Would like to add a thermal break (tape?) when furring 2×4 out to 2×6’s
Kivi
| Posted in Energy, Heating & Insulation on
Coordinating some work on a 55yr old house with 2×4 stud exterior walls with an exterior panel sheathing on the outside. The inside of the house is gutted and would like to furr out the 2×4 studs with 2×2’s to create 2×6 bays for new batt insulation and vapour barrier (this is a cold climate). I know there are other better options for accomplishing this but this is the method the homeowner and general contractor would like to use.
I was wondering if there was some type of adhesive insulating tape (or strip) that one could apply to the edges of the existing 2×4’s to create a bit of a thermal break… before the 2×2’s are nailed to them?
Kari
Replies
Anything that came as "tape"
Anything that came as "tape" would be too thin to be effective.
B/4 you and they make a decision.
Look up Mooney Wall here and consider an alternative that might give you so much more.
Thermal break
There are two things you should consider Aergel and using Owenings Corning Fanfold ridgid foam which comes in 1/4 and 3/8 thickness. I used the Fanfold. Aergel is more expensive.
Correction
It is Aerogel
Mark
Would you care to qualify how this qtr inch of foam is going to benefit the project?
thanks.
Second on the Mooney wall
This is an ideal application of the Mooney wall system.
I don't usually recommend it, because I think that adding foam to the outside makes more sense in most situations, because it insulates the entire wall including the header and sill plates. And, in most insulation upgrades, it doesn't make sense to gut the interior.
But, since you have already gutted the interior; the Mooney wall system makes perfect sense.
The Mooney wall consists of installing horizontal 2X2s at either 16 or 24-inches on center. And then insulating the deeper cavity, and installing drywall over the horizontal 2X2s.
I'd probably do a flash coat of foam before I installed the horizontal members, (this gives you a good air barrier). Then install the horizontal members, and do whatever modifications need to be done for the windows, and electrical. Then install either dense pack fiberglass, or cellulose blow in insulation, to fill the void before installing drywall.
This link is pretty explanatory: http://www.builditsolar.com/Projects/Conservation/MooneyWall/MooneyWall.htm
The most you're going to get out of any insulation is about R-10 per inch. If your tape is 1/4" thick (which is fairly thick for tape) that would be, AT MOST, about R-2.5 -- not much of an improvement over the R-6.5 or so you get from the wood alone. The Mooney wall scheme (horizontal 2x2s) is much better because the contact point between pieces is tiny.
Keep in mind that the wood framing only fills 9% of the cavity, so the effect of the tape will be on the order of 9% of 15% -- 1.35%.
Thermal Bridging is a bit of a myth
Maybe myth is not the right word. There is no such thing as thermal bridging in the sense that it is something magic. People think there is something special going on here. The facts are simple. Wood is a poor insulator, about 1 R per inch. So if you have a 6 inche wall all wall studs give you an R of 5 or so, vs the FG or whatever which gives you R 19 or 21. So you lose more heat at the stud than at points in the wall with greater R value. There is no thermal bridging just a lower R value. If someone developed a stud with an R value of 50, then people would say you have "thermal bridging" where ever you have insulation and not a stud. Kind of silly.
Therefore, you are not trying to "stop" thermal bridging, but trying to increase the insulation to the area of low R value, namely the stud. Therefore, if you add tape between the stud and the furring you are not "stoping" thermal bridging, you are just trying to add insulation. SO your tape would add only whatever the R value of the tape is.
DoRight, I see the point you are making, and you are probably correct that the term "thermal bridge" causes confusion. In my mind I always simply thought of it as the area in an assembly where heat transfer most easily takes place...hence the term " bridge", but no doubt one could eliminate the whole term.
The general contractor thought the whole idea of the mooney wall was silly..so they strapped directly atop the 2x4's. Clearly the tape idea did not seem valuable so I did not bother with that.
No question the homeowner will have a better wall assembly than he had originally so I will file away the idea of a Mooney wall for another time.
Again, thanks for the insights
Kari
You are welcome, and ....
You stated: "I always simply thought of it as the area in an assembly where heat transfer most easily takes place...hence the term " bridge", ..."
That is what I said, but with a lot more words. LOL.
But, that is the point. Wood framing offers a lower R value and thus transfers more heat at that point vs other better insulated areas. This is way adding tape between your framing plan would only improve effiency by the R value of the tape, which can't be much.
As for the contractors saying a Mooney wall is silly ... well that is silly of them to say if they are laughing at the added R value. Big if they laugh at the added expense and work .... well, I see the point. Personally, if I were doing new construction I would opt for 2x6 walls get R21 between studs and live with the "bridging / studs", just seems like the Mooney wall is a lot of work (the netting the glue the blower the framing ....). That said there is no doubt the Mooney can improve the effective R value of the wall, but.
So the contractors have a point, simply fur it out and go from an R 13 to R19 or 21. Done.
done? not necessarily
If there's no break from the cold or heat on the outside, then there should be on the inside-or
it (the cold/heat conducting wood) should be minimized-Mooney Wall.
Take a thermal imaging camera and have a look.
Add all those cold lines up and see the volume.
Might as well leave the entry doors open and rely on the storm doors. Heck, open the garage doors too and install visqueen.
no?
And what sized opening is a 5.5" fiberglass-insulated shell equivalent to, even if there are no studs? Nothing's perfect.
Breaking Bridges
I understand what DoRight is saying.
People use the term thermal bridge which does aptly describe the situation. And then they talk about breaking the thermal bridge as a remedy to the problem. However, breaking the thermal bridge has come to be widely perceived as eliminating it. This is where the confusion enters in. I believe it comes from the “bridge” analogy. With an actual transportation bridge, if you make just a short gap in it, you have effectively “broken” the bridge by making it impassible. Hence, it defeats the entire bridge.
But making a gap in a thermal bridge only defeats the part that was in the gap. The rest of the thermal bridge still functions. The only way to eliminate the thermal bridge is to eliminate the entire stud or other intervening wood structure that comprises it. But the terminology of “breaking the bridge” leads to the erroneous conclusion that just a thin break eliminates the bridge. And that leads to conclusions such as just a layer of tape eliminating thermal bridging by “breaking the bridge.”
Another way to look at it.
Thanks for your educational post. Another way to look at it is, if you were to take your stud and saw it in half length wise and nait the two pieces in the wall with a 1/4 inch or 1 inch space between teh two parts everyone would say "see no thermal bridging". Problem solved. But the reality is you still have a total of 5 1/2 inches of wood (2x6) with an R value of 5ish, and you have added a dead air space (or not so dead air space) between the tow pieces and air has an R value of ... what .. well some but not much. So much for breaking the "thermal bridge".
So people are not really addressing a thermal bridge as they are addressing a low insulation area of the wall.
People don't miss understand any of this. I am all in favor of increasing the R value area of a wall or ceiling in the area of structural material. However, I think the very nature of this thread illustrates confusion on this issue as the original poster was suggesting "breaking this bridge" with a tape. The result would only be the addition of whatever R value the tape would add. 1/4 inch of foam would only add say R1.5. That woudl still be defined as a bridge when compared to the cavity next door.
The only way to eliminate the bridging is to eliminate the wall (and the heat/cold on the other side). It's just that some parts of the wall are better insulators than others. As a % of heat loss the "body" of the wall is as significant as the studs, and the windows swamp both.
I don't believe you understand thermal bridging.
There is no bridging. It is just an area in the wall with low insulation, period. If you take a R5 stud and add 1 inch of foam with an R5 you end up with an R 10. Compare this to the R 21 in the cavity which after you add the R5 foam is now a total of R26. You still have a low insulation point in the wall, call it a thermal bridge if you must. You have not BROKEN any bridge you have just increased its insulation value. Plain and simple. It is not as if the stud is evil and sucks heat out of the house over a convenient bridge it is just a low R value point in the wall. So sure add foam add improve the R value, but the the bridge if you will is till there just a lesser amoutn of heat is lost.
Perhaps all sumantics (spelling), but people take about thermal bridging like it is mysterious or magic.
Air the base of all modern insulation.
All modern insulation is based on air bubbles, even the most expensive Aerogel.
Like Aerogel all the most useful insulation is based on millions of tiny air bubbles, with the likes of polystyrene merely tiny air bubbles each enclosed in a thin plastic shell.
An air gap at 19mm provides an excellant insulation, because the gap is not so wide that air circulation can start, that will transfer the heat from the warm side to the cold side.
The rate of heat transfer is directly proportional to both the temperature gradient (d theta/dx) and the area of contact (A). The constant of proportionality is the Thermal Conductivity of the material (k). Straight forward thermal science, you get that in second year mechanical engineering coursework--- and there you have it. If the studs, plus say overlaying 2x2 crosswise furring (google: Moody wall), having only have a 1-1/2" square contact points evaluted at say, r-7 and all of the rest is say battens at r-19: that is considerably less than studs having a much greater contact area at say, r-7, where you might have 2x6 studs running top to bottom as normal.. because the total contact area is FAR FAR less. Heat takes the path of least resistance, BUT the area of contact is all important here. The thickness matters too, but limiting the area of contact we can greatly reduce the overall conduction through the lower R-value studs (as opposed to the high R-value insulation). We're not getting a lot of convection or radiation in an insulation- filled stud bay cavity wall situation, conduction is the primary worry.
But no....you are not "breaking" thermal bridging, however WHAT YOU ARE DOING IS "BOTTLE NECKING" THE LEAST RESISTANCE PATH OF GREATER THERMAL CONDUCTIVITY OF THE WOOD STUDS (by limiting the area of contact)!!. It's straight forward, no?
Ps
Best be googling "Mooney Wall", as that guy moody had nothing to do with the idea.