Air-Seal an Attic Series: Insulation Ventilation in Rafter Bays
Add insulation without blocking air flow
In this video, Mike Guertin shows you how to install baffles that allow you to blow in the fiberglass or cellulose insulation without stopping up your soffit vents.
We’re going to add insulation to this attic eventually, so we want to put insulation blockers and air channels in the rafter bays. We want the most amount of insulation as far down as possible over the top plate of the wall. The insulation blockers should still allow the air to flow up under the roof sheathing and to the ridge so it cools the attic.
Store-bought baffles are OK, but they do need air-sealing
One way to do this is by using ventilation channels. They’re a piece of plastic or piece of foam which is designed to be installed between the rafters. It leaves a space that the air can flow up and through. To do it right, we must install a blocking panel at the very end underneath the ventilation channel and down to the top plate. This will stop any of the air from short-circuiting the ventilation channel and going through the insulation, where it can degrade the value of the insulation.
The blocking panel has already been cut to size to go between the top plate at the bottom and the underside of the roof sheathing at the top, and to fit between the rafters on either side. I need to carve out the shape of the ventilation channel. The cuts don’t need to be neat; as long as I’m within half an inch, the spray foam will fill the rest. The next thing to do is trim all the way around the perimeter a 45-degree angle off the face about half an inch back. That’s going to leave a channel for the spray-foam insulation to get a good bond between the insulation panel and the perimeter wood framing. Now I’m going to install the panel.
Plan ahead when working in tight spaces
You’re going to want to make sure you have a stapler and the spray foam ready because you don’t want to crawl out of the position more than once. I’m going to push the vent shoot so it’s a couple inches past the exterior of the wall. Then I can push the blocking panel in, get a good friction fit, and push it as far out to the exterior face of the wall as possible. A couple of staples will hold the vent shoot in place. I’m going to spray foam around the perimeter of that blocking panel. This is one way to make an insulation block and a ventilation panel.
Custom baffles add insulation for not much cost
I’ve got another custom way of making one out of some rigid foam. I’ll show you how to do that now. I’m going to make vent baffle out of a piece of rigid polyisocyanurate. I’ve already cut the sheet into 16-in.-wide pieces, and I’m going to cut halfway through the sheet about ¾ in.from each side. Flip the panel up, just like snapping drywall, and then fold the ear back. That becomes my spacer, and I use a piece of house-wrap tape to tape it down. Because this channel is going to fit in between these rafters, I’m going to need to be able to get a little bit more than the ¾ in. space that I have here. At the bottom I don’t have a problem because it’s going to sit on the exterior wall, the top plate of the exterior wall. That’ll be spaced down off of the top by about 3 in.. Up at the top I want to provide a more positive stop for my spacers. I’ll do that with a couple of extra scrap blocks that I’ll attach with house wrap tape. Then I have to make a notch at the bottom right-hand side. That’s going to wrap around the tail of the ceiling joist down below. I’m just eyeballing an inch and a half in to get this cut. Now it’s ready to insert.
I’m going to use a little nail to hold it in place until I can foam it in. Foam is important for two reasons: one, it’s going to help with the air seal so we don’t get any air seeping through the insulation and degrading it. The second reason is to keep the insulation from falling down into the soffit. Beautiful. Now we can either stuff some batt insulation down there or blow in some loose fiberglass and not worry about air leakage or about the insulation being degraded by short-circuiting air.
Videos in this series:
- Sealing Wire & Plumbing Penetrations
- Sealing a Ceiling Electrical Box
- Sealing Wall Intersections
- BONUS: Materials Overview
- Sealing Ductwork
- Sealing Soffits and Chases
- Insulation Ventilation in Rafter Bays
- Improving Attic Knee-Wall Insulation