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We have a loft room that is boarded and has insulated plasterboard attached to the rafters. There is a small gap between the bottom of the plasterboard and the flooring which I assume is compromising the insulation. I understand it is important not to block the space behind the plasterboard as this is needed to ventilate from the eaves, so I am reluctant to use expanding foam to fill this. What do people recommend?

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isherwood
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Drafts Loft
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    A picture of the area will help. What needs to be done to correct will depend on the size of the "small gap" I assume this is an "A" frame type of situation and base or skirting board is not a feasible option. – RMDman Nov 27 '22 at 22:53
  • Yes that’s right. – Drafts Loft Nov 28 '22 at 07:06
  • It’s about an inch wide or so. I can stick my fingers through – Drafts Loft Nov 28 '22 at 07:13
  • Added photo as requested. – Drafts Loft Nov 28 '22 at 17:42
  • There's almost _always_ a gap below drywall. It's not part of the building envelop and doesn't need to be sealed in any way. Please revise your post to explain the insulation and vapor barrier strategy employed behind the drywall. Proper answers depend on that information. – isherwood Nov 28 '22 at 18:02

2 Answers2

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That is normal, the gap helps prevent spills on the floor from soaking into the drywall.

To close the gap install a baseboard,

Jasen
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The best course of action would be to remove the plasterboard (as it seems to be improperly installed, horizontal, and unfinished) and tightly insulate atop the studs and therefore behind the plasterboard. Large sheet/panel Styrofoam is the best very long-term option.

However, if you're mostly concerned about aesthetics, then just a tightly fitted and caulked baseboard would fit the bill perfectly.

Eave ventilation is only important with a proper path to Gable Vents, behind the walls, or to a Ridge-Vent system with a proper path behind the ceiling only.

Iggy
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    There's nothing improper about horizontal drywall. Around here it's _all_ installed horizontally, saving garage firewalls where taping isn't planned. – isherwood Nov 28 '22 at 18:01
  • Because everyone else does it, doesn't mean it's right. 4-clicks down the scroll bar here https://diy.stackexchange.com/questions/28484/should-drywall-be-hung-horizontally-or-vertically will certainly get you started in understanding that there's nothing but wrong with horizontal. – Iggy Nov 28 '22 at 18:12
  • The plasterboard is insulated plasterboard, so there’s already insulation on the roof. The house is mid-terrace and I don’t believe there are gable vents across the block but there is ventilation at front and rear soffits. The latter is close to the gap and what I want to make sure not to block. – Drafts Loft Nov 28 '22 at 21:47
  • I’m not concerned about aesthetics. It’s a storage room within roof. What I’m worried about is cold air from the soffit vents entering the room. A skirting board would be possible, though awkward given the angle. I also wondered if it wouldn’t be a weak point in the envelope. – Drafts Loft Nov 28 '22 at 21:49
  • Then you'll be in good shape in just filling the gap and even covering it. A squirt of Big Gap Spray Foam on the back of baseboard as it's applied will squish nicely. Leaving the gap though, can introduce too much humidity and heat to cause condensation or ice dams in cold winter climates. – Iggy Nov 28 '22 at 21:55
  • But isn’t foam likely to block the air coming from the soffit vents? I thought this was important to ventilate the proper attic space (above my storage room). It’s a cold roof 1910s terrace. – Drafts Loft Nov 28 '22 at 22:19
  • Not as far as I see. I'm only talking about matching the depth of the plasterboard's insulation and not implying that you should fill the space behind the gap. It looks like there's cm's of airflow behind the gap. For example ----- 1) You can shove thinner Styrofoam sideways tightly and only a couple of cm's deep into the gap. 2) Then, cut it flush with the plasterboard above. 3) Then, smear its face with plaster or even caulk to successfully paint it...if desired. – Iggy Nov 30 '22 at 00:50
  • Okay, thanks. I was just worried about spray foam. – Drafts Loft Nov 30 '22 at 09:59