Insulating an RV/Boat garage in Port Townsend, Wa. – Is polyisocyanuate appropriate?
I am looking to insulate and add heat to an RV/Boat garage located in Port Townsend, Wa. (Zone 4 – Marine). Built in two sections, a back half was added to the original structure in the 1990s roughly doubling the size. Built on concrete slabs and with short cement foundation walls the structure measures 23’Wx 43’L. The walls are of 2×4 construction, 12’ in height. The exterior side of the walls are covered with T1-11 plywood siding. The interior walls are completely unfinished stud cavities, with exception of the back half addition to the structure. The lower sections of the interior walls of that 1990s addition are covered with 4×8 sheets of ¼” plywood. Curiously, the top 4’ of those stud walls, were left uncovered and in those areas black felt paper can be seen underlying the T1-11 exterior siding. A metal roof with a ridge vent has been recently installed on the underlying open trusses. While I am not looking to convert this to a living space, I would like to make it comfortable enough to use as a shop space during the long, damp, Pacific Northwest winters.
Toward that end, I am considering using 2” foil faced polyisocyanurate to create a continuous layer of insulation on the interior side of the 2×4 stud walls. I would then finish the wall with a layer of ¼” or ½” drywall. In the front half, the Poly would be applied directly to the exposed studs. In the back half, I propose covering the remaining 4’ of the exposed upper sections of those walls with plywood to match the 8’ lower sections. I would then have a continuous wall surface on which to apply the 2” foil faced Poly.
From reading through the many informative articles on this site, I believe that this approach would be viable but not having found any article that exactly describes this scenario, I do have lingering concerns.
First of all, should I be concerned about the lack of any Water Resistive Barrier under the T1-11 siding on the front half of the structure? Given that the foil faced Poly may act as a WRB, should there be a concern about having it on the interior side of the studs? In this scenario is it safe to assume that the T1-11 is sufficiently permeable to alleviate a potential of trapping moisture within the finished wall?
Conversely, does the presence of the black felt paper facing on the T1-11 on the walls of back section actually increase the possibility of trapping moisture? If as proposed, I complete the walls with plywood and proceed to cover the ply with foil faced Poly am I danger of creating a space sandwiched between two WRBs? Further, are there any other concerns that may be raised by applying the 2” Poly directly to the existing plywood?
I would also be interested in what you might think about the possibility of using Poly on the underside of the open trusses to create a ceiling. If that proves viable would it be advisable to add bats of insulation to the top side in order to increase the overall R-value of the structure?
Any advice that you may be able to offer would be greatly appreciated.
Sincerely,
Miles McRae
Seattle, Wa.
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Replies
T1-11 is usually both the structural sheathing and siding in one. #15 felt is not a vapor barrier, and cannot create a moisture trap. As it's moisture content increases, so does it's vapor permeance.
Even with no #15 felt/WRB/drain plane behind the T1-111 it's not going to be much of drying issue unless you're filling the stud bays with fluff and the siding is being wetted repeatedly with wind driven rain. The fact that the foil faced polyiso is a vapor barrier blocking drying toward the interior hardly matters. Port Townsend is in the rain-shadow of the Olympic range and even the amount of direct-wetting of the exterior is limited, and even installing batts would not likely increase the moisture content of the T1-11 by very much. The morning dew and sea-fog are the primary wetting events, which affects primarily the exterior surface, and dries as fast as any siding does.
FWIW: R13 batts + continous 1" polyiso on the interior would bring it up to IRC code minimum for zone 4C, whereas 2" polyiso on it's own does not.
As long as the attic space is vented to the exterior it's fine to use polyiso on the bottom chords, and batts above the polyiso. It's hard to get a good fit around truss elements with batts though- blown fiber would be a better bet.
Most polyiso does no have sufficient fire ratings to be left exposed to the occupied interior without a thermal barrier against ignition.
Dana,
Thanks for your timely reply. I hadn't considered adding batts in the stud walls due to concerns that had been raised regarding the lack of a WRB on the exterior. It is a relief to know that those concerns are largely unfounded given the relative permeance of the T1-11 siding. Am I correct in assuming that unfaced batts would be the best choice for application behind foil faced polyiso?
Adding R13 batts to the stud bays in the front half of the structure would be relatively straight forward, unfortunately the plywood covering the interior walls on the back half has been glued and nailed. Can you give me an idea about what the increase in R-value would be over over a single layer of 2" polyiso? It would require a major effort to remove that plywood but if the end result would be significantly better....now is the time!
Again, thanks so much for the advice.
The facing on the batts doesn't really matter when there's a true vapor barrier on the interior side, as long as you don't put a foil batt facer on the exterior, next to the felt.
The #15 felt is a WRB, but without a path to drain to the exterior it isn't doing much, but it will limit the rate at which the fiber insulation wicks moisture toward the interior. If your concerned, splitting high density R21s tight to the interior to leave a 0.75" air gap between the 1.75" split batt and #15 felt would be a very powerful capillary break.
A 2x4/R13 wall runs about R10 "whole wall", roughly the same as continuous 1.5" of foil faced polyiso facing an air gap.
How deep are the roof overhangs? With 2' overhangs filling the cavities with cellulose might be fine.
How in love are you with the look of the T1-11? Blowing the cavities full of cellulose or fiberglass, and installing 3/4" polyiso on the exterior, with vinyl siding or other inherently back ventilated siding just works, and may be cheaper than doing 2" of foil faced polyiso on the interior.
Given your very mild temperatures 38° average low in January, some jealous people may be tempted to tell you put on a jacket : ). How hours a month are you planning on heating the building? If it is only occasionally maybe you just want a wood stove and skip the insulation. If you do want to insulate fill the stud bays with Rockwool batts and cover it with drywall with one coat of mud. Note the Rockwool batts will not hold water and will dry should it get damp.
Walta
Walta,
You're not insinuating that we are weather wimps out here in the PNW are you? In the midst of winter, the days are short and the grey dampness makes the cold seem so much more penetrating! Seriously though...thanks for the advice. I now have all kinds of possibilities to consider.
Miles
I have relatives on Whidbey Island and in Port Orchard, and have spent my fair share of sea-fog rain winter days on the peninsula. It's a DAMP coolth going on for sure, but fortunately usually without extreme wind, (the tornado in Port Orchard a few weeks ago notwithstanding.)
The rain shadow zone from Port Angeles to Port Townsend is quite the anomaly for the region- the same slate skies as elsewhere in the region, but far less direct precipitation to deal with.