. (Kraft facers and mid density batts would be too vapor tight and too air-leaky for this stackup.) With the thermal bridging of the rafters factored in you're looking at a whole-assembly R of about R13-R14. From a budgetary reckoning point of view, closed cell foam is about a buck a square foot per inch of depth, so at 2" you'd be $2/foot. R15 rock wool (preferred) or HDR15 fiberglass at box-store pricing runs about $0.65 a square foot so with a sheet rock air-barrier on the interior you'd be at about $3/foot with this solution if you did the fiber as a DIY."
Woody,
Q. "Does the R 12-13 value Dana provides include the 2 inches of CC foam, 3 inches of Rock-wool and sheet rock air barrier?"
A. Yes, although Dana referred to R-13 or R-14, not R-12 or R-13. Dana wrote, "With the thermal bridging of the rafters factored in you're looking at a whole-assembly R of about R13-R14."
The "whole assembly R-value" that Dana referred to is the whole shooting match. This number is an estimate. Your actual whole-assembly R-value depends on some factors that are hard to pin down, including your framing factor (which is affected by the presence of double rafters and sistered rafters), and the quality of the insulation installation.
Could I use 2 inches of closed cell spray foam, R19 un-faced fiberglass batts and Thermax to finish my assembly to get my thermal barrier as well as additional R value?
Woody,
If you want to build an unvented insulated roof assembly in Zone 4 using a combination of spray foam and air-permeable insulation -- in your case, fiberglass batts -- then your spray foam layer needs to have a minimum R-value of R-15. Two inches of closed-cell spray foam has an R-value of about R-12 or R-13, so it is almost but not quite enough to meet the code requirement. You should really go with at least 2.5 inches of closed-cell spray foam.
More information is available here: How to Build an Insulated Cathedral Ceiling.
And the Thermax? Does that solve the ignition barrier, additional R value as well as the thermal bridging issues? Didn't see Thermax mentioned in above named article...
Woody,
Thermax is a brand name for polyisocyanurate, a type of rigid foam. There are many references to "rigid foam" in the article I linked to.
In general, I don't recommend that you create a "foam sandwich" with low-permeance foam on both sides of your roof assembly. It's usually best to install foam on just one side of the assembly, so that drying can occur in the other direction.