Old house attic polyiso rafter insulation
Hello,
I have been working on an attic renovation for about a year and am finally seeing the light at the end of the tunnel. However, I have some concern over some condensation I have been seeing in the last couple of weeks as the temperatures have risen to around 80 degrees F during the day.
I live in northern Kansas on the border between zones 4 and 5. I have a balloon framed house built in 1900 with a third floor walk up attic. The attic was plastered at one time, but had been removed by the time we bought the house 11 years ago. There was absolutely no insulation above the 2nd floor. The house was renovated in 1995 and it could easily been done at that time. We installed a furnace and central air system for the second floor with the duct work running behind where the knee walls used to be. We wrapped the duct work in fiberglass insulation since the HVAC company used uninsulated duct work. There are no soffits and no vents at the bottom of the 2×4 rafters that pass through the top plate. There is virtually no ridge on the house and most of the rafters terminate as they meet going up to the peak. The attic ceiling height is over 9 feet in the main area.
After researching insulation and reading several articles on this site I decided to go with 3 inches of polyiso insulation panels spray foamed between the rafters and an additional 2 inches below the rafters. I priced spray foaming the whole attic, but felt I could do a better job for the same money and end up with close to R30 instead of R13.
The panels between the rafters are made up of 2 inch fiberglass faced polyiso glued to a foil faced 1 inch sheet. I was able to buy the 2 inch at a large discount since it was left over from a new construction job. This leaves approximately 1/2 inch of air space between the insulation and the batten board and roof sheeting. I added an additional 2 inches under the rafters using the same fiberglass faced iso in one room and foil faced iso in the second room. I sealed the foil faced seams with foil tape and used silicone on the other. The goal of making a solid vapor barrier between the various layers of insulation.
A couple of weeks ago I noticed condensation and dripping around the seems on the foil faced polyiso. The worst was on an interior corner (valley) that I had not sealed up yet. We had remodeled a bathroom last year and I never installed the ventilation fan so I got that done to see if it helped. If I have 3 inches between the rafters and 2 inches below, should I ever have condensation between the layers. I am sure that at times there is 1/2 gap between the polyiso layers due to the rafters being slightly uneven. Most of the time it would be less the 1/8 inch. I am having the problem only in the foil faced room. I have not noticed any condensation in the other area. Yesterday we had temperatures around 80 with humidity. The attic windows were open so I know there was more humidity than normal. Some seams were actually dripping with condensate. Do I just need to continue to control moisture from the house and finish sealing the foil layer?
Side note. Our basement is limestone foundation with half crawl space and half dirt floor basement. We are planning to vapor barrier the whole basement and concrete the basement half. I am sure this is contributing to the house moisture as well.
Please help with any suggestions. Thank you.
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Replies
Code for a vent channel is 1” minimum. A 1/2” vent gap between the insulation and roof sheathing does not meet code. Most will tell you that 1” really isn’t enough of a gap for good ventilation, and I personally think of 1.5” as a minimum. That said, do have soffit vents and a ridge vent so that the vent channel you have can actually ventilate and allow OUTDOOR air to circulate from the eaves and up out the ridge? If you left a gap, but sealed the perimeter, then you don’t really have a ventilation channel.
“Cut and cobble” installations like you’ve done aren’t generally recommended due to the labor involved and the near impossibility to get a lasting air seal. You really can’t get as good a job as spray foam doing things this way. If you have a functional ventilation channel of sufficient size and with proper soffit and ridge vents, and a sealed inner polyiso layer over the rafters, you should be ok. The important part is that the vent channel be able to ventilate properly.
My guess is that your “vent channel” isn’t actually acting as such, and moisture made it ways into that area and froze. The “condensation” you’re now seeing is probably ice thawing within your assembly and running out any unsealed seams. You’re probably going to need to open up some of the insulation to see what is going on. I expect you’re going to have to work to improve the ventilation.
Bill
Thanks for the information Bill. I do have a couple of question on your answer.. The house roof has never had any ventalation, but also was never insulated. There are no soffits and the only air that can get under the sheeting would need to come from any gaps around the rafter ends where they pass through. The rafter bays mostly terminate before they reach the ridge due to they shape of the roof. Think of a pyramid with only a few of the bays ending at the top. How would a person ventilate those bays that don't go to the ridge? The condensation is only noticeable between the layers of polyiso. If I left a gap at the top and bottom of the inner layer, allowing air/humidity to enter and exit, I assume that would help my condensation problem. Would it negate very much of my insulating properties? Or could I install vapor barrier sheeting on the inside to prevent moisture from getting in to begin with? I attached a picture of the interior corner where I first noticed the condensation. I cut this corner instead of finishing it like the others since the stairs comes up right below it and I was going to finish it to maintain headspace. Again I have only noticed the moisture on the foil lined area.
Again, thank you for taking the time to help me and for anybody else that comments.
Lance
Don’t think you can seal out moisture. Moisture is really good at finding ways to get into things.
Before your house had insulation, air could freely circulate everywhere so you had lots of drying potential. Now that you have insulation, you’ve created cold condensing surfaces inside of assemblies and trapped air in some places, which can cause problems.
Regarding venting, you can use “regular” roof vents to vent channels that don’t go all the way to the ridge. Ideally you want to vent EVERY rafter cavity. If you put ventilation channels between layers of insulation, you essentially cancel out the insulating value of whichever insulation is short circuited by the vent channel. If your put a vent between two equal layers, you cut your total insulation in half as if you only had one of the two layers.
It’s possible that you’re only seeing moisture on the coil facing because the fiberglass mat facing vapor permeable, so the moisture could potentially be soaking into the polyiso itself, or possibly managing to dry through it (I wouldn’t count on that though).
If this were my house, I’d either try to get the vent channels functional or I’d go for closed cell spray foam.
Bill
What type of siding/cladding does this house have?
Are there any air barriers on the top plates of the balloon framing? If the wall cavities are uninsulated and open the half-inch air gap moisture in the wall will congregate in the tiny roof deck cavity with no way to exhaust out.
Whether sprayed or cut'n'cobble, high R/inch foam installed between framing is a waste. Despite the higher center-cavity R, the thermal bridging of the framing severely robs the performance. Do the math:
https://www.finehomebuilding.com/membership/pdf/184243/021269086NRGnerd.pdf
From a moisture point of view assembly would have been better off with zero foam insulation in the cavities and just the 2-3" of continuous air tight rigid insulation on the underside of the rafters, but with no good way to vent the valley a flash inch or two of closed cell foam filling the rest with fiber would be the safest bet, but make sure the roof deck has dried to <<15% moisture content before installing the foam.
Northern KS spans US climate zone 4A and zone 5A- which is your climate zone"
https://beta.greenbuildingadvisor.com/app/uploads/sites/default/files/images/DOE_climate_zone_map.preview.jpg
When going with a foam + fiber solution, in zone 4A it takes a minimum of 30% of the total R as air impermeable closed cell spray foam fully bonded to the underside of the roof deck to have dew point control at the foam/fiber boundary. Cut'n' cobbles ALWAYS leak air somewhere, eventually, putting the roof deck at risk. In zone 5A it takes a minimum of 40%.
With 2" of HFO blown closed cell polyurethane (R14) on the underside of the roof deck puts a 0.5-0.7 perm vapor retarder between the interior and the roof deck, and leaves 2.0" of remaining depth in a presumably full-dimension 2x4 cavity. Installing 2" wide edge strips for the rafters secured to the rafters with 1x furring through screwed to the edge strips with cap screws as a nailer for the ceiling gypsum brings it to 4.5". A cheap R19 batt compressed to 4.5" will perform at R16 (for R30 total at center cavity which is 47% low-permeance foam, which is PLENTY of dew point margin at the foam/fiber boundary for zone 5, and offers very good drying capacity for the structural rafters.
Backing of to just 1" of HFO blown foam (R7) leaves 3" of rafter depth, and is a 1-perm vapor retarder, sufficient for protecting the roof deck from interior moisture drives. Installing 2" polyiso edge strips secured with strips of half-inch CDX cap-screwed to the rafters as a nailer brings it to 5.5". An R19 batt compressed to 5.5" delivers R18 performance, for R25 total at center cavity, and a 28% foam/ total ratio. That's not quite meeting the IRC prescriptive ratio, but with a sheet of "smart" vapor retarder such as 2-mil nylon (Certainteed MemBrain) behind the ceiling gypsum the risk is low. Even without the vapor retarder the risk is low on sunnier roof pitches, the primary risk being fully shaded north facing pitches that get scant drying help from the sun.
See Table 3 of this document, a compilation of WUFI simulations of north facing roof pitches using different insulation strategies:
https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
Note, in that table in KC (zone 4A) or Chicago (5A) rows, at the "1" ccSPF + spray fiberglass" column is fully protective of the roof deck even at R38 total if dark shingles are used. This document doesn't address dew point at the foam/fiber boundary though, but the IRC gives guidance in TABLE R806.5:
https://up.codes/viewer/connecticut/irc-2015/chapter/8/roof-ceiling-construction#table_R806.5
The presumptive total-R in that table are the code-minimums prescribed in TABLE N1102.1.2:
https://up.codes/viewer/connecticut/irc-2015/chapter/11/re-energy-efficiency#11
The prescriptive total R in zones 4 & 5 is R49 for both, so the R15 for zone 4 in TABLE R806.5 is R15/R49= 30.6", and the R20 for zone 5 is R20/R49= 40.8%, both of which are reasonably conservative minimums.
At higher total- R the prescriptives in R806.5 are no longer valid- the ratio is what determines the average wintertime temp at the foam/fiber boundary, which is what matters for moisture accumulation in the assembly. But it's also true that at lower total- R it can be relaxed, as long as long as the minimum ratios are maintained.