Property advice: My attic flooring is wet. Why?

Thanks for your help,
Richard.

Hi Richard, 

Thanks for sending in your query. Your construction pal’s instincts were right that moisture may be related to warm air. What I believe he was referring to is condensation. Crudely put, condensation occurs when warm air meets a cold surface and cooling causes vapour to become liquid. Condensation will wet your rafters, underlay (roof felt) and sit on insulation. Damp will get under surfaces and onto storage boxes and make everything moist.

It may be possible that warm, moist air found its way through the opening in the ceilings via pipe and/or electrical penetrations. 

An uninsulated attic hatch may contribute to a greater degree, but the overall impact is likely to be minimal, unless the holes are gaping and you are producing very high levels of steam or moisture, which is unlikely. It would be much more likely that impact by condensation would manifest on ceilings on the habitable side of the room. You often see this represented as black speckling.

Coincidentally, I was at a talk lately where the man sitting beside me told me about the black speckling on his attic hatch. I suggested that the best thing to do was to build a little bulkhead over the attic hatch, on the attic side.

This may be easily done by cutting some insulation upstands, 200mm high, and outlining the hatch opening. Then cut a sheet of insulation and place it on top of the upstands like a lid. When you pull down your folding ladder, all you have to do is push off the insulating lid to gain entry and then replace it just before closing the attic hatch.

A number of proprietary systems will be much more thermally efficient, including an insulated attic hatch door, so have a chat with your construction pal. A good ‘measure twice, cut once’ chap would be my recommendation if you have budget constraints. Where cracks to the ceiling or holes in the ceilings are present, seal these.

You may have read my previous article and so might also be used to me having to making assumptions.

Why should today be any different? So, if you don’t mind, I will assume that you have a ‘cold roof’. That is, your building does not have an attic conversion, but has an open attic, which is insulated between the ceiling joists. You then placed your OSB board so that you could use the space for storage.

With such a roof, your eaves ventilation is designed to allow draft cross your attic, where air is drawn through at one side and enters on the opposite side. This ‘airing’ keeps moisture at bay.

Where airing/ventilation is interrupted — e.g. insulation covering the air vents in the soffit, no soffit ventilation, or excess storage items covering the attic area preventing a draw — it may be the case that air movement in the attic simply gets reduced or even eliminated. 

In Ireland, moisture in the air is pretty much constant, and in the warmer months moisture is held in the air in gaseous form, but when temperatures drop, vapour becomes liquid. Liquid will adhere to any fabric in the attic. 

It will roll under OSB, sit on insulation and storage materials and generally get everywhere. This moisture will hang around until it has the ability to escape. 

Ventilation

Ventilation is the answer here.

Air passing through the attic is essential to ensure that building fabric remains dry and robust. Timbers will resist for a substantial period, but, unlike timbers of the past (1800s and before), modern timbers simply are not as dense, and they may be susceptible to moisture. Incessant impact by moisture will eventually undermine timbers and may even cause rot. 

Clear all vents and where your storage is such that you disrupt air flow, introduce more ventilation into the roof plane by placing ventilation on the higher points of the roof to ensure the essential cross flow of air.

If your building is a dormer, with rooms in the roof and storage above, or storage in the crawl spaces to the side (behind knee walls), ventilation becomes much more nuanced and essential.

Cross ventilation is no longer simple to create. Eave vents are still a major player, and these interact with the aforementioned higher vents. The idea is to create looping of air movement, and this sucks out the moisture.

Higher vents need to be placed on the opposite side to each other to ensure full cross ventilation.

A little air is all that is needed to reduce moisture. There is a bit of working out to be done, but the good news is that vents may be retrofitted in most cases, it’s just a matter of throwing an expert eye over it. I would recommend consulting a chartered building surveyor before making any additions/changes.

Oriented strand boards

As an added precaution, you might consider lifting your OSB and placing additional timber laths on top of the joists to support the sheeting. But the key is to leave gaps between these added timbers to ensure air may get underneath to deal with any trapped moisture.

Moisture cannot dry out if ventilation can’t reach it. This will also ensure that attic insulation is not squashed or reduced. Squashed insulation impacts thermal performance.

In the meantime, pull back your OSB sheeting by standing it on end, and allow moisture to escape and the fabric to dry out and then look at working out your ventilation strategy. Hopefully some or all of these measures will address your attic issues.

• Brigid Browne is a chartered building surveyor and chair of the Southern Region of the Society of Chartered Surveyors Ireland. 
• She is the owner/ managing director of Cashel-based Fortress Planning which offers a range of services including assigned certifier, design certifier, building surveyor, and conservation consultancy to clients all over the country — www.fortressplanning.ie
• If you have a property related query or issue you would like to raise with Brigid, please email irishexaminerpropertyqueries@scsi.ie