Hello: our home is in Maryland (Zone 4). It is about 50 years old. The rooms are located above the garage. We recently saw mold forming in the ceiling of the garage, right under our rooms. On labor day weekend, I opened a section of the ceiling and found water condensation on the AC ducts dripping over the R-11 fiberglass batts. Some of the batts were soaking wet and moldy. So, I proceeded to remove the drywall and the old insulation for the rear of the garage and left the front mostly intact. There are water pipes running perpendicular to the joists and then parallel towards the front to supply the bathrooms. See a
ttached PDF for some pictures.
Since we moved to the house the temperature in the rooms has been unstable–cold in the winter, warm in the summer. I have neglected the insulation problem from the garage for way too long thinking it was a problem with the AC. We have done some improvement including adding insulation to the attic and the walls when we replaced our siding. Now is time tackle the garage ceiling.
For reference, I contacted a reputable insulation company. They recommended to open the whole garage because the vapor retardant of the old insulation is facing the wrong side and claim that blowing insulation in the bays is against code and will be a problem. Other factors they mentioned include dealing with the water pipes by making them part of the house envelope, and leaving an air gap because of the nails protruding from the floor above unless they install foam.
They quoted the options below:
1. Unfaced fiberglass vertically on the perimeter (R-30), unfaced R-38 batts to fill the drop ceiling of the garage rear, R-38 high density batts for front of the garage as well the front and rear cantilevers.
2. Spray closed cell foam (R-28 4″ of Icynene ProSeal LE 4) in the ceiling, and open cell foam (R-31 Icynene LDC50) in the cantilevers.
Option 1 replicates the installation that was there before but uses thicker materials R-30/38 vs R-11. It also adds vertical insulation on three sides of the garage. There is no vapor retardant with this solution.
Option 2 cost 2.4 times more that the fiberglass option for possible lower R-values but adds better air barrier. According to the company this is my best option because of the air barrier gain.
Given these options, I am considering installing mineral wool myself. However, I am hesitant because the difficulty on installing a proper vapor retardant (nails protruding from the floor above). Is one needed? On a side note, I insulated the main duct and accessible parts of the supply ducts using mastic, tape, and Reflectix. We have noticed an improvement in the air supply and temperature in the rooms, despite some hot and humid days.
GBA is great resource, I have learned a lot from the articles and the forums. Any recommendation on how to proceed? Your advice is greatly appreciated.
Thank you,
Jaime
Replies
Hi Jamie,
That's an interesting assembly. Hopefully there are some GBA users who have worked on a situation like yours that will offer some practical advice. In the meantime, here are some thoughts: You have plywood subfloor, which is a vapor retarder and I don't think you need anything more. I'm not sure what the concern about putting insulation against the nails is, perhaps that it will make installation difficult. The insulation needs to be tight to the subfloor. Also, you should have an airtight ceiling to minimize the amount of humid summer air reaching the cool ductwork.
This FHB article may be helpful: How to Insulate a Cold Floor
Thanks for the reference Brian. Does it makes sense to use Rockwool for this type of application? The cantilevers are 24-28" inches long. Is there a recommendation for how to fill up these cavity? Finally, is it really bad to dense pack cellulose over the existing fiberglass? From the FHB article, a vapor retardant exist above (plywood) and below the insulation (foam). Hopefully, other members can chime in.
It's fine to dense-pack cellulose over existing fiberglass. As it compresses the fiberglass, the total R of the fiberglass goes down, but it's R/inch goes up. Unless you need it to aid with air-sealing, it doesn't even need to be dense-packed in a zone 4A climate. In 20 years it might end up settling a half inch
Air dams need to be built where the bays cross the exterior wall, though in an air-leaky garage that is less critical than it would be in a basement.
I'm unaware of any codes that would bar blowing insulation into the joist bays above the garage. As long as the ceiling gypsum meets the required timed fire resistance assembly requirements it should be fine. If a contractor continues to insists that it's banned, get them to cite the chapter & verse.
Rock wool is pretty expensive compared to just blowing it full of cellulose, which is simple enough to do adequately as a DIY project. It looks like there is at least 3.5" of space above the ducts and 3.5" of space below, and the total R for the 7" would be about R25-R26 where the ducts pass, and at 18" it would run north of R60. From a thermal performance point of view it would more than pass on either an R-value or U-factor basis.
>"Since we moved to the house the temperature in the rooms has been unstable–cold in the winter, warm in the summer. I have neglected the insulation problem from the garage for way too long thinking it was a problem with the AC. "
DON"T expect insulating the garage to resolve the comfort issues- a large piece of it almost certainly IS an AC/heating system problem, but probably not what you think. The insulation and air sealing will definitely help, but it's not the magic bullet solution. It's highly likely that your heating & cooling system are simply too LARGE for bonus-room comfort.
Comfort problems in bonus rooms over garages are a classic example of what happens with gross system oversizing. When systems are oversized the rooms at the ends of the system suffer because the duty cycle is too low, and the thermostat gets satisified before the remote rooms are fully up/down to temp. That is even more exaggerated with bonus rooms or rooms with large cantilevers, since the heat loss characteristics differ from that of the fully-indoors rooms. With right sized mechanical systems the comfort problems created by those differences are dramatically muted, because the systems run nearly all the time when there is a significant load. When the duty cycle is low there is enough time in the dead-band between cycles for those rooms to rise or fall relative to the house average. When the duty cycle is 100% (as with right sized modulating systems) that doesn't happen at all.
In the Cleveland Ohio area Nate Adams has turned fixing these sorts of comfort issues into a business, and it's worth reviewing his free chapters from his book, as well as the short videos he keeps on his blog pages. He complains fairly regularly about having to scrap nearly-new systems that are way too oversized to be able to deliver comfort. See:
http://www.natethehousewhisperer.com/home-comfort-101.html
http://www.natethehousewhisperer.com/hvac-101.html
http://www.natethehousewhisperer.com/hvac-102.html
the FHB article Brian linked is very helpful. Here are two Building America guides with much of the same information:
https://basc.pnnl.gov/resource-guides/floor-above-garage
https://basc.pnnl.gov/resource-guides/cantilevered-floor
One thing that stands out in both situations: air sealing is very important. Not only will it improve the efficacy of the insulation by stopping hot or cold air from bypassing it, it will also prevent the contaminated air in the garage from reaching the room above. And as Brian mentioned, will also stop vapor laden air from reaching the ducts and the water pipes, mitigating condensation issues. If you want to stick to just mineral wool, you must have a comprehensive air sealing plan as well.
If you keep that air sealing in mind, Rockwool is perfectly acceptable in this application, as is unfaced fiberglass.
The ducts being so close to the garage make me nervous. If those ceiling joists are 2x6s, then there isn't a problem. But if those are 2x4 joists, then you only have room for about ~R15 of rockwool under the duct, which is not ideal and might not meet code depending on how your building inspector interprets it. Using a larger batt and compressing it might get you past the R19 requirement, but I am not familiar enough with how the numbers work in that situation to say for sure. If you have the room, I recommend adding 2x3 strapping perpendicular to the joists, that will get you to 5", and thus >R19, keeping the ducts nice and cozy. I might just be paranoid though. Edit: Alternately, you can bury the duct, as Dana suggests.
Brian mentioned that the insulation needs to be tight to the subfloor. He is likely thinking of section N1102.2.8(R402.2.8) in the 2018 IRC:
Floor framing-cavity insulation shall be installed to maintain permanent contact with the underside of the subfloor decking.
But there is an exception, and your contractors proposal in option one seems to meet the requirements:
Exception: As an alternative, the floor framing-cavity insulation shall be in contact with the topside of sheathing or continuous insulation installed on the bottom side of floor framing where combined with insulation that meets or exceeds the minimum wood frame wall R-value in Table N1102.1.2 and that extends from the bottom to the top of all perimeter floor framing members.
Since most of the sheetrock is already down why not attach an inch of rigid foam to garage ceiling followed by sheetrock (or maybe no sheetrock and just use foil faced rigid foam which is left exposed)? Then blow in insulation between the foam and the sub floor of the room above it?
Thanks all for the suggestions. I have some additional questions. The joists are 2x10 (actual measure 1.5 x 9.75"), mostly 16" OC with some variance of up to an inch. I took some additional pictures of the joist bays there is an approx. 1" gap between the gypsum and the joist. This means that there is about 3.25" above and 1" below the supply ducts.
We are planning to stay in this house for the long term. We have ruled out sprayed foam due to the degassing and cost. Cellulose seems like the best bang for the buck. Rookwool is appealing because of the fire, sound, and easy install. I am leaning towards Cellulose for the bays and Rockwool for the open area. I am also considering adding rigid foam blocks, especially for the cantilevers and the ceiling.
While we had some issues with comfort, the situation was not extreme. We have lived here for 12 years. We recently upgraded the AC. In the process the furnace was downsized from 120K to 80K BTU and the the outside unit is 3Ton 18 SEER. We noticed improvement in comfort, but probably should have gone with a smaller unit to have a constant duty cycle.
With this in mind, is it worth to take down the remaining drywall? It seems that cellulose will help with the air-sealing. As Danna said: "Air dams need to be built where the bays cross the exterior wall, though in an air-leaky garage that is less critical than it would be in a basement."
I am just wondering about diminishing returns of taking all the steps below:
1. Take down the remaining drywall.
2. Add 2x3 strapping as suggested by Aedi.
3. Install 1 or 1.5" rigid foam in the top and bottom of the cantilever and air-seal seams--this is hard to do because they are 2' deep, even harder where the duct terminates to supply rooms.
4. Fill the cantilever cavity with cellulose.
5. Close the cantilever with rigid foam and air-seal.
6. Install netting or 1-1.5" rigid foam board under the joists to create fillable bays while being careful to create a plenum where there are water pipes. (The contractor warned me about burying water pipes in insulation, also was against installing foam pipe wraps. They claim it increases the risk of pipes bursting. Is this right?)
7. Blown-in cellulose in all the bays.
8. Close-up with Gypsum.
Again, thanks for your wisdom.
Good morning, GBA. I would appreciate any recommendations based on my last posting. I am wondering what is a good 80% solution, the 100% solutions are hard to execute and maybe an overkill. Especially since this is a retrofit. I am using the FHB article as reference. Also, I appreciate any input on how to best proceed to insulate or not the water pipes. Thank you.