No wall insulation…now what?
Greetings,
We are currently planning a major interior renovation to our home and recently found out that the house is balloon framed with no exterior wall insulation. With that new knowledge, we now have a few questions about how to best insulate the cavity space. I have read numerous articles addressing the need for insulation and the various types of insulation to use in the space, but I am concerned with moisture and other issues that come with the different installation methods and types of insulation available such as spray foam (open & closed), fiberglass batts (faced & unfaced) and dense-pack cellulous etc.
The major problem I am now realizing is the concern about the balloon framed wall cavity being able to dry from both sides in the summer and winter after insulation is installed. The few contractors I asked and the various articles I read on how to achieve that makes my head spin.
The house is a 2-story wood balloon framed structure with a full unfinished uninsulated basement and a low sloped flat roof. The house was constructed 1907 and is located in New York, region 5. The interior surfaces of the exterior walls are lath and plaster on 2×4 studs with no insulation in the wall cavity. The exterior wall surface is diagonal wood sheathing (1×8?) not sure if a membrane layer is between the sheathing and the wood clapboard, which is now covered by ¾” rigid blue Styrofoam board and vinyl siding.
We are not planning on renovating anything on the outside of the house other than enclosing the porch to get additional living space. However, we are planning to gut the kitchen and bathroom on each floor to replace with new fixtures and cabinets etc. We also plan to replace most, if not all of the plumbing, electrical wire, outlets and switches throughout the house. The existing boilers and radiators will also be replaced with a new forced air HVAC system. The flat roof will be stripped and have new rigid insulation installed on the deck covered with an EPDM roofing membrane.
All windows were replaced five years ago keeping the original molding. However, we now plan to replace all window molding. Doors with new jambs, casing and baseboard molding will also be replaced. We plan to move out of the house while all of the renovation work takes place.
Originally, we were only going to remove the lath and plaster walls in the kitchen and bath rooms while keeping the remaining plaster walls in place and “fish” new electrical wires as needed. If necessary, we were going to laminate the plaster walls with drywall if they were too damaged from installing new wires and outlets.
My original intention to keep most the exterior plaster walls started to change once I found out that the exterior walls had no insulation or fire blocking. To date, we have never had any evidence of any moisture or draft issues, only high gas bills. However, I am now willing to remove all the exterior plaster walls, if a workable interior insulation design can be suggested that will not make me constantly worry about potential moisture and other issues that come with installing the different types of insulation available. We plan to live in the house for at least 30 years.
Any suggestions on how to correctly insulate the exterior walls from the inside living space would be very much appreciated, since we plan to start the work before winter. I do not need to have the most efficient design, but I do want to address the lack of insulation while we are renovating the interior of the house. Any advice would be welcomed. Thank you!
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Replies
William,
The most common way to insulate the walls of a house like the one you describe is by installing dense-packed cellulose in the stud cavities. This is routine work for any weatherization contractor. Lots of insulation contractors in the Northeast are familiar with this work.
The work can be performed from the interior or the exterior. It is not necessary to remove interior plaster to perform the work, although it will be necessary to drill holes in your wall. (The holes will later be patched.)
Any time you insulate walls that were formerly uninsulated, there is a chance that exterior paint won't last as long as it used to. That's a little sad, but it's a good trade-off. (The main reason that paint doesn't last quite as long is that you are no longer blasting your exterior sheathing with hot air all winter long. By reducing heat loss through your wall, you will ensure that your exterior sheathing and siding stay a little more damp than they used to.)
[P.S. When I first read William's question, I missed his reference to vinyl siding; clearly, vinyl siding doesn't need to be painted, so my comments on painting should be ignored.]
Martin might have missed the fact that you have 3/4" XPS in the exterior. That's less than the minimum 1" for zone 5. https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing
But it's close, so you might be OK, depending on 1) where you are in zone 5, 2) what your interior humidity ends up being in the winter, and 3) how good your air sealing is. One option that helps in marginal cases is to use a smart vapor retarder on the inside, such as MemBrain or Intello Plus. That can help you achieve good air sealing and reduce the water vapor diffusion getting into the wall, such that having 3/4" foam instead of 1" foam is not longer a problem.
Another option is "flash and batt" which Martin just wrote about:
https://www.greenbuildingadvisor.com/articles/dept/musings/flash-and-batt-insulation
Charlie,
You're right -- I did miss that. For some reason, I also assumed clapboards, not vinyl siding. I need to slow down.
There are a lot of ways to skin a cat. It's pretty easy to remove and re-install vinyl siding -- so another approach would be to temporarily remove the vinyl siding, add as much rigid foam as you want, and then reinstall the vinyl siding.
At only 3/4" XPS is more vapor-permeable than the plank sheathing unless it has a facer applied to take it's vapor permenance down. Typical specs for most 1.5lb density 1" XPS is around 1.5 perms max, which would make 3/4" XPS about 2 max, and probably between 1.5 and 2 typ.
That is not a moisture trap- the 1x planking has a vapor permeance of about 1 perm, which means the moisture can get through the XPS and into the air space behind the vinyl siding faster than the wood sheathing can deliver it.
If insulated with dense-packed cellulose the hygric buffering capacity of the cellulose is also protective of the wood sheathing.
Most 108 year old houses also have multiple layers of alkyd (sometimes leaded) paint on the interior plaster, which is also going to be a class-II vapor retarder, limiting the rate of moisture transfer into the assembly from wintertime moisture drives.
Bottom line, from a total drying capacity point of view the 3/4" XPS is not your problem, but the vapor retardency of the 5 layers of oil paint could be an issue if you make the exterior side TOO vapor tight. Simply piling the extra exterior foam may not be the right approach, since that makes drying toward the exterior that much slower. On sections where you're gutting the interior wall you can add a smart vapor retarder, use only standard interior latex paint and stack on the exterior foam to your heart's delight. But on the rest it may take a bit more analysis before adding more foam. Still, that's the least of your concerns.
The biggest thing to focus on is verifying presence and quality of the window flashing. Bits of wind driven bulk water getting into empty balloon framed stud bays dries quickly and causes no problems, but once you put fiber insulation in the cavities you have both moisture wicking toward the interior, and orders of magnitude slower drying.rates. You may have to pull the exterior trim to inspect & remediate the window flashing to guarantee that the bulk water is managed correctly before doing any of the insulation work.
Martin,
Thank you for the dense-packed cellulose recommendation, that is on our short list of insulation methods. No worries about the paint comment, we installed the vinyl siding to stop having to continue to paint the clapboard. Removing the existing vinyl siding then insulating with rigid foam over the clapboard and then re-install is a method I never thought of. That would eliminate the need to remove the existing plaster walls, the demo savings could be used towards the cost of the siding removal and rigid foam board. Hopefully, damage to the siding during removal and re-installation would be limited.
I only wish I knew we didn’t have exterior wall insulation when the vinyl siding was being installed, so we could have installed the insulation on the outside, which appears to be the correct method. The only downside I see in that method would be that I would not be able to address the lack of fire blocking in the balloon framing. Hopefully, others will learn from previous mistakes and the knowledge provided in the GBA forum, which I just recently found.
Charlie,
I only recently heard about "flash and batt" which is an interesting mix. If it is done mainly to reduce the cost of using thicker foam, would using Roxul in lieu fiberglass or cellulose be a benefit or just add to the cost?
Dana,
For what it is worth the 3/4” XPS did not have a facer and was not always installed with tight joints, according to installation pictures I took. I guess it was used as a level surface for installing the vinyl over the clapboard.
You are probably correct with your 5 layer paint comment, could even be more. I also noticed some plaster walls have what appears to be wallpaper that was painted over. Other walls have been laminated with drywall, the rest remain painted plaster.
I understand your concern with the potential lack of window flashing, something I did not think of. I’m sure flashing would not have been installed if they didn’t install insulation in the balloon framing cavities. The contractor that installed our replacement windows didn’t inspect for flashing, nor did they install any new flashing. Now that I recall, they didn’t insulate the window cavities where the pulleys, sash cords and weights were located, probably another mistake I inherited.
During our renovation, we have to remove several windows, so that will give me the opportunity to verify if any window flashing exists. I suppose the downside of waiting to check would be not knowing the actual window flashing condition now, which would delay the corrective action and insulation selection. Thank you for the observant comments and my upcoming weekend project of window inspections.
Project update ... after going back and forth, we decided to remove all of the lath and plaster on the exterior walls and ceilings. When exposed, we found the exterior wall cavities to be in good condition, so we plan to insulate with closed cell spray foam on the exterior walls, but would consider using Roxul if the foam price is over our budget. Would we be making a mistake not using the foam on the exterior walls and rim joists?
When we removed the plaster on the 2nd floor ceiling, we found that the ceiling joists were actually 2x4s (we replaced with 2x6s) that had separate roof joists above. The cockloft space between the ceiling joists and roof joists ranged from about 4 ft to 6" above the ceiling joists, since we have a low sloped flat roof. With the newly found space, we were able to locate the 2nd floor HVAC ducts above the ceiling. We also decided to replace the old asphalt roof with a new EPDM roof with 3.5" of rigid insulation above the roof sheathing. The balance of insulation will be installed underneath the roof deck. The plans call for using closed cell spray foam to make up the difference in order to get the R-38 value. Is foam the best option, if the space will not be vented?
In the unfinished basement the plans call for fiberglass batts to be installed between the 1st floor joists, is that the best option? The HVAC contractor plans to install two small supply ducts in the basement space that never previously had ducts or was conditioned, would that make a difference on the selection of insulation to be used between the floor joists for the 1st floor?
I am not certain that having an unvented space under the roof or using closed cell foam instead of open cell foam under the roof deck is the best method. Overall would our proposed insulation plan work or could it use some modifications?
All the best,
Bill
Q. "We decided to replace the old asphalt roof with a new EPDM roof with 3.5 inches of rigid insulation above the roof sheathing. The balance of insulation will be installed underneath the roof deck. The plans call for using closed-cell spray foam to make up the difference in order to get the R-38 value. Is foam the best option, if the space will not be vented?"
A. In your climate zone (Zone 5), you really need R-20 rigid foam above the roof sheathing. You can achieve that goal (sort of) if you use 3.5 inches of polyiso, but not if you use 3.5 inches of XPS (R-17.5) or EPS (R-14).
For more information on this issue, see Insulating Low-Slope Residential Roofs.
Ideally, you would install R-20 of rigid foam above the roof sheathing, and R-29 of cellulose, mineral wool, or fiberglass insulation beneath (and in direct contact with) the roof sheathing. If you insist on installing spray foam under the roof sheathing, make sure that it is open-cell spray foam, not closed-cell spray foam, so that the roof sheathing can dry inward if it ever gets wet.
Bill,
Q. "In the unfinished basement the plans call for fiberglass batts to be installed between the 1st floor joists. Is that the best option?"
A. No. It's always better to include the basement within the thermal envelope if you can. That means that you should insulate the basement walls, not the basement ceiling. For more information, see How to Insulate a Basement Wall.
Teminology: Roof joists? You are probably referring to rafters?
Don't put more than 2" of closed cell foam on the underside of the roof deck or you'll be heading into moisture-trap territory, and it would have to be installed in multiple passes with a cooling period in between.
With 3.5" of rigid polyiso above the roof deck you'd be at ~R20 (derated for climate & stackup) and 2" of closed cell foam under the roof deck for R13 you'd be at about R33. If you have a couple of inches of space left you can fill it with split or compressed batts or damp sprayed cellulose between the rafters and meet or beat your R38 goals without need of interior side vapor retarders, and without risk of moldy fiber insulation. If there's enough depth to get there with open cell foam or all damp-sprayed cellulose (and no closed cell) under the roof deck, that would be greener, and an even better drying path. In zone 5 with R20 above the roof deck you can safely install up to R30 of vapor open or air-permeable insulation below the roof deck without interior vapor retarders (still need an interior air barrier though, which COULD lean toward open cell foam rather than cellulose or batts if it isn't to be covered with gypsum board.)
On the walls a flash-inch of closed cell would provide more than enough air sealing, and would put a ~1 perm vapor retarder between the interior space and the sheathing. Fill the rest with blown/sprayed fiber insulation. If you installed 4" of closed cell it would have to be in two passes, and you'd be at ~0.25 perms, which is practically no drying capacity. The additional performance of the impressively higher center-cavity R with closed cell foam really adds only about R1 to the "whole-wall R" after factoring in the thermal bridging.
Use the closed cell foam budget for the basement, not the roof or walls, and insulate the foundation walls, not between the ceiling joists. It takes 2.5" to actually hit code min for new construction foundation insulation, but from a bang/buck perspective the limit is really 2". In most places you can meet fire code with intumescent paint rather than building out a fire rated thermal barrier assembly on the interior. It's damned near impossible to really air seal the basement ceiling, but spray foam on the walls does GREAT! And if the furnace & ducts are in the basement, the distribution losses accrue to the house rather than being lost to the outdoors. Another benefit of insulating & air sealing at the foundation walls is that the basement will be drier, with much lower mold risk or summertime dehumidification requirements.