Hot Topics: Three-Season Porch Insulation

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Original Post: Deck/Porch Insulation Question

asoule Member

We are building a 20x14 deck at our house in Minnesota. 14x14 of that space will be a three-season porch with vinyl windows. The floor will have 3/4" treated plywood with an LVT over it. Our plan is to use the porch mainly in the summer months.

Our builder asked if we would like to insulate underneath the porch with a spray foam insulation. Has anyone done this? How much warmth do you gain from this? If we can extend the use of our porch by a month or so it may be worth it. Realistically, without insulation in Minnesota this porch would be used mid-May through mid-September.

stickshift Group Moderator

Without a heat source, the insulation doesn't really do anything.

While my porch has (electric) heat, I refuse to turn it on to pay for it, so I get use maybe April to November with the door to the house left open. My porch is on a slab, for what that's worth.

Marq1 Member

Insulation itself does not provide warmth, it only reduces heat loss or gain.

Unless you are heating the area, it's not going to do a lot as the inside and outside temps will be the same.

asoule Member

Thanks for the responses. I know that the insulation will not heat the porch, but would it help maintain some heat or warmth to extend the length of the season we can use it?

Bud9051 Member

Yes, it would, but for the cost it might not be worth it. With lots of glass, insulating the floor may not help much. However, if that glass gets good sun, you may have more time to use that space than you think.

stickshift Group Moderator

I'm going to disagree with Bud on this one. I don't think insulating the floor is going to gain you any use of the space unless you find a way to add heat. On the other hand, if you do intend to add a heat source for even some of the time, then insulate away.

I'm basing this not only on my porch experience, but that of my parents' house, which has a design more like you're describing (with the floors, walls, and ceiling insulated but no source of heat).

Marq1 Member

The benefit of insulation is up, out and down! Floor insulation would provide the least benefit!

Bud9051 Member

Just to note—he isn't talking just insulation, but spray foam insulation which has the added benefit of air sealing. It also has the negative side of cost. But exposed to a dirt surface under there, the radiant heat loss would be significant.

Air sealing, insulating, and blocking the radiant heat loss will make a difference.

As for a source of heat, the house wall will be providing some and if there is some solar exposure there could be a significant amount of heat. I'm not suggesting it would be warm right through the winter, but it could provide a couple of extra weeks in each spring and fall along with some sunny days when temps are mild throughout the winter.

There is a less expensive approach of covering the bottom of those joists with a layer of foil-faced rigid foam board and using foil tape for edges and all seams. Add some caulking before that goes up, and most of the benefits of the spray foam will be achieved.

If the porch is low to the ground, then it becomes now or never. if there is a good space to get down there, then it could wait to see how that room performs.

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