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ThisOldCabinNJ
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# Posted: 22 Feb 2023 10:02am
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Hi folks,
I had a detailed message written about this and the site crashed as I click submit so here's the short version of my inquiry:
I have 2x4 stud walls placed in front of the original round log walls of my cabin. All the gaps are sealed. My plan is to insulate the entire cavity with rockwool. In many stud-cavities, you can ALMOST fit a 2x6 batt of insulation despite the studs being 2x4.
My father (who IS an engineer so there is a bit of merit behind his input) said to just use the 3.5" insulation batt and leave an air gap between the stud wall and the log. I am NOT ok with this. All of my field experience/education tells me otherwise. Now being an engineer, the old man does know a bit more about theory than I do so I don't want to completely discredit his input and I somewhat agree that air CAN be an insulator but I feel leaving an air gap between the back of the batt of insulation and the log won't work the way he thinks it will work. If I was able to hermetically seal off the void, sure they air gap would probably work as it should.
So I guess my question(s) is(are) this: What is right? What would you do? I really prefer to fill every nook and cranny with insulation. Not just jam it in, but fill the entire void leaving the insulation as fluffly as I can leave it; I realize there needs to be SOME air which is how insulation works: by trapping the air in the pockets of the insulation.
Does anyone find leaving an air gap is a good idea? The tops of the stud-walls will be 'open' to the "attic" area above it (it will actually be a conditioned loft area - too small for use as a room but perfect for use as storage like an attic, only difference is that is will be conditioned space - I plan to spray foam the roof rafters with 3" of foam) - and lastly if if makes a difference, i am using rockwool batts for insulation.
Thanks!
-al
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Aklogcabin
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# Posted: 22 Feb 2023 10:46am
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Well if that theory held true. Homes built with 2x6 walls with a purpose of better r value would only need 3-1/2" of insulation.
I would fill the bay
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Nate R
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# Posted: 22 Feb 2023 11:44am
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Yeah, given it's open at the top, your air gap isn't sealed from stack effect.....I'd fill it.
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Brettny
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# Posted: 22 Feb 2023 12:14pm
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Quoting: Nate R Yeah, given it's open at the top, your air gap isn't sealed from stack effect.....I'd fill it.
And to me if you didnt fill it it would be like insulating a interior wall.
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ICC
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# Posted: 22 Feb 2023 05:12pm
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When insulation is compressed to less thickness the r-value is reduced. I have no idea if rockwool reacts about the same as fiberglass batts, but when f'glass is compressed 3/4" that can equal a 10% reduction.
I believe that having an opening at the wall framing top is a bad idea. (1) If the place ever has a structural fire that will create a rapid fire spread. This was discovered decades ago when it was found to be a large factor in fire spread in balloon-framed buildings before fire blocking between floor levels was required. (2) There will be air circulation in such a gapped space.
Air is a good insulator but only when it is confined and not circulated. Warm air will rise, and air moves from somewhere to replace it. Even an airtight gapped space inside the wall cavity may have convection currents that result in unwanted heat transfer.
If the upper area is to be a conditioned space and you want to retain air movement between levels I suppose it would be better to use some ductwork to move air and be able to close that off as desired.
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NorthRick
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# Posted: 23 Feb 2023 03:56pm
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Did you buy the rockwool already? If not, go with blown cellulose. That would fill the nooks and crannies.
But, if I understand your description of the construction correctly, the 2x4 studs are in contact with the interior surface of each log. Meaning, if you fill the void with 3.5" insulation, the insulation will also contact the interior surface of each log. If that is the situation, you would not have a continuous air pocket from floor to ceiling. Just a series of isolated horizontal air pockets between each log.
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gcrank1
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# Posted: 23 Feb 2023 04:48pm
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Dont they make a 'runner strip' of batt insulation? Even if just available in f-glass get that to run in the 'grooves' and top up with the rockwool batts?
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Brettny
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# Posted: 23 Feb 2023 06:53pm
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Quoting: NorthRick Did you buy the rockwool already? If not, go with blown cellulose. That would fill the nooks and crannies
That's a great idea. If you can get acess to the top of the wall cavity then you can just blow the stuff in. They make a blown in fiberglass also.
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1tentman
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# Posted: 26 Feb 2023 12:25pm
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In your fist post you said you were going to spray foam the rafters, why not spray foam the walls also. If that does not fit your budget I can understand that because the foam is expensive. It sounds like this is a perfect job to use dense packed cellulose. Northrick suggested that you blow cellulose down the wall from the top opening. I agree with the use of cellulose , but I don't suggest you just blow it down the wall. The air from the blower cant get out of the cavity without coming back to the top and it will keep fluffing the insulation up, when the cavity is full it will look really good but after a year it will settle down about 12 to 15 inches. I would buy the netting and fasten it to the studs, . Get a blower from where you buy your cellulose. These will not be as strong as one that professionals use but it will work just fine, just a little slower. Also I would run the 2x4 up to the roof and fasten them to the rafters. Then run the netting up the wall to the roof and up the rafters to the peak and pack the whole thing with cellulose. You might have to work around some cross ties in the attic area I don't know how your cabin is built. Dense packed cellulose is the next best insulation, behind spray foam. This is how I insulated my cabin 10 years ago and I haven't had any problems yet.I have never worked with rockwool insulation, so i don't know how well it works. I will never use fiberglass again they should outlaw that stuff, it is junk in my opinion. Don't be afraid to try this its not that hard to do and you will love the results. If you need any more info I will be glad to help out. I would not suggest this method if I hadn't done it myself, and I am confident that you will like the results. Good Luck with your project.
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ThisOldCabinNJ
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# Posted: 28 Feb 2023 08:22am
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Hmmm , blown cellulose or any blown in insulation? Not a bad idea. It would certainly fill the uneven voids nicely and quickly. I don't know why i didn't think of that. I could even net behind the studs, "blow" in the loose fill, then install batts into the stud cavities.
Just curious: what's so bad about fiberglass insulation that it should be illegal? Other than you itch for the next 6 months or if it gets wet, it becomes trash...LOL Over the years i've become a big fan of the rockwool. Its just lava rock spun into a fiber similar in process to fiberglass. The batts are dense and cut super easily with a bread knife. I wonder if they make loose fill rockwool? They claim rockwool can be soaking wet and still remain as good as the day it was installed AFTER it dries back out. Fiberglass won't do that.
Spray foam is quite pricey HOWEVER after I considered the "cost" of my labor, cost of having to get scaffold, cost of time to install insulation into the roof and time (i work alone and slowly because i'm super anal-retentive) all equaled more than the metaphorical 10 minutes it would take a professional guy and a tank truck to come spray foam the roof. LOL.
I considered spray foaming behind the stud walls but what stopped me was the fact that I didn't want to destroy the interior log face in the event someone in the future wanted to remove the stud walls and refurbish/expose the logs. I'm just a steward of this log building until the next owner takes over. That said, I still do what I want, I just try to consider how my actions may affect the future. Also, if I have to replace a log, i feel the foam will make a mess for me to try and fix if its all foamed together?
Anyhoo, I'm going to look into said loose fill insulation. Even if I only loose fill behind the stud wall then use batt insulation or something.
As for the top of the wall, I do plan to cap it somehow. I'm 'terrified' of fire spreading through the walls but at the very least, its a one level cabin and the windows are only a few feet off the ground. As long as I wake up before the structure fire gets rolling, I should be able to just jump out a window 3 feet to the ground. But I don't ever want to have to experience that so prepare for the worst and hope for the best!
Thanks all for your input! 
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Tim_Ohio
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# Posted: 28 Feb 2023 09:25am
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Rockwool: The jury is still out as for it possibly being a carcinogen. For that reason, wear all safety gear possible. I installed it because it is less desirable to rodents nesting in it. I also did not use a vapor barrier because of it shedding water. Some say it still needs a barrier. Drywall and paint went over what I installed, so I felt that was good enough for a cabin. My problem with loose fill in a wall is that it will eventually settle and leave a space at the top of the wall. I've seen this to be a cause for condensation at the top of the wall where warm humid air meets a really cold surface. It's frustrating.
Tim_Ohio
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KinAlberta
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# Posted: 1 Mar 2023 06:21pm
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On Fibreglass:
Does Fiberglass Attic Insulation Really Lose R-Value? - Energy Vanguard
https://www.energyvanguard.com/blog/does-fiberglass-attic-insulation-really-lose-r-va lue/
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KinAlberta
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# Posted: 1 Mar 2023 06:35pm
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Any thoughts on one, dew points and two, whether an air gap might help or hinder the ability of the log walls to stay dry.
I’m thinking that an air gap would likely have both heat and trapped moisture circulating vertically and horizontally. Would that be good or bad?
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Tim_Ohio
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# Posted: 2 Mar 2023 09:58am
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Warm, moist air from a heated interior is what causes condensation in insulation, unless, of course there is a leak in the siding that allows wet weather conditions to bring in wet conditions. I'd install the best vapor barrier you can possibly create. Seal around electrical and everything if you intend to use bats or rolled.
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ThisOldCabinNJ
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# Posted: 3 Mar 2023 06:23am
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I planned on installing a vapor barrier. Maybe this isn’t common sense to other folks? I was always taught that moisture developed on the warm wall.
That said, I do have concerns with the logs getting any moisture, etc. has anyone heard of/used a product called BoraCare? It’s a mold and insect barrier you spray on. I was debating coating the logs before insulating as a protection barrier.
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Aklogcabin
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# Posted: 3 Mar 2023 09:05am
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Maybe as easy as just stapling some sill seal fiberglass and filling in the cavity and put the rest of the insulation in.
You do need to control moisture. Most applications of a vapor barrier, the visqueen is on the heated side of the room. This allows the walls to dry to the outside. Lumber absorbs moisture and so does sheetrock during higher humidity levels. The vapor barrier is a barrier that will not allow the moisture to travel through it into the walls. Also logs buildings have a tendency to have air leaks. If you have electrical power a bathroom exhaust fan with a humidistat control will exhaust humidity.
For what it's worth fiberglass insulation can be a bit scratchy but a paper jump suit n hat. A mask n gloves. A few hours later your done. I've done my share so not inexperienced.
Hope some of this helps.
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ThisOldCabinNJ
Member
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# Posted: 3 Mar 2023 11:00am
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Here's maybe/hopefully a better representation of what I plan to do....if anyone is interested in seeing a visual.
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ThisOldCabinNJ
Member
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# Posted: 3 Mar 2023 11:01am - Edited by: ThisOldCabinNJ
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Here's maybe/hopefully a better representation of what I plan to do....if anyone is interested in seeing a visual.
sorry for the poor quality. The diagram was created as a hi-resolution file that the site trashed.
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