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Old 11-11-2020, 11:51 AM   #1
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2018 16' Sport
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Rigid Foam Board Insulation Under Dinette

Have you installed rigid foam board insulation under the dinette (above the spare tire)?

In our 16RB the floor below the dinette table is very cold. It appears that I could install 1.5" or 2" rigid foam board insulation above the spare tire, against the underside of the dinette floor.

Have others done this? Did it make the floor feel warmer? Is water retention in or under the foam board a problem?

Depending on which foam board is used, I could get up to R13 with 2" insulation and still be able to stow the spare tire in the carrier without any modification.

Welcoming your thoughts on this project.
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Old 11-11-2020, 06:45 PM   #2
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I would look at Rockwool rather than rigid. You can stuff the Rockwool around and into tight spaces. It also doesn't mind moisture or heat. You might even be able to get the Rockwool with a facer on one side to keep the dust down. It is very easy to work with to get desired results. 3.5" should give you R14 and that can be reduced by pealing layers off the batts to fit your space. Rigid ISO will give you ~R6.5 per inch and Dow Pink or Blue rigid will give you R5 per inch.

You can use a tape to close the seams like Tuck tape or if you want to get fancy, you can use aluminum faced tape (very sticky).
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Old 11-11-2020, 07:10 PM   #3
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Rockwool would work. I would consider wrapping the batts in Reflectix with 3m spray glue. Reflectix has a water vapor transmission of .02. So should keep dust and moisture out. On our 55 I did use Foamular, leftovers off a commercial freezer job, underneath the subfloor with a layer of Reflectix against the bottom of the subfloor due to its high fire rating (post #53 of our rebuild thread). You’re on the right track. Either will work and hopefully get the results you’re looking for. Take care
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Old 11-13-2020, 10:58 AM   #4
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Thanks for responding, PB_NB and Bubba L.

I'm a bit confused. Are you talking about the Rockwool insulation that looks like typical loose bats of insulation?

The area under my dinette is wide open to the elements and 60-70 mph turbulent winds while towing. There is no belly pan under that area of our AS. Just a spare tire in a carrier. What would you recommend to hold the Rockwool insulation in place?

Rigid insulation could be held up against the dinette floor by some construction adhesive and upward pressure from the spare tire in the carrier.

Thanks again.
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Old 11-13-2020, 11:50 AM   #5
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Why couldn’t you cut a triangular piece of aluminum, turn the two side edges down an inch. I would pre-drill a few holes thru the aluminum, tap the holes with 8/32 and place stainless steel pan head screws. Sandwich the rockwool between the top of the aluminum sheet and the floor underside. I know that seems like a lot of effort, but you wouldn’t have to worry about it falling and could replace the insulation in the future if needed. Just a thought.
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Old 11-13-2020, 11:58 AM   #6
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Maybe the Foamular board insulation would be better. Along with the adhesive, you may consider pinning it to the bottom of the floor with a few screws and a 2” piece of aluminum as a washers.
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Old 11-13-2020, 12:11 PM   #7
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Quote:
Originally Posted by Runamuk13 View Post
Thanks for responding, PB_NB and Bubba L.

I'm a bit confused. Are you talking about the Rockwool insulation that looks like typical loose bats of insulation?

The area under my dinette is wide open to the elements and 60-70 mph turbulent winds while towing. There is no belly pan under that area of our AS. Just a spare tire in a carrier. What would you recommend to hold the Rockwool insulation in place?

Rigid insulation could be held up against the dinette floor by some construction adhesive and upward pressure from the spare tire in the carrier.

Thanks again.
Oh that does make a difference, I was envisioning the space above the floor.

The challenge with this would be water getting on the insulation. Rockwool (batts) would probably absorb some and eventually dry. Formular would also adsorb some but at a much slower rate. I have seen some applications where the Formular is completely saturated and weighs a ton. This may not affect the R-value but will make the insulation heavy. I think Bubba's idea of an aluminum sheet under the belly to support and contain your insulation would be clean and effective.
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Old 11-13-2020, 12:43 PM   #8
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Quote:
Originally Posted by PB_NB View Post
Oh that does make a difference, I was envisioning the space above the floor.

The challenge with this would be water getting on the insulation. Rockwool (batts) would probably absorb some and eventually dry. Formular would also adsorb some but at a much slower rate. I have seen some applications where the Formular is completely saturated and weighs a ton. This may not affect the R-value but will make the insulation heavy. I think Bubba's idea of an aluminum sheet under the belly to support and contain your insulation would be clean and effective.
Pre-cut the foamular and paint a couple of coats of System Three epoxy to keep moisture out? Shouldn’t affect the R-value.
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Old 11-24-2020, 11:34 AM   #9
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So ... I bit the bullet. Purchased a sheet of 2" foam board R-13 insulation. Removed the spare tire carrier. Cut 2 pieces (left side and right side) to fit snugly between the frame rails under the dinette area. And reinstalled the tire carrier.

We spent the weekend at Fort Worden, Port Townsend, WA. Low 40 degrees at night. I can't say the floor was any warmer than before the insulation. However, it didn't seem to suck the warmth out of your feet like it did before. So, I guess the insulation is helping some.

The 2" foam board nicely filled the space between the spare tire and under the floor. The spare tire isn't bouncing around down there loose anymore. I did not have to modify the carrier at all. Now, the slight upward pressure of the spare tire helps hold the foam board in place.
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Old 11-24-2020, 11:43 AM   #10
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Quote:
Originally Posted by Runamuk13 View Post
So ... I bit the bullet. Purchased a sheet of 2" foam board R-13 insulation. Removed the spare tire carrier. Cut 2 pieces (left side and right side) to fit snugly between the frame rails under the dinette area. And reinstalled the tire carrier.

We spent the weekend at Fort Worden, Port Townsend, WA. Low 40 degrees at night. I can't say the floor was any warmer than before the insulation. However, it didn't seem to suck the warmth out of your feet like it did before. So, I guess the insulation is helping some.

The 2" foam board nicely filled the space between the spare tire and under the floor. The spare tire isn't bouncing around down there loose anymore. I did not have to modify the carrier at all. Now, the slight upward pressure of the spare tire helps hold the foam board in place.
That's great, the tire helped out as well!

We really miss Fort Casey and Coupeville area since the pandemic, we usually go there once or twice per year. Last November we stayed at the little campsite next to the Ferry terminal. Haven't tried the Port Townsend side yet. Maybe next year!
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