Duct insulation in the Crawlspace

If you are involved with building a house, you will encounter challenges that you never thought could exist. You will think to yourself, "How can this be so difficult?" And when you try to figure out how to insulate a crawlspace and the ducts that run through it, you will be especially challenged. 

Remember one of the highest priorities in building John's house is to make it energy efficient. That means you try to minimize the energy losses from the home's shell, the thermal envelope. And that includes every side of the house including the dreaded crawlspace. Now we made the crawlspace a little less disgusting (dark and dank and spidery) because we did not enclose it. The house's foundation is on piers. So the space is open to the outside. I designed it that way because the house is very near a pond and a large creek and there is potential for flooding.  We had a torrential rainstorm 2-3 years ago that flooded most of the yard, it did not come to the area where the house is located. Still I thought it was prudent to build on piers to eliminate that potential problem.

We have insulated most of the joist bays in the crawlspace, but now we have insulate around the ducts. The ducts are already wrapped with what is listed as R-8 fiberglass though I don't believe it is truly R-8. If you google R-8 HVAC duct wrap you will see various products that are used to wrap ducts. These products are generally 2 inches thick and have foil-faced layer that is taped around the ducts. In order for the insulation to truly have that R-value (resistance to heat transfer) it cannot be compressed. It must be 2 inches thick, but you really can't wrap and tape it around the ducts and preserve that 2 inch space. It is usually compressed to about 1 inch which gives you at best an R-value of 4. I think that is much more realistic. 

You can see where I reinforced the taping on the duct insulation and how it compresses the insulation surrounding the ducts. I've made sure there are NO air leaks in the insulation surrounding the ducts. 

So here's the situation, you've got a house with the roof insulated > R-50, walls insulated > R-30, floors insulated >R-30 and your crawlspace ducts are insulated at R-4? Are you kidding me? That drives me crazy. I just can't see leaving them like that. So we are encapsulating the ducts in 1-2 layers of polyisocyanurate foam boards which will triple the effective R-value surrounding the ducts. It will not likely equal the insulating on the floor, but it will be close to R-20. 

This post aims to show you some of the details of enclosing the ducts. It is tedious work, not as itchy as working with the batt insulation. I hope I can explain these details so that they make sense. 

This photo demonstrates 3 things:

1. The joist bay is air sealed at the edges with Prosoco Joint and Seam filler so air from one bay cannot move from one joist bay to another.

2. The duct is very well air sealed to the subfloor.

3. We have a 2x4 in front of the duct which is to help with placement of the foam board which will enclose the duct. 

Back view of the same duct, now surrounded by polyiso foam boards.

Ok, this is view shows the same duct after we've applied the foam board onto the front. The foam board is glued on with construction adhesive, then foamed around the edges. Also the front of the board is taped to the subfloor and joist bays. So the likelihood of leakage around the foam boards is very low. 

You can see where we have installed some more boards around the duct and the joists. There will be longer boards parallel to the ducts on each side and underneath. Again, all the edges between the boards will be foamed and taped as well as I possibly can manage. This was a slightly more difficult duct because it had to dip down around the rim joist from the main part of the house and back up to the porch. It is the only duct in the enclosed porch which is just 8' x 16'. 

I'm off again tomorrow so I should be finished with this duct then. I hope this is helpful for someone working on their house. Good luck.