Seal and insulate 1960s attic?
First off I apologize if this is the wrong category.
I currently have an L shaped ranch style home that was built in 1961 with a 3:12 slope hip roof with 3″ overhangs. I currently have three gable vents, and no soffit venting. There is little to no insulation in the walls except on the south and west facing walls where I have opened them up over the years when doing renovations and installed R-13 fiberglass batt insulation. I would probably grade myself a II on install quality. In the attic there is blown in rockwool insulation about 6-8 inches deep.
I am about to undertake a home addition and convert the house to more of a U shape adding a master bed/bath as well as adding a second floor bonus room/bedroom/bath above the garage. This means that I will be closing off one end of the longer side of the L shape and losing the gable vent on that side. During this project I am also planning on replacing the asphalt shingle roof with a standing seam metal roof. On the new construction I am planning on using closed cell spray foam on all the walls, as well as creating a conditioned attic. as 75% of the ceilings will be vaulted.
My question is, should I just add additional attic venting at the end that is being closed off? Or would I be better off installing ridged foam on the roof deck and closing off the gable vents essentially converting the attic to a conditioned space?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I forgot to add, this is in central Texas so I am in climate zone 2A.
Striker,
In a hot, humid climate, there isn't any reason to invite outdoor air into your attic.
Q. "Should I just add additional attic venting at the end that is being closed off? Or would I be better off installing rigid foam on the roof deck and closing off the gable vents, essentially converting the attic to a conditioned space?"
A. It's no contest. Adding rigid foam above the roof sheathing to create a conditioned attic is by far the best of the two options.
Here are links to relevant articles for you to read:
Creating a Conditioned Attic
How to Install Rigid Foam On Top of Roof Sheathing
All About Attic Venting
Thanks Martin, I have read all of those articles and that is what prompted me to pose the question! I guess my only concern/issue is because the 60's era portion of the house isn't very well sealed, would it really work well and/or be worth the investment? I just go back and forth due to the fact that the attic already has horrible ventilation why not seal it up as best I can and insulate. Adding the rigid foam will also allow for more insulation where the truss meets the top plate and today there is almost 0 insulation in that area I am certain....
I'm trying to figure out how you have gable vents on a hipped roof (= no gables)?
You didn't ask but...
"On the new construction I am planning on using closed cell spray foam on all the walls..."
Closed cell foam in the walls is a waste of good foam, and one of the least-green options. R13 of closed cell foam in a 2x4 wall underperforms a full-fill R13 of half-pound open cell foam due to the higher thermal bridging (through 2" of wood unstead of 3.5") , uses more than twice the amount of polymer, and if blown with the industry standard HFC245fa (instead of the water used for blowing open cell), has a HUGE global warming impact by comparison. Even a full cavity fill of closed cell adds at best ~R1 to the "whole-wall R".
Instead, adding as little as 1/2" of continuous foil faced polyisocyanurate over the sheathing with an open cell foam cavity fill would add a R3 to the whole-wall R, at far lower financial & environmental cost than 3.25-3.5" of closed cell polyurethane in the cavities. Alternatively, 2x6 framing with 5.5" of open cell foam (R20-ish) adds about R4 to the whole-wall performance compared to a 2x4/R20 closed cell solution, using only about 2/3 the amount of polymer of even 2" of closed cell, and no HFC blowing agent.
See: http://www.finehomebuilding.com/membership/pdf/184243/021269086NRGnerd.pdf
In my neighborhood 3.5" of open cell runs about a buck a square foot, give or take, about the same as 1" of closed cell. At 5.5" of open cell runs about $1.60-1.75 per square foot, still less than even 2" of closed cell foam. The additional cost of the deeper framing makes it roughly equal in cost, but the 2x6 wall yields higher performance. Half inch polyiso sheathing is about a 30-40 cent per square foot cost-adder, and buys substantially more performance than closed cell between 2x4 studs.
Your money, your house, but applying the high R/inch foam budget for continuous layers on the exterior makes far more sense than installing it where it's thermally bridged by framing.
For financial rationality aspects, see also Table 2, p10 of this document:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
To hit the recommended ~R15 whole-wall for zone 2, an inch of exterior polyiso on a 2x4 open-cell wall gets you there, and is still less expensive than 3" of closed cell foam between studs.
Dana,
Attached is a picture of the roof, I was under the assumption this was a type of hip roof.
I do appreciate the advice on closed cell vs open! It is a little late in the game for me to change from 2x4 to 2x6 walls unfortunately though. I do like the idea of doing 1/2" foam board with open cell on the walls. I feel like I would still need to do closed cell on the new roof portions with 2x10 trusses though right? I could combine that with the foam on the roof as well.
The picture was definitely worth 1000 words- thanks! It's definitely a hipped roof, but with an "eyebrow" dormer feature extending the main ridge line added, probably for the purpose of attic venting. Pure hipped roofs continue the sloped ridge lines up to meet with the main horizontal ridge.
If you're putting at least R5 above the roof deck, you don't need any closed cell foam on the underside of the roof deck. See TABLE R806.5 in the IRC 2015:
https://up.codes/viewer/utah/irc-2015/chapter/8/roof-ceiling-construction#R806.5
Putting more than R5 above the roof can be pretty cheap if using reclaimed roofing foam. These folks in Round Rock (?) may have enough
of what looks like 3" (~R17-ish, if 2lb density fiber-face roofing iso) for your project:
https://austin.craigslist.org/bfs/d/4x-8-rigid-pole-barn/6446489439.html
There's at least one foam reclaimer outfit in San Antonio advertising used foam here:
https://austin.craigslist.org/mad/d/iso-rigid-foam-insulation/6496153672.html
There are probably others to be found too, running this search every week or two.
https://austin.craigslist.org/search/sss?query=rigid+insulation
So then I should (could?) for the old section, seal off the "eyebrow" dormer's, install rigid foam on the roof deck, peel and stick membrane(carslile WIP300HT ?), some 1x4 strips, then the metal roof. On the new portion, do the same on the roof, but use open cell foam on the walls and ceiling.
You could definitely amputate the eyebrow for a simpler roof line if going unvented. If the eyebrow is part of a pre-engineered truss it will need modifcation to the truss before lopping it off to preserve it's structure. If the hip ridges are continuous, hitting the main ridge at the back of the eyebrow it's a no-brainer to remove.
Most roofs with exterior foam would put the weather resistant membrane on the structural roof deck, below the foam. Depending on what the metal roofing manufacturer needs the foam can be held in place with furring/purlins (usually 2x, not 1x) through-screwed to the structural roof deck, or held down with a nailer deck through screwed to the structural deck. The nailer deck would normally need #30 felt and/or a manufacturer specified underlayment (usually as a slip surface for the roofing.)
Roofing foam under metal roofing is pretty common in commercial construction- I'm sure there will be people in Austin who know how to do it right.
With ~3" of roofing polyiso above the roof deck you can hit the IRC code-min on an R-value basis with 5.5" of open cell on the under side of the roof deck, but if you wanted to go higher R than that, you certainly can.
For what it's worth, I think this roof type is called a dutch gable or dutch hip.
This is the picture I see in my mind for the term "Dutch hip roof":
http://www.txhomes.us/wp-content/uploads/2015/07/Dutch-hip-roof.jpg
Apparently in the UK Striker's type of roof is called a "gablet", but in Oz it's called a "Dutch gable":
http://ausaca.blogspot.com/2008/12/
Looking at real-estate listings in NL there are plenty of examples of the Dutch hip (no gable):
https://www.vakantiemakelaar.nl/images/original/0/1/46001.jpg
http://www.lourenshoeve.nl/wp-content/uploads/2013/10/42_s-1.jpg
https://www.vakantiemakelaar.nl/images/original/0/1/42001.jpg
https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcR-L5EBFIadWpq87DcDP30sLRBE_7egQHRPsrixYmzGtcJya_m6
https://venumfilestore.blob.core.windows.net/pub/47f598be/b0e7/4474/9779/428ea188e318/$l/dsc_0933%20(2).jpg
http://www.watervillasfriesemeren.nl/sites/watervillasfriesemeren/files/styles/slide/public/images/de-galjoot_0.jpg?itok=w7-K4OSI
The "Dutch gable" seems pretty rare- didn't find a single house built that way (OK, it wasn't an exhaustive search :-) ) , leaving me wondering what's "Dutch" about those diminutive gables at the top ridge of hipped roofs?
At some point I learned to call those Dutch gables, until I worked on a project with the inspiration being a Japanese "Irimoya roof," which Google redirects to "East Asian hip-and-gable roof."
(The real inspiration for our project was the traditional Hawaiian roof--this is the result, if anyone is interested--one of Ecocor's first fully panelized Passivhauses: http://ecocor.us/projects/gerard-haus-in-progress.)
One last question, I can't easily and wasn't planning on adding any closed cell foam to the underside of the roof deck as that portion of the house isn't easily accessible and not a part of the addition/construction (we are living here during the construction) so I really only have about 6 inches of blow in rockwool. Is it still wise to put ISO on the roof deck if I am not insulating the direct underside or should I avoid insulating the roof deck?
Striker,
It's certainly possible to install rigid foam (for example, polyiso) above the roof sheathing, even if you can't install any fluffy insulation or open-cell spray foam on the underside of the roof sheathing. Ideally, if you go that route -- putting all of the new insulation above the roof sheathing -- the rigid foam would be thick enough to meet minimum code requirements in your area. (If you want thick rigid foam insulation above the roof sheathing, many builders find it easiest to purchase nailbase.)
But as long as your local code official agrees, even a thinner-than-code-minimum layer of rigid foam above the roof sheathing is better than nothing.
For more information, see this article: "How to Install Rigid Foam On Top of Roof Sheathing."
Yeah I would have 3 inches of polyiso on the roof deck so around ~R17 and then I have like I said about 6 inches of blown rockwool between the joists in the attic so somewhere between R16-21.0 which would meet the code requirement for the area, but the difference here is that I would have insulation on the roof deck, then a void and then the blown in insulation from the 60's. Should I be concerned with any condensation issues etc?
You can't count the attic floor R into the total from a code compliance point of view on an R-value basis, but in zone 2A there would be no condensation issues in the attic space with R17 above the roof deck, and R20 at the attic floor. Air sealing the attic from the outdoors would keep the average dew point of the attic space air at about the same level as the indoor air and even if there is some air leakage from the outdoors the average temperatures in the attic won't drop below the average outdoor dew point in summer.
With continuous insulation you can usually meet code on a U-factor basis at much lower R values than would be required for compliance on an R-value basis. The IRC code-min for zone 2 is U0.030 which is R33 "whole assembly", which includes the thermal bridging of rafters & joists, the R-value of the ceiling gypsum, the roof deck & roofing the interior & exterior air films, etc.. The ceiling gypsum & roof deck adds up to another R1, the interior + exterior air films + roofing adds another R1. If you count the air films on both the attic floor and under the roof deck you get another R1.5, so you're up to about R21.5 even BEFORE calculating the benefits of the R15- R20 between the joists (which I believe IS legitimate when meeting code on a U-factor basis.) That leaves only R11-R12-ish performance needed from the layer of joists + fluff, which isn't a big ask- even with 12" o.c. spacing on the joists you'd still make it. But with 5" of roofing polyiso you'd make it on a U-factor basis without needing ANYTHING from the attic floor level.
Striker,
For more information on one of Dana's points -- the one about "With continuous insulation you can usually meet code on a U-factor basis at much lower R values than would be required for compliance on an R-value basis" -- see this article: "Three Code-Approved Tricks for Reducing Insulation Thickness."