Garage wall insulation

[Geran]

Hi,

I was wondering if I could get some advise on the best way to insulate my garage walls. The walls are cinder block with a brick exterior. The garage will be cooled in the summer and heated in the winter as it will be my workshop and server room.

Outside wall

Back wall

Wall connected to house

I live in Washington, DC metro area (zone 4 not marine).

Any help would be greatly appreciated!

 

[Dana]

Ideally that question would have been asked BEFORE the framing went up.

Assuming those are 2x4s (?), a IRC code minimum for fully conditioned residential space in zone 4A is 2x4/R13 + R5 continuous insulation. With the intense summertime humidity and high moisture reservoir nature of brick cladding it's really good to have a vapor retarder between the outdoors and the air conditioned indoors to keep mold from growing on the cavity side of the wallboard from the high moisture content air.

If there is something like an inch of space between the stud maybe there's room to knock out a few studs and slip foil-faced rigid polyiso foam board (R6) there, fastening it tight to the CMU wall with a few cap screws. With polyiso it's good to stop an inch from the slab to avoid wicking up any moisture from the concrete, even if the concrete normally looks dry. Tape the seams of the polyiso with foil HVAC tape, seal the top & bottom edge of the foam board to the CMU with can foam or polyurethane caulk. This is a lot easier do do before the framing goes up.

If the gap is deeper than that, use thicker foam. 1.5" thick foil or plastic faced Type-I EPS would also run about R6, and is pretty cheap (even at box stores) and doesn't have the same moisture wicking issues of polyiso. Make it as snug a fit as possible, to allow reasonable fit for the cavity insulation.


Then, install batt insulation in the stud bays snugged up tight to the foam. Kraft faced is fine if that's any easier or cheaper, or unfaced. If fire resilience is a priority, unfaced rock wool batts would be best (but definitely not the cheapest.)

Foil facers are true vapor barriers, and will not allow moisture to pass through the assembly via vapor diffusion, so there should NOT be any true vapor barriers on the interior side or you'll have created a moisture trap. The foil facers will block the intense bursts of moisture loads that occur when the sun hits rain or dew wetted brick from reaching the cavities, and there is huge dew point margin against wintertime moisture accumulation at the foam/fiber insulation boundary- moisture won't accumulate to problematic levels over the winter when using only kraft facers or standard interior latex on wallboard as the interior side vapor retarder.

Most brick cladding on CMU walls have a 3/4"-1" cavity as a capillary break between the brick & CMU to keep moisture from wicking toward the interior. If your's doesn't have that gap installing fiber insulation in direct contact with the CMU without some sort capillary break is a disaster. But fiber insulation still needs an exterior side air barrier to perform to specification, so it might as well be R5 (or higher) foam board.

 

[Sponsor]

 

[Geran]

Ideally that question would have been asked BEFORE the framing went up.

That is my fault as I didn't realize that before putting up the wall. My wife would kill me if I decided to remove all the walls now so unfortunately I'm stuck with what I have.

Assuming those are 2x4s (?), a IRC code minimum for fully conditioned residential space in zone 4A is 2x4/R13 + R5 continuous insulation. With the intense summertime humidity and high moisture reservoir nature of brick cladding it's really good to have a vapor retarder between the outdoors and the air conditioned indoors to keep mold from growing on the cavity side of the wallboard from the high moisture content air.

Yes they are 2x4 walls at 16" on center. Good to know that I'm in zone 4A as I was trying to find out what that meant as I kept seeing it around.

If there is something like an inch of space between the stud maybe there's room to knock out a few studs and slip foil-faced rigid polyiso foam board (R6) there, fastening it tight to the CMU wall with a few cap screws. With polyiso it's good to stop an inch from the slab to avoid wicking up any moisture from the concrete, even if the concrete normally looks dry. Tape the seams of the polyiso with foil HVAC tape, seal the top & bottom edge of the foam board to the CMU with can foam or polyurethane caulk. This is a lot easier do do before the framing goes up.

I will check when I get home tonight if I have an inch between the stud and wall, if so I have opening where I can slide the foamboard behind the studs so that wouldn't require removing any studs. For the Polyiso, would the foil side face in the exterior or interior? And 1.25/1.5 inch cap screws would work with hollow cinder block?

If the gap is deeper than that, use thicker foam. 1.5" thick foil or plastic faced Type-I EPS would also run about R6, and is pretty cheap (even at box stores) and doesn't have the same moisture wicking issues of polyiso. Make it as snug a fit as possible, to allow reasonable fit for the cavity insulation.

What if the gap is less than an inch? Since .5" Polyiso is only R2.7, this wouldn't be enough, correct? If not, I'm assuming the next best thing would be ccSPF...if so would Demilec Heatlok HFO Pro or Heatlok HFO High Lift be better at 1/1.5" and the rest filled with either rock wool batts or wet sprayed cellulose?

Then, install batt insulation in the stud bays snugged up tight to the foam. Kraft faced is fine if that's any easier or cheaper, or unfaced. If fire resilience is a priority, unfaced rock wool batts would be best (but definitely not the cheapest.)

Fire resilience is a priority so I was planning on using rock wool batts.

Foil facers are true vapor barriers, and will not allow moisture to pass through the assembly via vapor diffusion, so there should NOT be any true vapor barriers on the interior side or you'll have created a moisture trap. The foil facers will block the intense bursts of moisture loads that occur when the sun hits rain or dew wetted brick from reaching the cavities, and there is huge dew point margin against wintertime moisture accumulation at the foam/fiber insulation boundary- moisture won't accumulate to problematic levels over the winter when using only kraft facers or standard interior latex on wallboard as the interior side vapor retarder.

If I can use foil faced sheets, which way should the foil face?

Most brick cladding on CMU walls have a 3/4"-1" cavity as a capillary break between the brick & CMU to keep moisture from wicking toward the interior. If your's doesn't have that gap installing fiber insulation in direct contact with the CMU without some sort capillary break is a disaster. But fiber insulation still needs an exterior side air barrier to perform to specification, so it might as well be R5 (or higher) foam board.

I cannot guarantee that I have the 3/4"-1" cavity between the brick and CMU so I would rather be safe than sorry and install whatever I need to on the inside.

 

[Dana]

Most foil faced polyiso has foil on both sides. If it's one side only, the foil goes against the block. If the gap is less than an inch, 3/4" will do. As long as the cap screws penetrate the masonry by a half inch it will work, but 3/4" is better. If you're going to cheat that, put some foam board construction adhesive (which uses solvents that don't damage the foam, unlike standard construction adhesive) in the holes prior to screwing it in.

The quicker/easier but more expensive/less-green way to go about it is to use closed cell polyurethane spray foam applied directly to the CMU, enough so that it touches the exterior side stud edges. Most closed cell spray polyurethane used in the use is blown with HFC245fa, and extremely powerful greenhouse gas (~1000x CO2), and costs about a buck a square foot per inch of depth, and runs R6-R6.5 @ 1". (This includes the DIY foam kits.) There are some closed cell foam vendors using HFO1234ze as the blowing agent, (includingn Demilec Heatlok HFO Pro & Heatlok HFO High Lift) which runs about $1.40 per square foot @ 1", and performs at about ~R7/inch. Either version runs less than 2 perms @ 1", most are around 1 perm @ 1", which is sufficiently vapor tight to not be an issue.

 

[Geran]

Most foil faced polyiso has foil on both sides. If it's one side only, the foil goes against the block. If the gap is less than an inch, 3/4" will do. As long as the cap screws penetrate the masonry by a half inch it will work, but 3/4" is better. If you're going to cheat that, put some foam board construction adhesive (which uses solvents that don't damage the foam, unlike standard construction adhesive) in the holes prior to screwing it in.

So I measured the gap and in some places it is just over an inch and others it is about 1/2". Looks like foil faced polyiso is out of the question.

The quicker/easier but more expensive/less-green way to go about it is to use closed cell polyurethane spray foam applied directly to the CMU, enough so that it touches the exterior side stud edges. Most closed cell spray polyurethane used in the use is blown with HFC245fa, and extremely powerful greenhouse gas (~1000x CO2), and costs about a buck a square foot per inch of depth, and runs R6-R6.5 @ 1". (This includes the DIY foam kits.) There are some closed cell foam vendors using HFO1234ze as the blowing agent, (includingn Demilec Heatlok HFO Pro & Heatlok HFO High Lift) which runs about $1.40 per square foot @ 1", and performs at about ~R7/inch. Either version runs less than 2 perms @ 1", most are around 1 perm @ 1", which is sufficiently vapor tight to not be an issue.

I'll get a company (or three) out to see how much it is for ccSPF before I start tearing out studs.

 

[Dana]

In my area if the job is under 1000 board-feet (1 " x 1 square foot) it can be somewhat higher than my budgetary $1.40 per square foot WAG, but it should be under $1.50 for an open easy-access wall, compared to the contortions and prep it takes to do basement band joists, etc. Good luck!

If price drives you toward rebuilding the studwall you might consider finding some reclaimed foam board from commercial building demolition & reroofing, which is dramatically cheaper than virgin stock goods. Using this search on your most-local craigslist is one way to find foam reclaimers near you. With as little as 2.5" of roofing polyiso (R14+) it would meet code-minimum without the rock wool as a "mass wall", due to the thermal mass benefits of the CMU block & concrete. Flipping the studs sideways and using them as furring through-screwed to the CMU (rather than cap screws) minimizes the amount of floor area that gets eaten up. Even 2" of foam board is enough to get there if you use split R15 rock wool between the furring. Some amount of pre-planning of the electrical wiring when going the furring route- you may want to route some channels for wires and pockets for electrical boxes ahead of time.

 

[Geran]

In my area if the job is under 1000 board-feet (1 " x 1 square foot) it can be somewhat higher than my budgetary $1.40 per square foot WAG, but it should be under $1.50 for an open easy-access wall, compared to the contortions and prep it takes to do basement band joists, etc. Good luck!

I'm trying to get three companies out here this week. When I got a quote back in 2010 for insulating my basement with 1" of closed-cell spray foam, it was $1390.00 for the walls, bands and ribbons. That comes out to about $1.16 per square foot, 9 years ago.

If price drives you toward rebuilding the studwall you might consider finding some reclaimed foam board from commercial building demolition & reroofing, which is dramatically cheaper than virgin stock goods. Using this search on your most-local craigslist is one way to find foam reclaimers near you. With as little as 2.5" of roofing polyiso (R14+) it would meet code-minimum without the rock wool as a "mass wall", due to the thermal mass benefits of the CMU block & concrete. Flipping the studs sideways and using them as furring through-screwed to the CMU (rather than cap screws) minimizes the amount of floor area that gets eaten up. Even 2" of foam board is enough to get there if you use split R15 rock wool between the furring. Some amount of pre-planning of the electrical wiring when going the furring route- you may want to route some channels for wires and pockets for electrical boxes ahead of time.

I mentioned this to the wife and she was not thrilled about it but said if it saves us money compared to spray foam than we'll do it. I found a couple reclaimers with polyiso boards...the one you linked and someone selling individuals boards for a couple more per board.

Couple of other questions:

1. If I do remove the walls and put up foam board, do they make shallow electrical boxes so I don't have to cut into the foam? Such as these (https://www.homedepot.com/p/Carlon-...and-Outlet-Box-Case-of-25-B108B-UPC/202197957)
2. When you say through-screwed, I assume you are referring to these (https://www.confast.com/product-1-4-x-5-flat-phillips-confast-concrete-screw) and having them sit flush or just below the surface of the wood?

 

[Geran]

So the wife has said no to spray foam due to price and removing the studs.

What would be my best option with it in it's current state? Could I just put up rockwool insulation without a vapor retarder and call it a day?

The whole garage won't be a conditioned space just one 7x10 room that will be my server room (need to keep the room between 75-80 degrees).

 

[Dana]

So the wife has said no to spray foam due to price and removing the studs.

What would be my best option with it in it's current state? Could I just put up rockwool insulation without a vapor retarder and call it a day?

The whole garage won't be a conditioned space just one 7x10 room that will be my server room (need to keep the room between 75-80 degrees).

The air gap between the studs and CMU is an issue. Batt insulation needs an air barrier on all 6 sides to fully perform to spec, though at the higher density you get with rock wool that performance hit is negligible IF (and only if) the batts are installed perfectly, with no voids where air can convect around the sides/ top / bottom of the batts. Without a firm exterior side to push/tuck the batt up against it's essentially impossible to get a sufficiently perfect fit at the edges and corners of the batts.

It might be possible to slip some 1/4" fan-fold XPS (with plastic facers) 0r foil faced fan-fold EPS siding underlayment in there without pulling any studs, which would provide a vapor retarder (or barrier, if foil faced) between the high moisture drive of the CMU and the moisture susceptible framing, then install batts thick enough to be a compression fit fully filling the space between the fan-fold and wallboard when the wallboard goes up. If you go that route be sure to tape all the seams of the fan-fold. Cheap "contractor roll" R19s would deliver about R15 when compressed to 4.0", but R20s would do a bit better. (When compressed to 3.5" in a 2x4 stud bay R19s perform at R13- it's essentially a "fluffed up" R13 batt, and not very air retardent.) If you're a perfectionist you'd take some contractor-roll R11s to stuff in the gap between the framing and the fan-fold too, though in the thinner gaps it might be able to fill in with strips of fan-fold foam.

Be sure to only use UN-perforated fan-fold foam- the perforated versions are for applications where it needs to be more vapor open, whereas your application needs to be fairly vapor-tight. Most fan-fold XPS has thin plastic facers to bring it down to under 2 perms, which is good enough here. Unperforated foil faced EPS is under 0.1 perms- a true vapor barrier, which would also be fine, but not absolutely necessary. While I generally don't like to use XPS (the least-green foam insulation in common use due to the HFC blowing agents used), the total quantity is low and it's efficacy as both a vapor retarder and capillary break makes it worth considering here, given the "no removing studs" constraint.

 

[Geran]

I'm assuming I should only do this in the space I want to condition, correct?

 

[Dana]

I'm assuming I should only do this in the space I want to condition, correct?

You should do it anywhere & everywhere there is studwall next to the CMU wall, not just the fully conditioned portions, otherwise the high moisture drive of the masonry will likely create a mold problem in the studwall cavities.

Whether in addition to the vapor-retardent fan-fold you also insulate the studwall in areas outside the fully conditioned space with batts is up to you, but if not, the partition wall to the less-conditioned space needs to be fully insulated. It's generally best to treat the entire exterior wall studwall the same, clearly defining the thermal & pressure boundary of the building as the exterior wall, even if/when there are other thermally isolated spaces contained inside the building that are separately conditioned.

 

[Geran]

Understood. I can install the foam board in most places and areas where I can't, I might try and do this:

This should work for places where I can't slide the foam in and would require me to cut it to fit it behind the studs.

 

[Dana]

Understood. I can install the foam board in most places and areas where I can't, I might try and do this:

This should work for places where I can't slide the foam in and would require me to cut it to fit it behind the studs.

If doing it with full sheets of cut'n'cobbled foam that don't quite fit behind the studs, go with half-inch foil faced polyiso, which is a lot greener, more vapor retardent, and flexible enough to slip fully behind the studs in some of it. With polyiso try to keep the cut bottom edge from resting on the slab- give it a half inch or so of air between the slab and foam behind the bottom plate of the studwall (make a few spacers), otherwise any liquid water seepage that makes it to the slab can potentially wick up into the polyiso, undercutting it's thermal performance.

At a half inch thickness XPS (pink, blue, green, sometimes gray) is a bit more vapor-open than ideal for this application, sometimes more vapor open than latex paint on wallboard. The foam should be more vapor tight than the paint to keep the moisture drive from the masonry from affecting the finish wall.

For cutting foam board cleanly, a 4"-5" steel wallboard taping knife sharpened on the edges works better than any utility knife, even for thin foam board, as shown in this video.

 

[Geran]

First thing I want to do is say thank you for all your help on my post and it has really helped me make my decision. Thank you for the video on cutting foam board, that looks so much easier than using a handsaw.

After much deliberation and talking with the wife (showing her the cost of everything), I have decided to remove all the studs that are currently up and put up 2.5" recycled polyiso that you mentioned.

I still have a couple questions though...

1. Should I remove the top and bottom plates that are already there? If the polyiso is 2.5" and a 2x4 is 1.5", I believe I wouldn't be able to install drywall correctly since the studs would overhang the plates.
2. If removing the plates, instead of putting spray foam in that inch gap. Could I lay down strips of 1" foamboard for the poly to sit on and have the new bottom plate sit on the foamboard to serve a sill-sealer?
3. Should I use foamboard construction adhesive to put the poly on the walls or could I use low expansion canned spray foam on the back of the foamboard and put it on the wall?

I might have more questions after these depending on the answers.

 

[Dana]

1: If you want to keep the plates as-is, go with 2" roofing polyiso (if they have it), and split some R15s into pairs of 1.75" thick batts to fit between your 1.5-2" deep stud bays. When compressed to 1.5" the rock wool is still going to perform at better than R6.5, so you'd be looking at something like R17+ at center cavity even with 2lb roofing foam, R19 if foil faced. The 1.5" of wood is good for about R2, so the framing fraction would be good for at least R12. With the R value of the masonry, wallboard, and air films it would come in under the code-max U0.060 (=R16.7 "whole-wall"). Even if it's a hair shy of meeting code on a center-cavity R-value basis it would still make it. Come to think of it, a "mass wall" would only need a continuous R13 (if the insulation is on the interior side of the masonry) to meet code, so it's really going to meet or beat code min performance with comfortable (but not huge) margins.

2: If removing the plates, install 1" EPS under both the wall polyiso and bottom plate (which would require whacking an inch off each stud), as a thermal and capillary break to protect the wood from wicking up any ground moisture or leakage. Leave a half-inch gap between the EPS and the masonry wall as a channel for any wall seepage.

3: A few blobs of foam board construction adhesive works. If you use a stud to press the foam toward the masonry wall only as far as necessary to align the stud to the top & bottom plates it will leave a thin drainage cavity between the masonry & foam, which would be ideal.

 

[Geran]

Understood for question 2 & 3.

For question #1: if I get it from here (https://washingtondc.craigslist.org/doc/mad/d/mechanicsville-factory-seconds-of/6931089232.html) at 2" foil faced, in theory I wouldn't need the rockwool insulation since it would be continuous R-13 if I read the code and your statement correctly.

Comfortability isn't my main concern. My main concern is that my server room with a bunch of computers in it will not over heat during the summer or become damp during the winter. Maybe I can add the extra rockwool just in that room and the rest of the garage can be just the 2" foam board.

 

[Dana]

Understood for question 2 & 3.

For question #1: if I get it from here (https://washingtondc.craigslist.org/doc/mad/d/mechanicsville-factory-seconds-of/6931089232.html) at 2" foil faced, in theory I wouldn't need the rockwool insulation since it would be continuous R-13 if I read the code and your statement correctly.

Comfortability isn't my main concern. My main concern is that my server room with a bunch of computers in it will not over heat during the summer or become damp during the winter. Maybe I can add the extra rockwool just in that room and the rest of the garage can be just the 2" foam board.

That's corrrect, for a mass wall in zone 4A the R13 on it's own would meet code minimum. With the air films in the cavity and any amount of air gap on the masonry side it'll even beat that, if it's shiny aluminum on both sides, due to the low emmissivity /high reflectivity of the aluminum.

See the MASS WALL R-VALUE column in TABLE 1102.1.2. The "8/13" indicates a continuous R8 would cut it if more than half the R was on the exterior, but R13 is enough if more than half is on the interior. In your case all the insulation is on the interior, so it takes at least R13. If thermally bridged by stud framing extending through the foam layer you can't use that- it has to be a continuous layer (which it will be.)