ROI and comfort gain of deeper walls when using ZIP R6
Zone 5
Retrofitting a porch, 23’x15’, sloped roof, to create a four-season room. Three exterior walls, lots of glass (continuous rows of windows and patio door, with knee wall and wall spaces at the two gables). The house is from the 1970s, with 2×4 walls filled with fiberglass; the roof is R49. Yet, the new four-season room has its own roof, which will have either a nominal R38 closed-cell spray foam or exterior plus interior insulation (for an R value yet to be determined). The windows will be good dual pane, but nothing crazy in terms of U factor (unfortunately).
The new room will be connected to the rest of the house with a 6’ open door. It will have, however, its own heating and air conditioning, with a dedicated one-zone mini split.
The current structure is built with 4”x4” posts and double 2”x4” top and bottom plates. My initial plan was to reframe for 6” walls, using Zip R6 sheathing on the outside. A couple of contractors have said that the added costs for materials and labor would not be worth it, and that I should stay with the 4×4 and 2×4 framing. It’s not that I will be able to use much of the existing, but, for example, the top plate on the longest side could remain; the job would be a bit easier. And every 2×6 is more expensive than every 2×4; etc.
I wonder whether, in my situation, there is a noticeable ROI from the extra expenses of having 6” walls (plus the R6, 1.5” sheathing). AND I wonder if staying with 4” walls (plus the R6) would mean a loss in COMFORT. Under either scenario, my plan is to work on air sealing personally, using excellent tapes and the like. The insulation in the walls and in the floor, also, is a part of the job I would like to take care of myself, so as to detail things very carefully.
Comfort is a major consideration for me. If I have nothing to gain in that respect, I’m tempted to stay with the 2×4 framing. Unfortunately, the township wants to know such details at this time; otherwise, I’d be giving myself more time to think about this, and calculate costs.
Any thoughts? Thanks!
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Replies
My wild guess is the walls are mostly windows with a U value between .26 and .3 so the R value of the window is more or less R3. If the 3 walls are say 70% glass the average for the wall is going to be in the 4s if the rest of the wall is R60 or R13 the average is not changing much. When it is -20° outside it is not going to be a pleasant space.
If you buy fewer, smaller and better windows say 20% of the walls your new room will be very comfortable but you will not call it a “four season room” and better walls have a chance for ROI after 15 years or so.
How sure are you there is a foundation under this new room? all to often they start out as slab on grade patios.
Walta
Thank you Walta. You're giving me awful news, as the windows have already been ordered. You're right on the R value of the glass: ~R3.3. At a quick calculation, the glass will be 45% of the wall surface, maybe less (there is going to be a knee wall, and walls above the windows/door on two of the three sides; that is, I am not going to have floor-to-ceiling glass). The floor is on a solid foundation, and one that allegedly includes a 1" (I think) of rigid insulation; I am going to raise the floor, and that will create a floor cavity of approx 7", which will be filled with insulation.
Does the above change things in your view? Will the room be comfortable year around? (As I mentioned, there is going to be a minisplit for heating and AC, but I'd like to get the best results from the insulation. Shall I go for 6" walls (plus the R6 sheathing), then, to increase comfort, or do you still think that will not make much of a difference?
As further info: the wall portions will have R21 if 4" walls (R6 Zip plus R15 Rockwool), or R29-31 (R6 Zip plus R23 Rockwool or R25 XPS).
You'd probably gain a little with the exterior rigid foam, but probably not enough to make a huge difference in comfort since the windows will probably dominate the overall heat loss of the space as Walta mentioned. Where the exterior rigid foam would help is to cut down on total energy losses, so it would save you some money on heating, it just wouldn't do anything for the cold draft that will likely be coming off the window.
If you have a hydronic system, radiators along the bottom of the windows would help a lot with comfort, but it's not going to be justifiable cost wise to install an entire hydronic system just for this space. Some electric radiant heat panels in the ceiling might help though, but I've never tried that for something like this. Either way, you'd be trading off increased energy consumption ALL the time to gain some comfort for the relatively poorly insulated space, which is something we generally try to avoid with green and energy efficient buildings.
Bill
Thank you, Bill. Definitely, I want to try NOT to rely too much on the HVAC system to compensate for a poorly isolated space. Did you have a chance to look at my response to Walda? The windows will be approximately 45% of the wall space. The wall portions will have R21 if 4" walls (R6 Zip plus R15 Rockwool), or R29-31 (R6 Zip plus R23 Rockwool or R25 XPS). The roof/ceiling hopefully will be R43 or so, with R20 exterior. The floor will be maybe R33.
The draft from the windows worries me a lot! (All but two are picture windows, fortunately; but there is a patio door.) When you talk about such drafts, are you primarily referring to the window as constructed, or to the window installation (which I am hoping to supervise carefully)?
At this point, I am not questioning the use of the ZIP R6 -- that's decided. I am wondering whether by also having 6" framing rather than 4" framing I'd be gaining in comfort.
Matt2021,
There is no discernible advantage to upgrading to a 2"x6" wall. The Zip-R will help reduce thermal bridging, which will be a benefit worth having.
Thank you, Malcolm! When you say "discernible advantage" you are referring to comfort as well?
Could I ask for your opinion on what was brought up above, regarding comfort? I have already ordered the windows; so, in that respect there is no turning back. I wonder whether the pessimism about comfort might be mitigated by the further details I offered (45% or less will be windows/patio door, not the 70% of Walda's "wild guess"; floor and ceiling insulation will be decent; great attention will be dedicated -- by me -- to the window installation, and to the air sealing of the room).
Thanks!
Matt,
Both comfort and energy use. Erik has posted a good overview of the savings or lack of, and from a comfort perspective it's the windows that will govern.
Bill, as further follow-up to your suggestion, above, regarding radiators along the bottom of the windows, I have been thinking that, perhaps, I could investigate the (relatively easy) possibility of extending the house's HVAC system, which is forced air, to the new room, with maybe vents in a couple of places and one return, IN ADDITION TO the dedicated mini-split system the new room will have. I am probably going to check on that with the HVAC company that replaced my furnace and central A/C (and that, I believe, sized it carefully for the house WITHOUT the new room's added volume); yet, I'd love to also hear your thoughts. Is mixing two HVAC zones always a bad idea for example?
Thanks!
My wild guess is the houses furnace is oversized and can easily heat the room. Given the amount of glass it seems unlikely you have enough excess cooling capacity for the room.
I see no reason to add a return given that there is 6-foot opening back to the house.
The problem with the plan is the new room with all the glass will change temp much faster than the rest of the home. My old house had a similar room I found a thermostat that had an optional remote sensor and the thermostat used an averaged of the two sensors to operate the equipment.
Walta
Thanks Walta! Actually, I’m pretty sure that our furnace (and AC) is not oversized. The whole system was installed as part of a state-sponsored program, following a door-blower test, sealing of the attic and upgrade to R-49 for the attic. So, that’s my concern: that adding a 24x15 room, and one that has an 11 ft wall on one side, and — as you know — one with so many windows, might mess up the system for the whole house. As I mentioned, the room will have its dedicated mini-split system. I just wonder whether connecting the room also to the house’s main system, so as to have a couple of vents under some windows, would make sense.
By the way, for whatever is worth it (and to be more precise than what I was able to be above), the windows will have a U factor of .26 (R-3.8) and solar heat gain of .23.
I am trying not to be negative the bright side is the windows are smaller than I had imagined.
I know you paid for the windows and you are not getting a refund but consider not installing all of them. If you did half 22% window is a well-lit room by any standard.
I am sure it will be a great space if the plan is for a green house that fine but is that what you really want?
When I visit homes with 3 and 4 season rooms they don’t strike me as places where people spend much time.
I am frugal by nature and the heating and cooling bills that come with that much glass would make me crazy but that is me and you have to do you.
Walta
Thanks again, Walta! I definitely do not want a room like those three- and four-season rooms you mention. This room will become an integral part of the kitchen; it will be used ALL THE TIME. As the space will be open, it will benefit a bit from the house heating and air conditioning; yet, absolutely, it needs its own HVAC -- hence, the minisplit. I really want to insulate it as well as possible, however, and nor rely excessively on the minisplit. The draft from the windows, which Bill mentions above, now really worries me. And there seems to be a consensus that doing more with the walls and their insulation is not going to help much, if at all. I am a bit at a loss, to be honest. Not installing some of the windows... Maybe some day I could eliminate two of them, and replace them with wall...? It's very difficult for me to envision that -- or any other -- scenario, to be honest.
I'm going to vote for "no discernible difference".
So to summarize, you have:
- roughly (assume 10' walls) 247 ft^2 windows R3.3
- 303 ft^2 non-window wall ~R21
- 345 ft^2 ceiling (~R38)
- 345 ft^2 flooring (~R30)
By my calcs, that is a total R of R11.3 for the R21 wall, total R11.8 for the R30 wall. That is a change of ~R0.5. Someone that is willing to put in more work than me can suggest a real wall temp on the insulated wall section. The wall surface temp on the insulated wall section in the only real variable a human will feel here that I can see.
Can I suggest some storm windows? They could make a difference. Adding R1 to the windows to approximate storm windows:
- R21 walls, R3.3 windows - total R11.3
- R21 walls, R4.3 windows (extra R1 from storm windows) - total R13.4
- That is a change of ~R2.1
Again, surface temps are what you feel for comfort, but at least R values can inform the approximate magnitude of the change.
Erik, Thank you SO much! This definitely helps putting things in perspective: switching to 6" walls is not worth it.
I saw a helpful slide presentation on surface temperatures, and I know that my windows are not at what they should optimally be.
I will definitely look at the possibility of adding storm windows in the winter. I guess that is something that would be best addressed at the time when the walls are framed and windows installed, right?
By the way, I will be adding shades, and I understand that good cellular shades may add an R-1.
I would certainly want to consider storm windows at design time. This is the time to figure out how to make install easier (drip edges, assuming outdoor storm windows, where mounting screws go, integration with trim, etc.). I don't think it really makes a difference, thermally.
Thanks! I will definitely look into that.
Incidentally, I ran across a discussion - on this forum - on the R value gain that shades can offer (especially certain cellular ones, and especially when using tracks, they can add even an R2-3 apparently). Of course, the whole point of having all that glass is to enjoy the view; yet, in the evening and night such shades might be quite effective probably.
Malcom: Thanks for the clarification. It's very helpful.
Erik: In fact, at a more precise calculation of the areas, I have 165 sq ft of windows and 195 sq ft of wall (the roof has a slope, shed-style; so, one of the longest walls is much lower than the other (the other, higher one, of course, does not factor into the calculation, as it is not an exterior wall). Out of the three external walls, 46% of the surface is windows, and 54% wall.
In any event, all of this is extremely helpful and illuminating. While there are things I cannot change, I'll definitely try to work in the right directions, without wasting energy and money into changes that have no detectable impact, such as thicker walls.