New siding for an old house, and a couple other questions
I asked a couple questions a while back regarding installing rigid over existing clapboards with a WRB and an inch or so of rigid insulation – I couldn’t seem to respond to those answers but I have a couple more to ask If I use semi-permeable polyiso board over the existing wood siding so it can dry to in both directions, can a fluid applied WRB be applied to the outside of the rigid board. Alternately, If I put either housewrap or fluid applied under the rigid, should the joints be taped? Reading on Dupont’s website they recommend not taping the joints.
And a couple other unrelated questions: our house is a 1 1/2 story, balloon framed, gable front. The eaves on the side are about 24″ above the second floor level. Above the second floor is (mostly) open cell spray foam, below is blown in cellulose. Does this cause any problems with moisture in the walls since air moves through some areas more that others?
Lastly, We have original 1920 windows with storm windows which frost and condensate very badly on this inside of the storms in the winter because the old wood windows are so leaky. Can old windows be effectively weatherstripped and sealed to avoid (or at least minimize) this or do I just need to go with new windows?
Thanks.
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Replies
Michael,
Q. "If I use semi-permeable polyiso board over the existing wood siding so it can dry to in both directions, can a fluid applied WRB be applied to the outside of the rigid board?"
A. First of all, we need to know your climate zone or geographical location to answer this question.
Second, it's usually a bad idea to assume that a wall with exterior rigid foam will be able to dry to the exterior. It's safer to design the wall so that the rigid foam layer is thick enough to keep the wall sheathing above the dew point during the winter. Here is a link to an article that explains this design principle: Calculating the Minimum Thickness of Rigid Foam Sheathing.
Third, fluid-applied WRBs are vapor-permeable, so the use of a fluid-applied WRB won't affect drying rates very much. Most brands of fluid-applied WRBs can be applied over rigid foam, but if you have any doubts on this matter, you should contact the manufacturer of the fluid-applied WRB.
Q. "Alternately, If I put either housewrap or fluid-applied under the rigid, should the joints be taped?"
A. The manufacturers of most brands of housewrap require the housewrap seams to be taped. If you are talking about the rigid foam layer, it's always a good idea to tape the seams of rigid foam, because the tape improves airtightness.
Q. "Above the second floor is (mostly) open cell spray foam, below is blown in cellulose. Does this cause any problems with moisture in the walls since air moves through some areas more that others?"
A. If you have walls with air leaks, you may have moisture problems in your walls. You need to pay attention to airtightness in all of your walls, regardless of what type of insulation you use. If you suspect that your walls are leaky, a blower door test might be useful.
Q. "We have original 1920 windows with storm windows which frost and condensate very badly on this inside of the storms in the winter because the old wood windows are so leaky. Can old windows be effectively weatherstripped and sealed to avoid (or at least minimize) this or do I just need to go with new windows?"
A. It's possible to add weatherstripping to old windows. For more information on this issue, see What Should I Do With My Old Windows?.
Climate Zone 4 - mixed/humid (forgot to mention that) which is why I want it to dry both directions. In climate zone 4 any thickness of rigid will work to keep the walls dry.
The walls are leaky because it's an old house. Air comes in the outlet covers, window weight pulleys, etc. This will of course be (hopefully) solved if I add a WRB and rigid insulation to the exterior.
there haven't been any issues as it's been this way for years - I was just hoping we didn't cause any problems by spray foaming only the upper couple feet of the side walls while leaving the cellulose on the first level. We converted our attic to and unvented/conditioned attic so the side walls and eaves were also spray foamed.
One more follow up. Everything I've read is about WRB and rigid insulation being applied over OSB/plywood/etc. as in new home construction. Is there any problem with leaving the existing wood clapboards so I can maintain the cellulose and use them as the sheathing - applying the WRB or rigid directly to it (depending on where I put the WRB). And with using the siding as the sheathing and leaving the existing window framing intact (just building them out to accommodate the depth of the new rigid and siding) - does that dictate where the WRB needs to go? Above or behind the rigid. Thanks again.
The most important thing to get right is to attend to the bulk water handling details of the windows. Uninsulated wall cavities have many times the drying capacity of cavities stuffed with cellulose or open cell foam. If you get that right, flashing the windows such that they direct bulk water to the outside of your WRB, with sufficient drain-space between the WRB and the layer immedtiately to the exterior you should be able to breathe easy.
I know of an instance in a zone 6 location where they retrofitted a balloon framed antique air sealed it the best they could, put 1/4" fan-fold XPS foam over the exterior of 100 year old clapboards + plank sheathing with vinyl siding over that. They dense packed it with fiberglass and didn't even bother with interior vapor retarders (which is technically a code violation for zone 6). As part of the retrofit they installed replacement windows, flashed properly to the exterior of the WRB. Shortly after the initial retrofit they did a thermal scan with an IR camera (once it was cold enough), fixed a few spots, and used dense-pack in feed-backs to fix a couple of thermal bypasses, but that's it. And that is at sheathing foam WAY below the R7.5 prescriptive in the IRC for zone 6! They put 1/4" drywall over the existing plaster & lath, which probably had layers of vapor retardent, even leaded, oil paint which may have made a difference but it's been 14 winters so far, no hint of any moisture problems creeping in.
But installing proper flashing when they did the replacement windows was a critical detail, and would be for you too.
Michael,
Adding spray foam insulation in some portions of your walls, while leaving cellulose insulation in other areas, will not cause problems.
If you want to install rigid foam on the exterior side of existing clapboard siding, you need to come up with a way to minimize convective loops or air leakage paths between the rigid foam and the siding. If I were you, I would use canned spray foam as an adhesive at the perimeter of each piece of rigid foam, to try to limit these types of air movement.
If you are adding rigid foam to the exterior of your siding, and you are keeping your existing windows, you need to come up with a water-management plan to handle the window sill extensions. This can be tricky. Each house is different, and the risk depends on exposure to wind-driven rain and the depth of your roof overhangs. It's usually impossible to retrofit sill-pan flashing in these cases, so you end up with less-than-optimal flashing details at the windows.
Regardless of how you proceed, think these details through carefully. You're going to need something like peel-and-stick flashing covered with Azek at each window sill, and you need a way to direct the water that dribbles down to jamb extensions onto the sill extension -- and out.
I guess I'm confused about using the spray foam around each panel to keep air from circulating between the clapboards and the rigid insulation. If the WRB is installed to the outside of the rigid and all joint in the rigid taped, wouldn't t his eliminate any problems?
Dana mentions using 1/4" fan-fold XPS in a cold climate. Would I be safer in climate zone 4 to just add the the WRB to the clapboards and forget the rigid insulation? Just at furring over the WRB and install the fiber cement boards (at least I'm hoping to use fiber cement boards. This would reduce problems with building out the windows. I won't gain any R value or reduce any thermal bridging but I might sleep better knowing the walls can breathe.
Michael,
Q. "If the WRB is installed to the outside of the rigid [foam] and all joints in the rigid [are] taped, wouldn't this eliminate any problems?"
A. Not necessarily. Even if the rigid foam seams are taped, (a) these taped seams may not stay airtight if the foam expands and contracts over time, and (b) there may still be communication between the outdoor air and the trapped air that is between the rigid foam and the siding, unless you have a good plan to address the perimeter of the walls (corners, tops, and bottoms).
Q. "Dana mentions using 1/4-inch fan-fold XPS in a cold climate. Would I be safer in climate zone 4 to just add the WRB to the clapboards and forget the rigid insulation?"
A. I'm not sure what you mean by "safer." The rigid foam will add R-value to the wall assembly, and will definitely lower the risk for moisture accumulation (assuming that you include good flashing details). Research shows that walls with exterior rigid foam have sheathing that stays dryer than walls without rigid foam.
Q. "Just at furring over the WRB and install the fiber cement boards (at least I'm hoping to use fiber cement boards) -- this would reduce problems with building out the windows."
A. That's true.
Q. "I might sleep better knowing the walls can breathe."
A. Walls don't need to breathe. If you prefer dry sheathing to damp sheathing, then rigid foam moves you in the right direction -- again, assuming that your flashing details are good.
You'll be better off using rigid insulation, but using way more than 1/4" fan-fold. Current IRC code min for zone 4 would be 2x4/R13 + R5 continous insulation. It only takes R2.5 on the exterior of 2x4 framing filled with fluff or o.c. foam to have minimally adequate dew point control at the sheathing, and doubling that gives you signficant margin, provided the bulk water handling details are up to snuff.
An inch of foil-faced polyiso would get you to code min, and it's comparatively easy to air-seal foil facers with high quality aluminum tapes (eg Nashua 324a , which is available even at the box stores.)
If an inch is too much, 1/2" is enough for dew point control on milled 2x4s, but take it to 3/4" for full dimension 2x4s.
I was thinking a full 1" anyway which gets me R5 or better (possibly even 1 1/2"). And after doing some research on Building Science Corporation's website it seems to make sense to use unfaced polyiso in a mixed-humid climate. If I use a fluid applied WRB outside the rigid foam, would the WRB provide enough protection to eliminate any air issues (since fluid provides a better air barrier).
I've done quite a bit of research on how/where to apply rigid foam to an old house with WRB's, etc., and this is the first I've heard of possible issues with air trapped between the foam and clapboards. But maybe the other websites are assuming everything is installed perfectly. I could take the old clapboards of but then the expense goes way up because I have re-insulate the entire house.
And I just thought of another possible issue. Would the existing clapboard siding with layers of paint - maybe lead, maybe oil based - be too impermeable (as opposed to say plywood) and cause issues with that layer in the middle of the assembly?
A lot of questions again but I really want to do this right the first time.
The facer does not matter AT ALL if you have an inch or more of foam, and 1-1.5lb density 1" or 1.5" foil-faced polyiso is much easier to find than unfaced polyiso in those thicknesses. To be sufficiently rugged for handling unfaced polyiso starts at ~2lbs density and up.
Concerns about trapped air are unfounded.
The vapor permeance of the leaded & oil exterior paint is indeed quite low, but doesn't matter if you have sufficient exterior R.
Dana,
You write that "concerns about trapped air are unfounded." That's true, of course, if it really is "trapped air."
I was explaining to Michael that he really does need to install the rigid foam in such a way that the air behind it is "trapped" -- and that there are no pathways that connect this air to the exterior.
I was responding to:
"...this is the first I've heard of possible issues with air trapped between the foam and clapboards."
There are no issues of concern with trapped air, but it's important to block the potential thermal bypass air current paths created by the surface profile of a clapboarded wall. Sealing the gaps between rigid panels & clapboards with can-foam at least every 4-8' is a good idea to block potential air currents during high wind.
Even clapboards with 10 coats of oil paint are rarely very air tight, though it could become pretty-tight with a spray on WRB. Most antique clapboards have some splits, and some separation at the overlaps. But those paths are pretty small in area compared to the lateral channels you would get with rigid board overlaying clapboards.
Dana,
Thanks for the clarification. We're on the same page.
So a spray on WRB either applied to the clapboards or over the rigid won't be enough to stop air flow? Still need to seal all joints with can spray foam? If I do that, can the rigid insulation act as the air barrier? And, you've both mentioned using foil faced polyiso - after researching information from Building Science Corporation in a mixed-humid climate they recommend a permeable or semi-permeable rigid insulation to allow drying to the inside and the outside. Are you saying that doesn't matter if the rigid is thick enough? Or with the possible lead/oil paint on the clapboards I don't have bi-directional drying anyway?
Michael,
Q. "So a spray-on WRB either applied to the clapboards or over the rigid won't be enough to stop air flow?"
A. It will be enough to stop air flow through the clapboards. But you are planning to install rigid foam on the exterior side of the clapboards, so you still need to worry about air flow through the channels between the bumpy WRB and the rigid foam -- because air flow through these channels undermines the thermal performance of your rigid foam layer.
Q. "Still need to seal all joints with caned spray foam?"
A. I recommend that you seal the rigid foam seams with canned spray foam, caulk, or high quality tape.
Q. "If I do that, can the rigid insulation act as the air barrier?"
A. Yes, although some builders worry that rigid foam may shrink, reducing its effectiveness as an air barrier.
Q. "You've both mentioned using foil-faced polyiso - after researching information from Building Science Corporation in a mixed-humid climate they recommend a permeable or semi-permeable rigid insulation to allow drying to the inside and the outside. Are you saying that doesn't matter if the rigid is thick enough?"
A. I disagree with the Building Science Corporation on this point. It doesn't matter if the rigid foam is vapor-permeable or vapor-impermeable. As my article on this topic (Calculating the Minimum Thickness of Rigid Foam Sheathing) makes clear, in Climate Zone 4 (not marine), you can choose to install rigid foam of any thickness, without worrying about any minimum thickness rules.
Q. "Or with the possible lead/oil paint on the clapboards I don't have bi-directional drying anyway?"
A. The paint layers won't be a significant factor. The main water entry mechanism will be wind-driven rain, so pay attention to your flashing details, and you'll be fine.
A couple more questions. I've been rethinking leaving the existing siding. It will be expensive and a mess but I might remove the old siding, spray foam the stud cavities from the outside, install sheathing, maybe 1/2" or so of rigid, air barrier then the siding. In the end it will be a better install than trying to work over the existing clapboards. I also plan to rebuild the 1920 windows and install new storm windows. I already have an unvented/conditioned attic and a conditioned crawl space. If I go this route, or even if I leave the siding and install an air barrier, at what point do I need an air exchanger?
Second question: Martin, in a previous exchange you said that an air space behind the fiber cement board is required. Vinyl is the only siding option that doesn't require an air space. The reps at Hardie Plank tell me that with up to 1" of rigid and air space is not needed. So why is the air space needed?
Michael,
If you plan to install a layer of exterior rigid foam, I think that installing spray foam between the studs is a waste of spray foam. It's expensive, it's often environmentally damaging (due to the blowing agents used for most types of closed-cell spray foam), and it doesn't result in much of an R-value improvement over dense-packed cellulose.
If you decide to insulate your stud bays with dense-packed cellulose, just make sure that your exterior rigid foam layer is thick enough to keep the sheathing above the dew point in winter. More information here: Calculating the Minimum Thickness of Rigid Foam Sheathing.
Q. "If I go this route [insulating and air sealing the walls], or even if I leave the siding and install an air barrier, at what point do I need an air exchanger?"
A. If you are sealing air leaks in your house -- and you should be -- then your house needs a mechanical ventilation system. It doesn't have to be an HRV or an ERV, though. For more information, see Designing a Good Ventilation System.
Q. "The reps at Hardie Plank tell me that with up to 1 inch of rigid foam, an air space is not needed. So why is the air space needed?"
A. With most types of siding other than vinyl siding, I strongly recommend that builders include a ventilated rainscreen gap between the siding and the WRB. That said, such a gap is not required by code, and in many cases it is not required by siding manufacturers. You can make your own decision; before deciding, however, you may want to read this article: All About Rainscreens.
I'm in climate zone 4 so I don't need any rigid outside the sheathing. So do think I would be better off with spray foam between the studs and skip the rigid - or cellulose + rigid? If I use foam it will be medium density open-cell water-blown spray foam.
An IRC 2015 code minimum wall in climate zone 4 is 2x6/R20, or 2x4/R13 + R5 continuous insulation, either of which comes in with a whole-wall-R of about R15 (R15.5 -ish if including air films)
With balloon framing and full-dimension studs, 1" plank sheathing you could get to code-minimum performance with cellulose or open cell foam cavity fill by using 1" ZIP-R (R3) sheathing rather than half-inch OSB. If it's milled 2x4s (3.5" nominal cavity depth) you can do it with 1.5" (R6) ZIP-R. Or, you could add a full inch of foil-faced polyiso over the plank sheathing (no ZIP) detailing the foam as the air barrier.
I hadn't thought about using the ZIP system but I will definitely look into it. It doesn't seem to be used frequently in my area. Wall studs are full 4", the spray foam I would use is R5.46 per inch so getting the R value is not a problem. Just thought of this. If I remove the old clapboards this will be some extra prying and banging on the house. Do you think this would cause any problems with the original plaster on the interior? New cracks, etc.
What is the R Value requirement for unvented attic roofs. Had ours spray foamed a couple years ago and while it easily meets the climate zone 4 R15 requirement for roof condensation, I'm not sure it meets energy code.
Marin, I agree a rainscreen is a better approach, but I'm trying to keep my walls from getting too thick so the siding doesn't stick out several inches past the existing cmu foundation. What are your thoughts on the Benjamin Obdyke Slicker rainscreen? You can get it as thin as 1/4". If I do remove the old siding (already dreading mess), re-insulate the stud cavities with either foam or cellulose, a the 1" ZIP sheathing and then 1/4" Slicker under under the siding - have I covered all the bases? And the siding wouldn't hang out over the foundation too much.
Michael,
I love to see siding overhanging the foundation. That's much better than the other way around (a foundation which is proud of the plane of the siding). But it's your house, so you can decide what looks best.
Benjamin Obdyke Slicker is a good product -- much better than no rainscreen.
Q. "If I do remove the old siding (already dreading mess), re-insulate the stud cavities with either foam or cellulose, a the 1" ZIP sheathing and then 1/4" Slicker under under the siding - have I covered all the bases?"
A. I'm not sure what "covering all the bases" means. You should remember to tape the seams of the Zip sheathing with Zip System tape. And if you switched from Zip sheathing to Zip-R sheathing, you would reduce the thermal bridging through the studs -- so that would be better than what you are suggesting.
I want the siding to overhang the foundation but not stick out past it several inches (on a horizontal plane) so the foundation looks too small for the house. I had planned on the Zip-R sheathing, that's why I mentioned it being 1" thick. By covering all the bases I just meant R-value covered, rain screen/air space covered, etc.