Attic Insulation for Old House With AC
Hi all,
Thanks for providing such a wealth of information here. I’ve read a ton, but it seems there are many rules for many cases. I have a question regarding our 110yr old house outside of Boston, zone 5. It has a hip roof, 900sqft unfinished walk-up attic. It currently has very old insulation in the attic floor. The floor itself is just old boards/planks. The attic also has a tstat controlled exhaust fan. There is one dormer and i leave the window open in the summer thinking it’ll help the fan draw air through rather than cool air up from the living space. There is minimal soffit venting, only four vent/grates total and mostly painted over. The walls of the house have blown-in insulation.
Next week we’re having central air conditioning installed and the plan is to have the air handler and ducting in the attic. After this job, we will address insulating the attic.
I have two things to figure out:
1. How best to insulate the attic. The goal being to save energy and increase comfort in winter heating and also summer cooling. It seems insulating the roof itself to contain the AC equipment within the building envelope would be best. Does this mean we’d be “sealing” the attic/house? Is this possible with an old structure? If not, how best to vent? In the short term, should i kill the exhaust fan?
2. The ceiling of our basement (under the first floor wood flooring) there is fiberglass batt insulation. Mice have made a home of it and its a mess now. I’ve read mixed things on how necessary this is. The basement is unfinished, but generally clean and dry and we use the space. It is a fieldstone foundation with concrete floor. Should we simply replace the existing insulation or consider a new plan?
Thanks in advance!
Dan
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Replies
Dan,
Q. "It seems insulating the roof itself to contain the AC equipment within the building envelope would be best. Does this mean we’d be 'sealing' the attic/house?"
A. Yes. You will be converting your vented unconditioned attic into an unvented conditioned attic.
Q. "Is this possible with an old structure?"
A. Yes. Here is a link to an article that explains the work: "Creating a Conditioned Attic."
Q. "In the short term, should I kill the exhaust fan?"
A. Yes. More information here: "Fans in the Attic: Do They Help or Do They Hurt?"
Q. "The ceiling of our basement (under the first floor wood flooring) there is fiberglass batt insulation. Mice have made a home of it and its a mess now. I’ve read mixed things on how necessary this is. The basement is unfinished, but generally clean and dry and we use the space. It is a fieldstone foundation with concrete floor. Should we simply replace the existing insulation or consider a new plan?"
A. The best approach is to remove the mice-infested insulation, and to insulate the interior of your basement walls with closed-cell spray foam. More information here: "How to Insulate a Basement Wall."
Thanks for the info. Our roof is in good shape so i think our plan would include closed cell spray foam directly to the underside of the roof, no bay vents (because air-impermeable insulation), probably with furring strips to deepen the bays, and then rigid insulation boards under that. No soffit vents, no ridge vents, no exhaust fan. Then remove any existing insulation from the floor i'd imagine. Sound reasonable?
The article says with air-impermeable close cell insulation, there's no need to add air vents/returns to the attic. Is there such a thing as too sealed of an attic? Does that air need to turn over with the rest of the house?
Thanks again,
Dan
How deep are the rafters, and how much closed cell foam is being proposed for the roof deck?
The R value of insulation on the attic floor doesn't count toward energy code c0mpliance, but it's still doing something, and doesn't need to be removed except to allow the closed cell foam to be applied over the top plates of the exterior walls at the eaves. It would only be important to remove it if it had a substantially high-R (R30+) AND the "...walk-up attic...." was well air sealed from the rooms below, which it clearly isn't.
Will you be re-roofing any time soon?
I'll have to check, but the rafters aren't very deep. I'd guess 8". It's old lumber that is actually 2" wide. I was thinking we'd just fur out until there was enough depth to get the desired R value combined with the rigid board. How should i consider the trade between adding spray foam depth and rigid insulation depth? Is it necessary to cover the rigid board insulation with anything?
That's good to know we can leave the old insulation in the attic floor. I guess it'd only help a bit from the living space perspective. Guessing, the floor joists are like 8". I'll check. It's very loosely full of what looks like old dark brown batty stuff. It also looks like insulation was once nailed to the roof rafters. It all fell down, but i can see bits and pieces on the nails. I can't find anything like it in a quick google.
It's a walk-up attic with a door to access the stairs. But yes, surely not sealed air tight.
A new roof is not in the plan, but maybe we could swing it in a couple years. I believe the current roof is about 8yrs old.
Thanks!
Installing rigid foam board under the rafters of an unvented roof potentially creates a moisture trap, so you have to be a bit careful here. A full fill of closed cell foam is expensive, and very low permeance.
In zone 5 as long as more than 40% of the total R is closed cell foam applied to the underside of the roof deck the remainder can be a more vapor permeable insulation. Using 2lb density HFO blown foam you'd hit R20 in about 3", which leaves 5" of rafter depth for the remaining R29 needed for a code-min R49. If you furred it out to 6.75" with 1" of polyiso + 1x furring "Bonfiglioli Strips" on the rafter edges you could compress R30 rock wool batts into that 6.75" and hit it pretty much right on the money for R49 performance.
Alternatively, 4" of HFO blown foam (for R28) leaving 4" of remaining depth, furred out to 5.5" with 3/4 foam board + 1x furring Bonfiglioli strips, (or simply adding a 2x2 on the rafter edges) would allow one to install R20-R21 fiberglass or R23 rock wool, with a considerable amount of dew point margin at the foam/fiber boundary.
https://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf
The Bonfiglioli approach is fairly labor intensive to do on a hipped roof, but at $1.25- $1.40 per square foot per inch of depth doing it with 7" of HFO blown foam would be pretty expensive, and in the end not all that green, with a high polymer per R, and a thermal bridging performance hit due to the short 7" path through the rafter. The 4" foam and furring out with a 2x2 is probably more financially rational if paying someone to do it.
That's very very interesting. I like the idea of having someone coming in and sealing the attic in a few inches of spray foam and then DIY'ing the rest. That'd be a satisfying project (in the late fall).
Thank you
Note that code allows you to use any combination of open cell spray, closed cell spray and rigid.
Deleted
Additional info: the attic rafters are good ol' proper 6x2"s. So not very deep.
An insulation contractor stopped by for a quote. He set the insulation goal at R38 which is for existing structures. It's not as good as R49 of course, but its a solution that might fit in the existing rafters making for low complexity and price.
He quoted a full bay fill of closed foam for about 75c per sqft-inch. Seems like a really good rate. It'd be skimmed to the face of the rafters so the ceiling could potentially be finished some day. Maybe rigid could be added to this option?
He also quoted open cell foam for a full bay plus extra inches beyond, covering all the faces of the all the rafters for about 2/3rds the price of the closed cell foam.
Both options would seal from top plates up. I'd have to have the attic exhaust fan hole patched separately. His quotes include flame retardant paint.
If i got to R38 using only spray foam on the inside, maybe i could just install rigid insulation under the shingles the next time the roof is done and call it good. What are you thoughts given this relatively lower than expected estimate? Thanks again
edit: plot twist. We assumed they were 2x6 rafters but i just measured and they're actually between 2x4 3/4 and 2x5". Not deep enough to hit even R38 with closed cell as i understand it. hrmm
Assuming 16oc with real 2x which works out to around 18% framing factor on a hip roof.
Spraying 4.5" of cc SPF gives you an effective R22 roof (R30 center of cavity).
Even if you had real 2x6 and went with 6" of cc SPF, that would only get you up to R26. That extra bit of spray foam would just cost you money and not improve your insulation much.
If you want higher performance, go with 3" or 4" of cc SPF for now and install 2" of rigid insulation on top at the next re-roof, or under the rafters before you finish the celing. That would get your roof up to R35.
>"He quoted a full bay fill of closed foam for about 75c per sqft-inch. Seems like a really good rate."
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>"What are you thoughts given this relatively lower than expected estimate?"
Caveat emptor. Most installers charge about a buck a board foot for the envir0nmentally damaging HFC blown foam, and more than $1.25/board-foot for the more benign HFO blown stuff. At 75 cent a bit more than half the cost of a middle of the road bid the more environmentally benign HFO blown closed cell foams.
There are strong odds that it's blown with HFC245fa, a powerful greenhouse gas (about 1000 x CO2 @ 100 years). HFC blown closed cell foam would has to be installed in three lifts of 2" each with a cooling period between lifts to be fire-safe while curing, and to avoid quality issues. Only HFO blown foams would be safe to install 5-6" in a single pass.
Not to mention that closed cell foam between framing is a waste, since it's being robbed of it's potential performance by the thermal bridging of the studs. It's better from price/performance perspective to spend some of the cavity fill foam cash on thermally breaking the studs. See:
https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste
The framing fractions of most roofs are lower than walls, but you have 16" o.c. rafters rather than the current standard 24", and 2" not 1.5" wide rafters, so it's still a real hit.
If not furring out the full-dimension 2x6 to something deeper, installing 2" of HFC blownfoam would be about R12-R13, which is sufficient dew point control for up to ~R18-R20 of fluff in the remaining 4". Even if you were compressing R23 rock wool (nominally 5.5") into the (nearly) 4" depth it's performance would be a hair shy of a perfectly safe R18 for about R30-R31 at center cavity.
If furring out the rafter edges with Bonfiglioli strips (1x furring + "1 polyiso foam) sufficient for a ~5.5" cavity depth beyond the 2" spray foam layer, installing R20 fiberglass batts would deliver R32-R33 at center cavity, but boost the rafter framing fraction from about R7 to R13+ , cutting the thermal bridging by nearly half. This is higher performance than 6" of HFC blown foam between full dimension 2x6 rafters.
At 2" of closed cell foam the vapor retardency of the foam is low enough to fully protect the roof deck in a zone 5 climate. See Table 3 of this document:
https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
In particular, seen the Boston, Chicago, and Boulder rows at the intersection of the 2" ccSPF + spray fiberglass column. Those simulations were at R38 center cavity, and you'd be a bit lower than that, which makes the roof deck a hint warmer/drier/safer. When it's time to re-roof the vapor retardency of 2" of close cell foam is still high enough to be a reasonable drying path after installing any amount above-deck foam or true vapor barrier full membrane shingle underlayments, whereas 6" of closed cell foam is getting close to true vapor barrier territory.
I asked the contractor and he said that its HFO. He also claimed that its R7.5/in which from what i gather is unlikely. He does have a lowish number of reviews and they're all good. And he has a gallery full of pics that look like mostly commercial and new construction, but also some renovation work. He said he normally fills the gap in the soffit area with open cell foam to create a structure, and then seals it all over with the closed cell. He also quoted a 2" HFO install and it scales with his other estimates.
Wouldn't 2" of HFO + 4" of rockwool have the same thermal issues as 5" of HFO in terms of the framing losses?
It seems like no matter what, 2" of HFO is the foundation of this. It does the sealing and dew point control. Beyond that i could:
A) simply fill it all up with HFO
B) add open cell filled beyond the rafter depth
C) furr it out to 6" or so and add rockwool
D) furr it out to 6" or so and add fiberglass
E) Bonfiglioli strip it plus fiberglass
Under what circumstances could i use rigid insulation boards under the rafters to add Rvalue as well as reduce framing losses? Maybe that'd be easier to install the furring, but probably more expensive?
I'm working with about 1160sqft of roof, so not very big. But it is hipped and has a dormer so a little fiddly.
The AC system is now installed in the attic and working well. I also disabled the attic roof vent.
My next step is to get a roofer to close up the attic fan hole and shingle it up. I'm still trying to decide on an insulation plan. I'm leaning toward doing closed cell as soon as possible to get something up there. (It's amazing how little is in the attic floor). Then i can work on adding to it. I also think that adding rigid insulation under the shingles is a great way to go, but i would hold off on that for a few years.
How good/bad is it to have "some" insulation in the attic floor and "some" in the roof? I'm sure it isn't purely additive, but it's gotta be better than only the floor insulation i've had so far. It would at least seal the attic.
How about the plan of doing 2" closed cell, then dense pack cellulose blown in over rigid foam under the rafters? Seems labor intensive.
Or what if i just filled the bays with closed cell for now and then did the under-shingle rigid foam in a few years? It's tough working with these shallow rafters. (4.75-5" deep).
Thanks again.
>"How good/bad is it to have "some" insulation in the attic floor and "some" in the roof?"
It's not bad at all, with caveats. In zone 5/Beantown type climates as long as at least half the R-value or more is at the roof deck the humidity levels in the attic in winter won't be too high.
>"How about the plan of doing 2" closed cell, then dense pack cellulose blown in over rigid foam under the rafters? "
2" of closed cell is at most R14 (which would require HFO blownfoam), which would only be sufficient dew point control for about R20 of insulation to the interior side. If layering on rigid foam board with fluff between the foam board and spray foamed roof deck there is a risk of trapping moisture in the rafters, since there will be effectively no drying path unless using mid-density unfaced EPS.
The 2" of HFO blown foam in a full dimension 2x6 cavity leave 4" of cellulose for about R15. So to limit the moisture-trapping potential the interior side foam should be limited to about R5 or so, preferably something at least somewhat permeable to water vapor. That could be 1.5" of UNFACED Type-II EPS (~R6.3, ~1.5-2 perms).
If you want to punt and finish the game later, go for at least R20 (3" of HFO blown foam) any more of that has rapidly diminishing returns on cost, both environmental & financial. With R20 you have sufficient dew point control for up to R30 on the interior side, which would allow you to get to current code-min performance.
>"Seems labor intensive."
Yes it is- dense packing cellulose in this case isn't going to be "worth it" for just the 3-4". It's cheaper and better to just install rock wool batts as a compression fit for when the foam board goes up. With 3" of HFO blown foam and 3" of rock wool you'd be looking at about R13 for the rock wool, and you could then install up to 4" of Type VIII roofing EPS (R17, ~0.8 perms) on the underside.
Places like Insulation Depot/Nationwide Foam in Framingham or Green Insulation Group in Worcester almost always have stacks of 2" - 4" EPS scavenged from commercial building demolition & reroofing projects, at a very deep discount, as well as virgin stock factory goods rejected for blemishes or dimensional error. They won't always know the density- so where it matters be prepared to weigh a sheet and do the math, but it's almost always Type-VIII which is nominally 1.25lbs per cubic foot. A 2" x 4' x 8' sheet is 5.3 cubic feet, so Type VIII would weigh 6.7 lbs +/- 10% a 4" x 4' x 8' sheet is 10.7 cubic feet, or 13.4lbs give or take. Type II would weigh closer to 8lbs for 2", 16lbs for 4". In your case even Type-II would be OK at 4" as long as there is at least 3" of HFO blown foam on the underside of the roof deck.
Very interesting, thanks. I'll do a "foundation" of 3" HFO then. This only leaves me with 1.75-2" of remaining rafter depth, though. can rockwool be smooshed in there? Is it worth it? Looks like i might be doing some furring.. not the end of the world. Maybe i should just plan on 3" HFO plus furring out to get enough rockwool depth.
Actually, after a calc, it'd take some serious furring to get enough depth to get enough rockwool in there to get to R39 on top of the 3" of HFO. Then still have the thermal shorts of the rafters. I guess that's where the beauty of the insulated furring solution mentioned above. Good news: i measured my rafters to be 20"oc and not 16". My roof will probably collapse, but it has fewer thermal shorts! :)
3" of HFO at 6.5R = 19.5
8" of rockwool at 2.5R = 20
Maybe i really need the under shingle stuff in the equation or perhaps its just not in the cards for this old house. Or maybe i could do a little in the attic floor? gah.
Thanks again for your input. I think we're close to a plan!
With odd rafter spacing, I would just leave the gap empty or get the rest filled with open cell foam.
Getting batts in there is a lot of work for not much extra R value.
About the only way this is worth it is to cross strap the roof at with 2x3 or 2x4 on edge and dense pack the cavity. This would get you around R35 whole assembly roof.
>"3" of HFO at 6.5R = 19.5
>8" of rockwool at 2.5R = 20
Most HFO blown foam is closer to R7/inch for R20 - R21.
Unlike loose fill rock wool, most rock wool batts currently sold in the US are about R4.2/inch.
It's possible to split 3.5" R15 rock wool batts into a pair of ~1.75" thick batts good for R7-R8, but it's a lot of work for not much gain.
But installing 2" wide strips of 1.5" polyiso cap nailed on the rafter edges to deepen the cavity depth up to 3-3.5" would be enough to get the full R15 with less mess, bringing the center-cavity R up to about R35, and an R8.5-R9 thermal break over the rafters (depending on foam type). Ripping a few hundred feet of edge strip from used 1.5" roofing polyiso goes quickly on a table saw if you can do it in a place where you can deal with the mess. Cap-nailing it in place is also pretty quick. With R15 fiber/R20 foam 57% of the total R is on the exterior of the fiber, which is enough to not be concerned about the vapor retardency issues- even if the rock wool were left uncovered it wouldn't present much of a moisture accumulation problem in your climate zone. But with the rock wool on the attic floor a layer of MemBrain up against the ceiling is cheap insurance.
If going that route keep track of the temperature and humidity in the attic space. If it turns out to be too cool/humid up there in winter it's easy enough to vacuum out some of the loose fill rock wool if absolutely necessary.
>"Maybe i really need the under shingle stuff in the equation or perhaps its just not in the cards for this old house. Or maybe i could do a little in the attic floor?"
Given that..
>"...we’re having central air conditioning installed and the plan is to have the air handler and ducting in the attic."
Insulating at the attic floor would be an actively BAD idea, both for cooling season efficiency, and wintertime comfort. Cascading cold air coming out of the AC vents feels good in summer, but when it's 0F outside and it's 10F in the attic you'll be feeling the convective draft, and may even end up with copious condensation inside the ducts during cold snaps.
>"... i measured my rafters to be 20"oc and not 16". My roof will probably collapse..."
It won't collapse. A full dimension 2x6 is at least as strong as a milled 2x8, and milled 2x8 rafters 24" o.c. are pretty common all over MA. Even milled 2x6 (1.5" x5.5") rafters 19.2" o.c. can handle a significant snow load.
Interesting point regarding the rafter spacing and the impact on under-shingle boards.. wow the plot keeps thickening.
The spray foam guy did say he could open cell it out well beyond the rafter depth. That'd get me depth and cover the rafters themselves. Maybe 3" of closed cell HFO and then 4" of open cell? That'd get me to:
3" x R6.5 closed cell = 19.5
4" (2" of which in bay, 2" beyond rafter) x R3.7 = 14.8
=> R34.3 plus come rafter coverage.
It would be messy looking, but that's ok i guess. I'd have to consider how to mount basic lighting. It wouldn't require any work from me which is nice :)
Thanks again
I didn’t see any discussion on how to provide the needed thermal or ignition barriers for the spray foam.
Given that you have an air handler in the space, you will need a thermal barrier. This is commonly 1/2” drywall or fire retardant paint applied to the foam.
Read here: https://www.greenbuildingadvisor.com/article/thermal-barriers-and-ignition-barriers-for-spray-foam
The least work and fastest approach, but most expensive, is closed cell to 40%R, open cell over the rafters at 60%R, fire retardant paint.
In my house I did 3” of HFO closed cell, r30 rockwool, with 1” polyiso furring. Normal open bays go fast, but complicated sections are a drag. I have collar ties, framed knee walls and 3 dormers. I was concerned about the air quality risks of two foam operations and thermal barrier paint. There were a few times along way I wished I’d spent the money and risked it though.
The 20” on center might be a issue for batts if you have a lot of SF to cover in terms of yields. Are they definitely 20” OC and not 20” bays?
Good point. I believe the quote mentioned a flame retardant paint. I've also read that some open cell foams have "built in" flame retardant? Do these qualify as an ignition barrier?
In terms of thermal barrier, if the attic is unused, is the 2nd story ceiling drywall sufficient? Actually some of these paints (like DC315) claim to be thermal barriers as well. Is that so?
Right now i'm leaning toward 3" closed cell and then some amount of open cell on top and over the rafters. Probably 4-5". For that reason i can't easily do drywall.
Thanks
With a walk up attic that can be used for things like storage you will need a thermal barrier. DC315 in the correct thickness is an approved thermal barrier on many foams. That being said, there isn’t actually a super accurate way to control the thickness of these coatings.
A number of foams meet the ignition barrier requirement without coating, but I am not aware of any foams that meet the thermal barrier requirement.
Hey all,
Thanks for all the great info so far. I have (hopefully) one last confirmation. I have the attic fan schedule to be removed this week. I also have a spray foam contractor lined up with a really good quote. It works out to $0.66c/sqft/in total including:
3" closed cell spray foam, HFO ($0.90/sqft/in)
4" open cell spray foam (this will cover 2" beyond the 5" rafters) ($0.30/sqft/in)
DC315 paint ($0.75/sqft)
The attic will be sealed
This will get to me to R35 which is a big jump over the current zero. I'll leave whatever is in the attic floor alone. It's not too much though. I can add under-shingle rigid insulation later if need be.
And one more question:
What's the best way to mount electrical wiring and light fixtures up off the spray foam since there will be no exposed wood? Should i nail on some 2x4"s that can poke through the insulation as mounting points/surfaces? They'd be thermal shorts but there wouldn't be too many.
This is a very permanent install so i'm a little nervous! :)
>"What's the best way to mount electrical wiring and light fixtures up off the spray foam since there will be no exposed wood? Should i nail on some 2x4"s that can poke through the insulation as mounting points/surfaces? They'd be thermal shorts but there wouldn't be too many."
Why do the lights have to be mounted on the ceiling? Open cell foam and most intumescent paints are a fairly reflective off-white. Floor mounted outdoor yard type fixtures or indoor up-lighting fixtures directed at the ceiling yields better quality low/no glare nearly shadow free ambient lighting.
That's a good idea. Would greatly simplify things. Thanks
I like Dana's application of lighting design applied within an encapsulated attic! I would want a smoke detector installed up there though. I can't remember what code is. Some sort of framing extension prior to install would take care of this.
Talk to the installers about what their install schedule is like. Ideal would be to get an IR camera to inspect for air leaks after the closed cell foam. There should be a time where it is safe for you to enter, but they are still running their exhaust fan, which will do a good job showing leaks if the outdoor temp has a reasonable difference with the indoor. Any gaps found can be sealed with one part foam at that time if they are in potentially moisture sensitive areas. Otherwise, flag the locations for coverage with open cell. Personally inspect the thickness of the closed cell foam. Check for leaks after the open cell as well.
Are they doing .5lb OCspf? You may ask about pricing and their experience with .75lb OCspf as some installers report they can put it down in a much more controlled/flatter manner and it is not significantly more expensive. DC315 will apply better to flatter foam.
Because you are totally covering your framing, ask if they are going to install depth sticks on the top edge of the framing to confirm foam thickness as they install. Similarly, it may be advantageous to install something like 4" drywall screws in two points on several of the rafters so you can find framing without digging. This would allow hanging things with long screws in the future if needed.
See if it is possible to extend the foam all the way down to the top of the wall or ceiling on the floor below. The floor would need to be cut back if it extends all the way out. This will seal the rim joist or if balloon framed, allow sealing of the top of the wall cavities.
Hi all, well its been a year, but things are starting to come together. Attic fan is deleted and hole patched and i dropped all the electrical down to expose some outlets and lights. Insulation is lined up in the next couple weeks. Final plan is what we came up with last year, 3"/R21 of 2lb HFO CC and 4"/R15 0.5lb OC. The plan is to sort of fill up the soffits with open cell to create a foudnation and then seal with CC and then the OC on top of that. Then DC315 thermal barrier paint. I'm a little unsure of how exactly the roof/wall sealing will happen so i guess i'm trusting them a bit. This outta be much better than before at least. Thanks for the info and i'll post an update when its all done. Thanks!
Sounds like a good plan until I got to this:
"The plan is to sort of fill up the soffits with open cell to create a foundation and then seal with CC and then the OC on top of that."
Can you expand on what this means? I am imagining foam flowing out of the old soffit vents.
Hey all,
Welp, the deed is done. My attic now looks like the inside of a giant igloo. Attached are a few pics.
Last question: i've read the material on the remaining attic floor insulation. Being that mine is pretty sparse, old, and flattened its probably about as good as gone. But now that its sort of bright and comfy up there, i may replace the flooring with decking. When doing so i'll remove the funky old insulation. Is there anything to keep in mind with redoing the floor regards to maintaining air flow between the attic and house below? I obviously want to avoid sealing it.
Thanks a lot!