Is wind washing a problem in a vented roof with mineral wool? & Can we mix in Polyiso?
Hello Martin,
First, thank you so much for all the excellent answers and articles you have posted. They have been a great help in making decisions about how to build our “tiny home on wheels” house. We are hoping you can provide some advice about our specific roof insulation assembly.
We are in the Pacific Northwest, marine climate zone 4c (for now, but we are trying to build with the idea that we could also live in a colder climate, such as Eastern Washington, climate zone 5b if we choose).
So far, we have a vented roof assembly with 5” rafters (spruce car decking), roof sheathing, grace ice and water shield, and a metal roof. Our walls are 2×4 and will likely be around R15. Our floor is insulated with foam and mineral wool (R28).
For the roof insulation, our plan was to put 3.5” of mineral wool in the bays and a 1” continuous sheet of foil-faced polyiso foam board sealed with foil tape on the interior of the rafters, making the assembly R21 with a thermal break and leaving a 1.5” space for the vent. There will be no penetrations of the polyiso foam board and we intend for this layer to provide an air seal, separating the living space and the roof assembly.
1. Should we be concerned about wind washing?
Reading more about the site-built baffles you suggest, I’m wondering if it would be better for us to include a ½” foam layer above the mineral wool to prevent wind washing or if making side-running baffles from wood just to keep the mineral wool in place is sufficient. How important is it to have an air sealed vent when using mineral wool?
2. We were recently gifted some 2” foil-faced polyiso “scraps” that would fill about half of the bays of our roof in the “pancake method” of infill to a 4” in-bay thickness, leaving us with a 1” space for the vent. Do you see any problem with 1) mixing these two methods, 2) placing the “pancakes” tightly, but without an air seal around them 2) back to back foil pancaked as a place where water might condense?
3. If you think it is ok to mix the two methods, would you suggest having the foam or mineral wool be above the sleeping loft/kitchen (higher humidity area), in the event of air leakage into the roof assembly?
4. Any other concerns/recommendations about these ideas?
Thank you!!
Laura
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
FYI, Martin is on vacation until August 8th.
Hi again Martin, I'm sure you must be busy just getting back from vacation, but my question is really buried under here, so I wanted to let you know that I'd still love to hear what you think. I've been waiting to move ahead until I hear your advice
Laura,
Here in Vermont, we don't use the term "car decking," but I was under the impression that "car decking" usually refers to 2-inch-thick tongue-and-groove lumber. Why would you use tongue-and-groove lumber for rafters?
Q. “Should we be concerned about wind washing?”
A. Tiny houses are usually easy to heat, so I wouldn't overthink things. It's best to have an air barrier above your fluffy insulation (to separate the fluffy insulation from the ventilation channel), but in a tiny house, you can get away with a few details that aren't perfect.
Q. “We were recently gifted some 2-inch foil-faced polyiso 'scraps' that would fill about half of the bays of our roof in the 'pancake method' of infill to a 4-inch in-bay thickness, leaving us with a 1-inch space for the vent. Do you see any problem with mixing these two methods?”
A. No.
Q "Do you see any problem with placing the 'pancakes' tightly, but without an air seal around them?"
A. Yes, that approach isn't good. You need to install these pancakes with attention to airtightness.
Q. "Do you see any problem with the back-to-back foil (pancaked) as a place where water might condense?”
A. No.
Q. “If you think it is OK to mix the two methods, would you suggest having the foam or mineral wool be above the sleeping loft/kitchen (higher humidity area), in the event of air leakage into the roof assembly?”
A. As long as you have an airtight ceiling, it makes no difference which type of insulation is where.
I, for one, don't like that exposed polyiso under the rafters...
Laura,
I assume that you plan to install a ceiling (ideally, 1/2-inch drywall) on the interior side of the polyiso shown in the drawing.
If for some reason you planned to leave the polyiso exposed -- then Ethan is right. That would be a fire hazard.
Thanks so much for your response, Martin!
Regarding the "car decking," we used 2x6 t&g spruce and cut off the tongues because we wanted to light weight of the spruce in the roof. In retrospect, this was much more work than it was worth because it made other steps in the building more complicated.
When you say to install the pancakes with "attention to airtightness," do you mean that the foam pancakes should be snuggly fit into the rafter bays or that each pancake needs to be sealed (like with spray foam or tape)?
Ethan, I forgot to draw in the t&g pine ceiling boards that will go below the polyiso. Sorry, that wasn't clear.
If I may ask for a little more advice:
For our wall assembly, we have 2x4 walls with plywood, building wrap, and cedar siding currently installed. We are considering finishing with just insulation and then t&g pine on the interior, as a few builders in our area have advised against a plastic air/vapor retarder and we don't want to paint or install sheet rock. We were thinking since our walls are so thin, maybe we could push moisture through them by heating. Is this a bad idea?
We will be heating with a wood stove primarily.
For the insulation, we are considering sheep's wool, cellulose, or mineral wool. What would be your recommendation?
Concerns about insulation types:
Mineral wool: - dusting in the living space (could we put up a breathable air barrier like paper or cotton sheets? - formaldehyde (company says its gone by installation time, but would prefer to see that from a third party source)
Cellulose: Would it get damp and not dry out?
Sheep's wool: Expensive, slim data on performance
Laura,
Q. "When you say to install the pancakes with 'attention to airtightness,' do you mean that the foam pancakes should be snugly fit into the rafter bays or that each pancake needs to be sealed (like with spray foam or tape)?"
A. The perimeter of each piece of rigid foam should be sealed with caulk, canned spray foam, or tape. For more information, see Cut-and-Cobble Insulation.
Q. "For our wall assembly, we have 2x4 walls with plywood, building wrap, and cedar siding currently installed. We are considering finishing with just insulation and then T&G pine on the interior, as a few builders in our area have advised against a plastic air/vapor retarder and we don't want to paint or install sheetrock."
A. That's a big mistake. You need an interior air barrier, and T&G pine boards leak like a sieve. For more information, see Questions and Answers About Air Barriers.
Q. "We were thinking since our walls are so thin, maybe we could push moisture through them by heating."
A. I'm not sure what you mean. Moisture can move through walls by diffusion -- you can limit this movement with a vapor retarder (for example, interior vapor retarder paint) -- or by piggybacking on exfiltrating air -- you can limit this movement by including an interior air barrier.
Q. "For the insulation, we are considering sheep's wool, cellulose, or mineral wool. What would be your recommendation?"
A. Of those three options, cellulose or mineral wool make the most sense.
Q. "Mineral wool: - dusting in the living space."
A. This won't be a problem, because you will be installing an interior air barrier between the insulation and the living space.
Q. "Formaldehyde?"
A. See this article: Mineral Wool Makers Dropping Formaldehyde Binders.
Q. "Cellulose: Would it get damp and not dry out?"
A. If your cellulose is getting damp, you have made a serious error in your wall design. You want to design your wall to stay dry.