Polyisocyanurate Radiant Barrier Rigid Foam Insulation-Condensation Worries
I live in Lake Elsinore, CA. In the summer, it can be dry and hot (110 deg. F), and in the winter it can be around 27 deg. F. There are some humid days, but very few.
In my woodshop, I have walls that have a 1″ thick stucco/ scratchcoat layer over tar paper directly over the 2×4 studs. It is currently uninsulated and there is no drywall on the interior.
During the summer, the sun heats this thick scratchcoat/ stucco layer and it re-radiates into the shop during the afternoon and well into the evening, making it unbearable to be in there. I want to use a radiant barrier near the tar paper (toward the exterior) to reflect this radiant heat back outside before it makes it into the shop.
My plan is to cut polyisocyanurate radiant barrier foam sheets (RMax Thermasheath 3) to fit in each stud bay, leaving a 3/4″ air gap between the tar paper and the rigid foam. I would like to caulk/ spray foam the gaps to air seal this 3/4″ air space completely around the edges where the tar paper meets the studs, and where the foam sheets meet the studs, but leave the cavity filled only with air to keep the radiant barrier effective. Then, I’d like to either use fiberglass batt or rigid foam (sealed w/ spray foam) to fill the rest of the wall cavity before drywalling. I want no air movement in the stud bay to reduce the effective R-value.
Here are the planned layers, from Exterior to Interior: Unpainted Stucco, Scratchcoat, Tar Paper, 3/4″ sealed air gap, 1″ polyiso foil faced rigid foam (sealed in bay toward exterior), additional 2-1/2″ batt or rigid foam insulation cut to fit and sealed, possible 1″ sheet of rigid foam over studs, 5/8″ drywall, primer, paint.
Do I need to worry about any condensation issues in the 3/4″ airspace, or anywhere else? Is it a bad idea to add a sheet of foam over the studs for a thermal break on the interior with this assembly? Thanks, guys!
Replies
Pretty good plan
Condensation would be a wintertime issue for you, if it happened. If you put a thin layer of foam in the stud bays, and nothing else, the interior surface of the foam could get cold enough to be a condensing surface for interior humidity. The solutions would be to install thicker foam, raise the interior temperature, or reduce the interior humidity. In this case, the first one is the only one I would do, at least as a starting point.
Depending on who you believe, you need 3/4" or more of airspace in front of a radiant barrier in order for it to work well. If you have 3-1/2" of stud depth, maybe the best thing is to install 2-1/2" into the cavities followed by another inch or more over the studs, with taped seams. That will give you a ~R-21 with very little moisture movement into the wall from the inside.
Of course another approach is to skip all the foam and just insulate with cellulose or fiberglass. followed by drywall. With stucco and without sheathing I'd be a little hesitant to recommend that, especially in my climate, but it might work in yours.
IMO you should be clear on humidity and dewpoint and know how to measure and evaluate them in your own building before deciding on an insulation strategy.
Airspace and condensation
Thanks David! I think using only foam is the smart thing here, because if that fiberglass gets wet, it'll hold moisture and the only way for it to dry out would be toward the interior drywall, assuming I didn't put an extra 1" layer rigid foam over the interior studs. If I used all foam, I suppose I could do that too.
I have the 3/4" airspace between the tar paper and the polyiso foam by using 1"x1" foam blocks in the corners, which I would like totally caulked and sealed for no air movement in the void to increase the R-value. But if water condenses on the foil surface, the "drying path" available would be through the tar paper/ stucco layer toward outside.
The stud bays behind the 1" polyiso radiant barrier I can totally seal with ridid foam and spray foam. Then, your suggestion of the 1" interior rigid foam for the thermal break sounds good. Since everything is sealed between the polyiso and the interior foam layer, I want to make sure I'm not setting myself up for issues.
"Totally sealed" sounds like the key here. Luckily, I don't live in Florida!
Thanks,
Bobby
Do you cook or shower in the building? Host large parties every other day? Run a humidifier? If not, you have nothing to worry about -- no source of moisture to cause condensation.
If you do run a humidifier, you just need to be reasonable about its operation, never actively humidifying above 40%, maybe 30% if you get a really cold (15F) stretch of 2-3 days.
Your plan sounds good to me.
Ok Thats a Relief!
Thanks for the well-thought out replies! I would be the only source of moisture in the woodshop (latent heat) but nothing else really. The only source would be grains of moisture in the air naturally. I thought about using batt insulation but I think I'll use foam in each bay that is perfectly sealed (for acoustic reasons as well) so there is no fiberglass potentially holding moisture without the ability to dry out. Plus, I get my radiant barrier which is my main goal.
By sealing everything I eliminate unwanted airflow, and a path for bugs and things. But whenever anything is sealed, thought should be given to source and direction of moisture, and if there is a path to dry it out. Fear of double vapor barriers, I guess.
By adding a 1" foam layer over the interior studs, my only fear is that the 2x4's moisture contect would have only the path towards the tar paper to dry out.
Since I have cast iron table tops in the shop, if anything I'd like to dehumidify the shop but there wouldn't be a humidifier in operation. When it rains (like today, but it's not very often in CA), the tar paper could have moisture on it, but I figure it can dry out through the stucco/ scratchcoat layer toward the outdoors, same w/ any condensation on the radiant barrier polyiso foam that faces the 3/4" toward the outside.
With all the differing opinions, and theories that sound good, I don't want to mis-read anything and wind up with a moldy mess!
Bobby
It's important to realize
that you are completely changing the dynamics of the building by enclosing the stud bays from the inside. Right now, if the stucco is getting wet anywhere, and some of that moisture is coming thru to the backside, it dries out to the interior very easily. Add foam and you will force it to dry back out to the exterior. One of the keys is adequate roof overhangs, good flashing at windows, etc., so that the stucco takes on a minimum of moisture.
In general your climate sounds very dry, which is a big advantage.
Unless you have serious problems with roof leakage or poor flashing around windows or some such, there's no way that mold is going to be a problem in this structure. Any moisture that gets in through the stucco can get out through the stucco. If you use fiberglass on the inside there would be no reason for it to ever get wet, and if it did somehow it would dry towards the inside just fine.
This post is two years old, but I thought I should reply -- in case anyone else is thinking of installing foil-faced polyiso foam insulation panels in their home or basement, interior or exterior.
We are in the middle of a ground-up remodel project, for a house in Massachusetts. I purchased ~2" thick DOW TUFF-R polyiso foam panels from Home Depot, to insulate the basement. The panels are foil-faced on both sides. I stacked them vertically in the basement, leaning against one of the stud walls.
When I returned a few days later, there was a puddle of water under the panels. I thought, "Oh no! A water leak!", and examined the pipes in the area. No luck. Then I picked up the panels to move them, and noticed that the foil surface was wet. Voila! Moisture was condensing on the thin layer of aluminum foil, which is very insulated on one side.
I checked the weather data for the area. The spring, summer and fall in eastern Massachusetts can be humid -- averaging 70% on many days. In the evening, the humidity and "dew point" rise and the air temperature falls, until they are only a few degrees apart. In the early morning, the air temperature starts to rise -- but the insulated foil surface on the polyiso panels lags behind, and condensation occurs. My car is also covered with dew on most mornings during this season.
Key point: check the weather data in your area. Many parts of the US are humid in the spring, summer and fall. You don't want large panels of insulation in your house to create puddles of condensation.
Our solution: Moisure in a basement is inevitable, so we installed multiple layers of protection. (1) The concrete walls were covered with a thick layer of poly sheet, which was well-sealed top and bottom, and at the seams. This will keep moisture from migrating from the concrete wall into the stud bays and the foil on the back side of the polyiso panels. (2) Each stud bay was sealed around the perimeter with flexible foam, to create an isolated pocket of air. (3) The stud bays were filled with the foil-faced polyiso panels, which were sealed around the perimeter with flexible foam. (4) The top of the concrete wall, the mud sill and rim joist were covered with spray foam -- a key insulation step, which also helped seal the top of each stud bay. (5) Air is a major source of humidity in the basement (along with the rest of the house) so we insatlled an Aprilaire dehimidification system. This will create a large gap between the air temperature and dew point, to block condensation problems on the interior foil side of the polyiso panels. A dehumidification system also reduces the need for air conditioning and uses 1/3 of the energy -- for an overall reduction in AC costs in the summer.
Key concern: I do not see how condensation could be stopped on foil-faced polyiso panels if they are mounted on the exterior walls. What would block condensation on the large foil surface facing the house sheathing? (On the exterior side, condensation may also occur -- but should evaportate as the siding warms during the day.)
I hope this is helpful. Please let me know if you have any questions.
Mark