Roof insulation and condensation issues

[Jimothy]

Hi all,

we're in the process of purchasing a new house at the moment, but the roof felt has been exhibiting quite alot of condensation. I'd noticed this when viewing the house, but as all the visible timbers and panelling seemed dry and in good condition, didn't give it much thought.

The survey picked it up, though, and is suggesting "further investigation" with a view to increased ventilation.

At the same time, though, the roof joists have been insulated with 100mm of fibreglass and then boarded, the roof has a skylight in it, and shelving has been put all around, making the roofspace a very handy storage space.

I'm wondering if there's any reason why I shouldn't insulate the underside of the roof (between the rafters) to:

a) reduce the temperature gradient, and thereby reduce condensation
b) avoid having to bury the nice useful boarding under piles more insulation to bring it up to standard.

Everywhere I look, I see "cold roof" (ceiling level insulation) and "warm roof"(rafter level insulation) solutions discussed, but is there any middle ground where the loft is insulated at both levels to provide a "buffer zone" temperature-wise, whilst retaining maximum usability?

Thanks in advance for any thoughts.
Jim

 

[^woody^]

The survey picked it up, though, and is suggesting "further investigation"

What a joke. The purpose of a survey is to advise not recommend more investigation for a simple thing which the surveyor should know about

You need to increase the insulation at the ceiling level to prevent more warm air entering the loft space to condense on the felt. About 300mm would be good

This may be enough

If not, then open the some laps of the felt with your hand or wedge with some bits of timber. Search for previous posts on this. No need to add specific ventilation

You should not insulate at rafter level else you trap warm air between roof and ceiling

 

[Jimothy]

Thanks for your reply, I don't really understand why adding more insulation would help prevent the moisture getting in there, though. Yes, warmth permeates through the ceiling from below, but surely not moist air itself? I assume that moisture gets in there as and when the hatch is opened, and the air from the house can mix with that in the loft space.

Anyway, I spoke to the surveyor today, who said that as the roof has both ridge- ands eave-vents there really shouldn't really be any lack of ventilation, but he thinks that maybe the problem is that some of the vents are blocked, or simply that the panelling in the loft space is interupting the airflow within it. Unfortunately, because of the crawling boards are limited to the centre of the roof, he couldn't assess the eave vents themselves and suggested I get a roofer to come and examine it all.

 

[^woody^]

All air carries moisture - 40-95% normally.

The heat going through the ceiling heats the air up, and then when the warm air hits the cold felt, it condenses.

If the air remains cool, then the dew point is lowered and it does not condense so readily on cold felt