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Question DetailsAsked on 5/17/2016

What kind of insulation goes under metal roof?

House has unfinished sun room built on one end of deck which has a metal roof. The previous owners installed sheet-rock on ceiling and walls. However, it seems to have 3 or 4 small areas of light mold on the ceiling, which I have been told is condensation from not insulating the metal roof. What kind of insulation is required?

I still need to install some kind of flooring over the original deck flooring. How should this be done?

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First, you should check (may mean coring out a piece of the mold if you can't see in on top of the ceiling) to be sure it is on the inside surface only, not from moisture in the "attic" dripping on the drywall and causing the mold.

If due to too cold a ceiling - tearing off good condition roofing and putting down rigid foam insulation is pretty extreme a measure - and that would assume that you have a "warm" ceiling - that the "attic" or rafter space is not open to outside air as it normally would be. If the "attic" is open to outside air, then the only place insulation would be any use if on top of the ceiling, not under the roofing, because a "cold" attic has outside air in it so insulating the roofing serves very little winter benefit, though it can reduce solar heat passthrough in summer.

Since you said light mold, it might be that installing a fan (fixed ceiling fan or portable upright) would ventilate it and remove moisture enough to solve the problem - especially if this room is used daily. If you do that, I would combine it with using Kilz on the mold areas after the mold is bleached and washed off, the repainting with a semi-gloss exterior paint with mildew preventative additive in it or with Kilz finish color paint. In a minor case that itself might take care of it when combined with periodic (say 1-2 times a year) washing with Dawn or such to remove the dust which helps start mold.

If a "warm" attic - not open to the outside, then in theory you can spray foam on the underside of the roof for insulation - but you have to be VERY sure that there are no penetrations to the house because warm moist household air moving up into the attic can cause condensation and mold if the roof is well insulated unless the attic is kept well ventilated, which is an energy waster. I would seriously doubt, for this sort of room, that it is a "warm" ceiling.

You don't say what type of access you have up there - if just the rafter space (say 4-10 inches deep) with no "attic" to walk around and work in, then about all you can reasonably do in most cases is, if at least 6-10 inch deep rafters, use an extended placing tool and working from the eaves snake roll fiberglass batting up in there. Can be easy to do at times, pretty nasty if the drywallers missed the rafters with a lot of fasteners because they snag the insulation as it goes in. Can be done in that case, just more labor intensive and you have popouts from fastener removal to patch and repaint. You need to keep a good 2-3 inch or so air gap above the insulation (under the roof) for ventilation (again assuming this is a ventilated or "cold" roof) - and not likely to help much unless there are ridge vents to promote free airflow through the attic. Unless your rafters are 8-10" deep so you can get 5-7 inches of insulation in there, in fact, the requirement for airflow to ridge vents to prevent condensation may well negate the benefit of a single thickness of insulation. 4" deep rafters do not allow enough room for meaningful insulation. My recommendation - some contractors promote this, but do NOT pack the space tight with blown-in insulation - an invitation to trouble.

Also, if the drywall was put up without a vapor barrier above it, insulating could cause condensation issues in the attic due to moisture passing through the drywall from the house.

More I think about this, I would say you need an Architect or Energy Efficiency consultant to advise on what would be best for your particular circumstances, to avoid doing something that might make it worse.


Flooring - you need to block moisture and airflow from outside to inside through the deck, so normally in mild climate areas you would put a vapor barrier over the decking and airtight sealed to the perimeter walls, then exterior grade plywood (I recommend treated or marine grade plywood for this sort of application) subflooring, then your flooring over that - and on a deck the flooring should not be heavy like stone or tile, both for weight and because decks/porches generally move too much so it will crack up in short order. If you do not want a cold floor (if in an area when it is cold outside when you will be using the room) then generally you have to put outside-surface painted exterior (preferably treated) or marine plywood UNDER the deck joists (assuming it is wood) with vapor barrier between the plywood and the joists and plywood joints caulked during installation, then non-organic (like batt fiberglass) insulation between the joists, flooring sheathing (again should be exterior or exterior/interior grade), then the flooring - no vapor barrier under this sheathing if insulating the joists area. You have to be careful about blocking off all entry points around the joists to prevent vermin entry, and of course unless the deck joists and posts/piers are in good shape you would normally replace them before doing this.

You also have to be careful to put in moisture gaps and flashing and watershield anywhere deck wood passes from "outside" to "inside" conditions, so prevent wet exposed deck wood from soaking through into the "interior" area and causing mildew and rot. You really need some expert advice like from an architect on thise issues to be sure the details are correct to avoid rot issues.

Answered 4 years ago by LCD

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